Managing Building Moisture

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Managing BuildingMoisture


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American Standard Inc. 1998

SYS-AM-15 i

Managing BuildingMoisture

by Dennis Stanke, staff engineer

La Crosse, Wisconsin

Bruce Bradway, senior airside applications engineer

Lexington, Kentucky

with Art Hallstrom, airside applications engineering manager

Lexington Kentucky

Nan Bailey, information designer

La Crosse, Wisconsin

Special thanks to: J. David Odom III, vice president CH2M Hill and his staff in

Orlando, Florida, for permission to adapt text and illustrations from the

CH2M Hill manual Preventing Indoor Air Quality Problems in Hot,

Humid Climates: Design and Construction Guidelines, CH2M Hill,

1996.


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ii American Standard Inc. 1998SYS-AM-15

Contents

Preface ......................................................................................................... iv

Introduction ............................................................................................. 1

Good Reasons for Dry Buildings ........................................ 2Better Indoor Air Quality/Better Health ......................................................2

Reduced Building System Deterioration ................................................... 3

More Comfortable Space Conditions ......................................................... 3

Moisture Sources ............................................................................... 4

Liquid-Water Sources ............................................................................. 4Weather .................................................................................................. 5

Ground Water ......................................................................................... 5

Interior Leaks.......................................................................................... 6

Cleaning ................................................................................................. 6

Water-Vapor Sources ...................................................................................6

Vapor-Pressure Diffusion ........................................................................ 7

People .................................................................................................. 10

Evaporation .......................................................................................... 10

Combustion .......................................................................................... 12

Infiltration .............................................................................................. 12

Ventilation ............................................................................................. 14

Condensation ............................................................................................. 15

Moisture and Building Envelope ....................................... 17Prevent Liquid-Water Intrusion ................................................................17

Minimize Vapor-Pressure Diffusion .........................................................17

Minimize Infiltration ................................................................................... 18

Summary ..................................................................................................... 20

Moisture and Occupied Spaces ......................................... 21Minimize Liquid-Water Sources ...............................................................21

Prevent Unplanned Condensation ........................................................... 21

Dehumidify Spaces .................................................................................... 22

Account for All Loads............................................................................23

Part-Load Control ................................................................................. 23

Unoccupied Control .............................................................................. 24

System Monitoring ................................................................................25

Summary ..................................................................................................... 25


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SYS-AM-15 iii

Contents

Equipment-Room Moisture ....................................................26Minimize Moisture Sources ...................................................................... 26

Prevent Unplanned Condensation ........................................................... 27

Raise Surface Temperature and Vapor Seal ........................................ 27

Lower Equipment-Room Dew Point ..................................................... 27

Dehumidify Ventilation Air ........................................................................28

Equipment-Room Design Examples ........................................................30

A Poor Design: Negative Pressurization ..............................................30

A Better Design: Positive Return-Air Pressurization ............................32

Best Design: Positive Supply-Air Pressurization ..................................34

Summary ..................................................................................................... 35

Moisture and Chillers ................................................................... 36

Moisture and Air-Handling Units .......................................37Condensate Collection Pans ....................................................................37

Size Coil to Limit Carryover ..................................................................37

Slope to Prevent Standing Water ......................................................... 38

Drain to Prevent Flooding.....................................................................39

Drain-Line Seals ......................................................................................... 39

External Condensation .............................................................................. 41

Seal Penetrations and Joints................................................................ 42

Improve Unit Insulation .........................................................................43

Internal Condensation ............................................................................... 45Seal Unit Penetrations and Joints ........................................................ 46

Lower Equipment-Room Dew Point ..................................................... 46

In Conclusion ....................................................................................... 47Acknowledgments ................................................................................ 47


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iv American Standard Inc. 1998SYS-AM-15

Preface

Uncontrolled moisture within a building can contribute to structural damage,

occupant discomfort and unacceptable indoor air quality. Moisture is often

overlooked or underestimated during HVAC system design and operation, and it

can cause significant problems in the building envelope, occupied spaces and

mechanical-equipment rooms.

This manual helps HVAC system designers identify and quantify moisture

sources. It also presents moisture-management techniques related to the

building envelope, the occupied space and the mechanical-equipment room.

Moisture problems can occur in buildings in any geographic location. The

solutions identified in this manual apply to building-moisture management in any

climate; however, the concepts are especially applicable for buildings located inhumid climates.

The Trane Company, in proposing these system design and application

concepts, assumes no responsibility for the performance or desirability of any

resulting system design. System design is the prerogative and responsibility of

the system designer.


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Introduction

Uncontrolled moisture in buildings can cause serious problems for building

occupants, furnishings and structure. Microbial growth, encouraged by high

relative humidity, leads to poor indoor air quality and building deterioration.

Poor IAQ results in discomfort, health problems and could ultimately lower

productivity and spawn lawsuits. Uncontrolled moisture can also stain wall

surfaces, damage paint, corrode metal surfaces and accelerate the deterioration

of building furnishings and structural materials. Buildings located in climates with

long periods of high ambient temperature and high, absolute humidity (hot,

humid climates) are particularly prone to uncontrolled moisture. Figure 1

illustrates humid climate and fringe climate regions within the U.S.

Figure 1 Hot, Humid Climates

This manual discusses design considerations and HVAC operating techniques

that help control moisture within occupied buildings once it enters the building. l


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Good Reasons forDry Buildings

Controlling building moisture takes time and money. How do building

occupants and building owners benefit from these expenditures?

Better Indoor Air Quality/Better HealthBuilding components (walls, floors and ceilings) and building furnishings (wall

coverings, carpets, furniture and stored materials) provide ideal amplification

sites for microbial growth. Microbial growthfungi, bacteria and dust mites, for

instancecan produce odors, allergens, and in some cases, toxins.

Odors cause discomfort and long-term exposure to allergens and toxins can lead

to health problems such as asthma and lung disease. Also, musty-smelling

buildings cannot command high rent or a high resale price.

For microbial growth to occur, certain conditions must be present

w A source of food

w Temperature between 40F100F

w Adequate moisture, usually 70% RH or higher

w A source of mold or mildew spores

These conditions can certainly be present in a building. Materials used in

building construction, building furnishings, stored materials (books and papers)

and accumulated dirt can all become food sources for microbial growth. Typical

indoor temperatures fall in the middle of the microbial growth temperature range.

Uncontrolled, indoor relative humidity can easily rise above the 70% RH needed

to encourage microbial growth, especially in hot, humid climates. Spores, ofcourse, are present everywhere in both indoor and outdoor air as well as in

building materials and furnishings.

Of the four conditions for microbial growth, relative humidity is most easily

controlled. Maintaining indoor relative humidity below 60% RH, as required by

ASHRAE 62-1989 (Figure 2), limits the potential for microbial growth in buildings.

Figure 2 ASHRAE-Recommended Humidity Levels


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Good Reasons forDry Buildings

Reduced Building System DeteriorationThe same fungi (mold and mildew) that cause people discomfort and/or harm

can also cause building materials, furnishings and structure to prematurely

corrode and/or degenerate. Premature failure of walls, ceilings and floors and

irreversible damage to carpets, wall coverings and furnishings can result.

The same rationale holds true for the air handling equipment and duct system.

Wet insulation can lead to corrosion and/or deterioration, shortening the useful

life and effectiveness of the air-distribution system.

Deterioration in buildings increases maintenance and operation costs. Figure 3

illustrates the relationship between the effects of relative humidity and building

operation, maintenance and repair costs. Maintenance includes normal cleaning

and periodic replacement of damaged furnishings, such as moldy carpet and

wallpaper. Building operational costs include the cost of energy.

Figure 3 Humidity and Building Costs

More Comfortable Space ConditionsControlling indoor relative humidity to an acceptable level results in consistent

thermal comfort within the occupied spaces. Thermal comfort reduces occupant

complaints and improves worker productivity, and increases both rental potential

and market value. l


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Moisture Sources

Moisture enters the building as liquid water or as water vapor. It causes trouble

in either form and changes readily from vapor to liquid through condensation. It

must be properly managed to avoid trouble. Lets look at possible moisture

sources and techniques to minimize the impact of each.

Liquid-Water SourcesLiquid water damages furnishings and building structure, supports microbial

growth and provides surface wetness for evaporation, a source of water vapor

and increased indoor moisture load. Common liquid-water sources include the

weather (rain, fog and snow), ground water, leaking pipes and equipment, and

wet cleaning processes (see Figure 4). Perhaps the most troublesome source of

liquid water, condensed water vapor, is discussed separately on the following

pages.

Figure 4 Common Liquid-Water Sources in Buildings


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Moisture Sources

Weather

During building construction, prior to completion of the roof and walls, building

materials and the partially completed building structure may become saturated

with rainwater or snow. Protect building materials from rain, snow and

condensation (which can form inside equipment wrapped with a vapor retarder

such as plastic) during construction. For best results, store materials within

covered structures. If materials become wet, dry them quickly and completely or

replace them. Mold can grow within 24 hours on wet materials. Discard visibly

moldy materials and replace with new, dry materials.

During normal operation, rain may enter the building structure through roof and

wall leaks or openings. Design windows and walls to minimize leakage and

control water with internal drainage schemes. Roof design and construction

practices must result in a leakproof membrane that drains properly. Roofs need

proper maintenance to assure long-term integrity. Repair leaky walls and roofs

quickly to prevent water damage and to avoid high latent loads indoors.

The outdoor intake airflow may entrain rain droplets or snowflakes and carry

them into the duct system. Design outdoor air intakes to limit rainwater

entrainment, using rain hoods and moisture eliminators sized to avoid high intake

velocity. If the design allows water droplets to penetrate the intake, provide for

indoor drainage (drain pans for instance); in other words, manage the water flow

once it enters the building.

Rain hoods and moisture eliminators cannot stop entrained snow. Prevent

possible filter damage or internal flooding using a heating coil to melt the snow ora large plenum to allow the snow to settle, melt and evaporate before it reaches

the filters.

Fog also enters the building through the outdoor air intake. Fog droplets, too

small to stop at a louver or moisture eliminator, usually evaporate quickly within

the HVAC system, causing little or no surface wetting. However, the evaporated

droplets certainly add to the indoor moisture load and must be accounted for in

the design and operation of the system.

Ground Water

Ground water may seep into the building through very small cracks in basement

walls and floors. Be sure that surface water and subsurface water drains away

from the building, not toward it. Design the floor to limit water intrusion via cracks

and joints. Manage any water that penetrates the floor using proper slopes and

drains.


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Interior Leaks

Leaking appliances, valves and pipes can quickly wet large areas of interior

structure. Liquid water within walls and concealed areas, through capillary action

and surface tension, can travel long distances and result in widespread, long-

term wetting.

Plan and follow maintenance procedures to assure speedy location and repair of

any water leaks within the building. Accidental spills and floods should be

cleaned up quickly. If porous materials become wet, dry them completely within

24 hours or consider replacing them.

Cleaning

Cleaning processes, such as floor mopping and carpet shampooing, result in

large wet areas. Carpet shampooing in particular increases moisture content in

carpet fibers (and in the accumulated dirt beneath the carpet).

Use wet cleaning processes cautiously (or not at all). Take steps to dry wet-

cleaned surfaces within 24 hours. For shampooed carpets, assure speedy

evaporation by providing adequate air motion and dehumidification during drying.

If a cooling coil controlled by a thermostat provides the necessary

dehumidification, auxiliary heat may be needed to maintain a sufficient cooling

coil load for continuous dehumidifying capacity.

Water-Vapor SourcesA high indoor water-vapor level elevates the indoor dew point and relative

humidity, and it contributes to the latent load on HVAC equipment. High dew

point increases the likelihood of unplanned condensation; high relative humidity

can result in occupant discomfort, increased dust mite population, and

accelerated microbial growth. HVAC-equipment capacity must match total (latent

plus sensible) load.

Common water-vapor sources in buildings (see Figure 5) include vapor-pressure

diffusion from outside, evaporation from people, wet surfaces and processes,

generation from combustion, infiltration from outside and introduction from

outside via ventilation airflow.

Moisture Sources


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Figure 5 Common Water-Vapor Sources

Vapor-Pressure Diffusion

Water vapor moves through solid materials in direct proportion to the difference

in vapor pressure between the opposite sides of the material and the resistance

of the material to water-vapor flow (the permeance of the material).

Moisture Sources


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The moisture load (grains of water per hour) added to a building from outside

by vapor-pressure diffusion can be found using Equation 1.

Equation 1

Wp = P A (VPo VPi)

Where

Wp = moisture load (gr/h)

P = permeance factor (gr/h ft2 in. Hg)

A = surface area (ft2)

VPo = outdoor vapor pressure (in. Hg)

VPi = indoor vapor pressure (in. Hg)

The table below (from 1997 ASHRAE Fundamentals Handbook) shows

permeance factors for some common construction materials. The permeance

factor for a composite wall (Pc) can be found as the reciprocal of the sum of

permeance reciprocals.

Common Construction Materials Permeance*

hardwood siding (1/8") 11.00

air space (1.0") 120.00

polyethylene vapor retarder (0.002") 0.16

insulating board sheathing (1.0") 50.00

fibrous insulation (6.0") 19.00

gypsum wallboard (3/8") 50.00

paint, commercial latex 6.28

vinyl wallpaper 0.23

*Permeance factor = gr/h ft2 in. Hg

Consider a composite wall that includes wallboard (P = 50), fibrous insulation

(P = 19), exterior sheathing (P = 50), and hardboard siding (P = 11), and has a

high composite permeance [Pc = 1 (1 50 + 1 19 +1 50 + 1 11) = 5.448];

i.e. water vapor can diffuse through the wall quite readily. The same composite

wall with a polyethylene vapor retarder (P = 0.16), has a much lower its

composite permeance (Pc = 0.155); i.e. water-vapor diffusion through this wall is

much more difficult.

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Figure 6 shows the composite walls described above. On a hot, humid day in

Jacksonville, outdoor water-vapor pressure exceeds indoor water-vapor pressure

more significantly (VPo VPi = 1.03 - 0.39 = 0.64 in. Hg) than on a cooler day

(VPo VPi = 0.60 0.39 = 0.21 in. Hg).

Applying the permeable wall structure (Pc = 5.435) to a small building with

4000 ft2 of exterior wall surface, and solving Equation 1, we find that the moisture

load due to vapor-pressure diffusion can be quite high (Wp = 5.448 4000 0.64

= 13,900 gr/h) on the hot day and lower on the cooler day (Wp = 5.448 4000

0.21 = 4580 gr/h).

Adding a vapor retarder results in a less permeable wall structure (Pc = 0.155)

and significantly lower moisture load on both the hot day (Wp = 0.155 4000 0.64 = 400 gr/h) and on the cooler day (Wp = 0.155 4000 0.21 = 130 gr/h).

As this example illustrates, moisture load due to vapor-pressure diffusion can be

significant through a simple wall. The addition of an exterior vapor barrier

reduces it considerably but cannot eliminate it.

Use a vapor retarder in floors, walls and ceilings to minimize moisture transfer

due to vapor-pressure diffusion. Locate the vapor retarder within walls and

ceilings, near the warm side, to avoid unplanned condensation within the

structure.

Moisture Sources

Figure 6 Vapor Diffusion Through Wall Structure


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In humid climates and other predominantly cooling climates, use low-

permeance materials near the outside surface (the warm side) to keep

summertime outdoor water vapor away from cold, interior surfaces. When the

building includes an attic, be sure to include a continuous vapor retarder on the

attic side of the insulation.

In mixed climates, for buildings with low wintertime inside relative humidity

(35% RH or less), use low-permeance materials near the outside surface, as

recommended for cooling climates.

In predominantly heating climates, use low-permeance materials near the inside

surface (the warm side) to keep wintertime indoor water vapor away from cold

exterior surfaces.

People

Building occupants produce water vapor at different rates, depending upon their

activity level, via respiration and perspiration. According to the 1997 ASHRAE

Fundamentals Handbook, an adult working at a desk introduces a latent load of

155 Btu/h, and an active athlete contributes 1000 Btu/h.

Since water vapor contains approximately 0.14 Btu/gr, an office worker

contributes 1100 gr/h (Wo = 155 / 0.14) to the indoor moisture load while a

volleyball player contributes approximately 7100 gr/h (Wo = 1000 / 0.14).

Although it can be significant, many designers erroneously consider respirationas the sole source of moisture load in buildings. Design the HVAC system and

equipment with sufficient capacity to satisfy the total moisture load, including the

people-related moisture load as one of many sources.

Evaporation

Wet surfaces add moisture to indoor air through evaporation. Evaporation occurs

when the air vapor pressure (VPa) is less than the vapor pressure of the

saturated air at the wet surface (VPs).

Wet surfaces, found throughout the building, may be planned (pools, aquariums

and fountains) or unplanned (such as leaky pipes and unplanned condensation).

Cooking processes and live plants add water vapor via evaporation. On rainy orsnowy days, building occupants carry a significant amount of moisture into the

building on their shoes and clothing.

Equation 2 can be used to calculate the moisture load due to evaporation from

liquid-water surfaces.

Moisture Sources


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Equation 2

We = H A (VPsVPa) 7000/1060

Where

We = moisture load from evaporation (gr/h)

H = latent heat transfer rate (Btu/h ft2 in. Hg, see Figure 7)

A = water surface area (ft2)

VPs = saturated vapor pressure of air at the water surface

temperature (in. Hg)

VPa = vapor pressure of space air (in. Hg)

7000 = definition of grains (gr/lb)

1060 = latent heat of vaporization at 75F (Btu/lb)

Figure 7 shows latent heat transfer rate (H) related to air stream velocity. For

example, a large aquarium in an office with typical 50-fpm transverse airflow

(perpendicular to surface) transfers latent heat to the air at a rate of (H = 250

Btu/h ft2 in. Hg). If the space is 72F, 50% RH (VPa = 0.39), and 8 ft2 of water

surface at 78F (VPs = 0.96 in. wg) is exposed, the evaporation moisture load

can be found using Equation 2 (We = 250 8 0.57 7000 / 1060 = 7500 gr/h).

In another example, a 4-ft diameter puddle (A = 12.5 ft2) of condensate at 80F

(VPs = 1.03) on the floor of an equipment room at 85F, 50% RH (VPa = 0.60),

contributes 8900 gr/h (We = 250 12.5 0.43 7000 / 1060) to the equipment

room.

Figure 7 Latent Heat Transfer from Water Surface

(with respect to the research by W.H. Carrier in 1918)

Moisture Sources


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Depending on the situation, moisture load due to evaporation from water

surfaces may be significant. Design the HVAC system with sufficient capacity to

satisfy the moisture load from all sources, including evaporation from planned

water surfaces (pools and fountains) and predictable liquid-water sources

(rainwater in entryways, shampooed carpets). Minimize unplanned evaporation

sources (liquid water from leaky pipes, leaky roofs, and spills) by quickly

eliminating the source and removing the liquid water.

Combustion

Combustion liberates water vapor. Remember from high-school chemistry: the

two products of complete combustion are carbon dioxide (CO2) and water (H2O).Equation 3 can be used to calculate the rate of moisture generation from an

open gas flame. Open-flame heaters, boilers and appliances, as well as open-

flame cooking surfaces, can produce significant water vapor.

Indoor combustion processes must be considered when calculating indoor

moisture load. If an unvented gas griddle consumes natural gas at a known rate

(G = 6.7 ft3/h), the combustion moisture load can be easily found (Wc = 6.7 650

= 4400 gr/h).

Equation 3

Wc = G K

WhereWc = moisture load from combustion (gr/h)

G = gas firing rate (ft3/h)

K = 650 gr/ft3 for natural gas

1300 gr/ft3 for propane

If possible, use vented combustion processes to eliminate combustion moisture

load and other products of combustion. Dehumidify to remove moisture from

unvented combustion processes within the building.

Infiltration

No building is airtight. Outdoor air enters (infiltration) and indoor air leaves

(exfiltration) via countless little openings in the building envelope as well as large

intentional openings such as doors and windows.

Driven by differential pressures imposed by mechanical ventilation, wind and

stack effect, the infiltration (exfiltration) air carries water vapor with it. Air passes

through any available penetration in wall, floor or ceiling.

Moisture Sources


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Elevator shafts act as chimneys, reducing pressure on lower floors. Gaps can

be found at wall-floor or wall-ceiling joints, between wallboard and electrical

fixtures on perimeter walls, at electrical wire and conduit penetrations, at pipe

and duct penetrations, and at window and door penetrations. Air also comes

and goes through open exterior doorways and windows. Whenever an exterior

door opens in Florida, cooled indoor air spills out of the building at floor level,

only to be replaced by an inflow of warm, moist air at door-top level.

How much water vapor moves with the air? Building moisture load due to

infiltration can be calculated using Equation 4.

Equation 4

Wi = A r 60 Va (HRo HRi)

Where

Wi = moisture load due to infiltration (gr/h)

A = area of opening (ft2)

r = density of outdoor air (lb/ft3)

60 = minutes per hour (m/h)

Va = air velocity through opening (fpm)

HRo = outdoor air humidity ratio (gr/lb)

HRi = indoor air humidity ratio (gr/lb)

Equation 4 includes air velocity and the total area of all openings in the envelope.

Values for these variables may be estimated, either at normal or extreme

differential (inside-to-outside) building pressure. However, it may be more

practical to estimate total envelope airflow (Qe = A Va) as follows.

Building tightness specifications often rate building leakage at a differential

pressure of 0.30 in. wc. At a positive differential pressure of 0.30, the 1997

ASHRAE Fundamentals Handbook, Chapter 25, estimates typical exfiltration

airflow per square foot of exterior wall surface for tight, average and leaky walls

(Q = 0.10, 0.30 and 0.60 cfm/ft2, respectively).

If we assume an exponential relationship between differential pressure and

airflow (Q = k P0.65), established statistically by various researchers, we can find

the flow coefficient (k) for each wall construction category, as shown in Figure 8.

Moisture Sources


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Figure 8 Leakage Airflow

Assuming our example building in Jacksonville uses average wall construction

and operates at a slightly negative pressure (DP = 0.05 in. wc.), we can calculate

infiltration airflow (Q = 0.66 (0.05)0.65 = 0.094 cfm/ft2) per square foot of exterior

wall surface or total infiltration through the envelope (Qe = Q Ae = 0.094 4000

= 376 cfm).

In Jacksonville, Florida, when outdoor conditions are hot (96F, 80% RH, HRo =

156 gr/lb) and indoor conditions are comfortable (72F, 50% RH, HRi = 58 gr/lb),

we can use Equation 4 (with A Va = Qe) to calculate the infiltration moisture load

(Wi = 376 0.069 60 (156 58) = 153,000 gr/h).

Even very low infiltration airflow can contribute significantly to moisture load and

must be considered during design. Seal air leaks due to cracks/gaps/holes

where air and moisture can enter, paying particular attention to wall penetrations.

In hot, humid climates, design to minimize infiltration by maintaining indoor

spaces at a slightly higher static pressure than outdoor atmospheric pressure;

operate with positive building pressure.

Ventilation

HVAC equipment draws in outdoor air for ventilation and to replace indoor air

removed by exhaust fans and combustion processes. Ventilation air can contain

considerable moisture, especially in hot, humid climates. Water vapor in the

ventilation air often represents the single largest source of moisture in a building.

We can use ventilation airflow (Qv = A Va) in Equation 4 to find ventilation

moisture load Wv.

Given ventilation airflow (Qv = 2000 cfm), ventilation moisture load on a hot,

humid day in Jacksonville can be calculated (Wv = 2000 0.069 60 (156 58)

= 820,000 gr/h).

Moisture Sources

Finding Worst Case

Design Condition

Most designers use the ambient

weather data presented in the

1997 ASHRAE Fundamentals

Handbook to determine total

cooling load due to ventilation.

Traditionally, designers used peak

dry-bulb temperature and mean-

coincident wet-bulb temperature

as worst-case design condition.

A designer in Jacksonville, using

the 0.4% annual peak dry bulb

(96F) and mean coincident wet

bulb (76F) from 1997 ASHRAE

Fundamentals Handbook, can first

find outdoor and indoor air

enthalpy (ho = 39.2 and hi = 26.4

Btu/lb, respectively), then find the

associated air conditioning load.

For our example building,

ventilation air adds 9.6 tons

(Qt = 4.5 2000 (39.2 26.4)/

12000 = 9.6) to the building air

conditioning load at this design

condition. Is this the worst-case

ventilation air load? No!

The 1997 ASHRAE Fundamentals

Handbookalso presents annual

peak dew point and mean

coincident dry-bulb temperatures.

Using the 0.4% annual-peak dew

point (76F) and mean coincident

dry-bulb (84F) temperatures,

both outdoor air enthalpy (ho =

41.6 Btu/lb) and ventilation load

(Qt = 11.4 tons) increase.

Ventilation air on a warm, rainy

day represents more total coolingload than the same volume of

ventilation air on a hot, sunny day.


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Contrast the moisture load in Jacksonville to that in Denver. On a typical hot day

in Denver, each pound of ambient air at 93F, 10% RH contains only 24 grains

of water vapor. Again, given ventilation airflow (Qv = 2000 cfm), ventilation

moisture load can be calculated using Equation 4 (Wv = 2000 0.071 60

(24 58) = -290,000 gr/h). Note that a negative ventilation moisture load results.

In Denver, ventilation air can actually remove moisture from the building rather

than add it.

Most designers use ambient weather data (see side-bar) to estimate ventilation

air conditions and ventilation moisture load. Ambient weather data describes

historical conditions in a general geographical region. However, local ventilation-

air moisture content may be even higher than indicated by ambient conditions.

For instance, a roof-mounted intake in Jacksonville may introduce very warm air

(110F) with very high moisture content (170 gr/lb or more), especially when the

sun reappears after a rain shower. Since outdoor airflow cannot be lower than

the minimum required for proper ventilation, the air introduced for ventilation

must be dehumidified.

Select and operate HVAC equipment and systems to dehumidify the ventilation

air at all load conditions.

CondensationCondensate forms whenever moist air contacts a surface at a temperature below

the dew point (Figure 9).

Figure 9 Surface Condensation

Moisture Sources


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Inside the exterior walls, water vapor can enter by vapor-pressure diffusion, by

exfiltration or infiltration, or by evaporation from liquid-water leaksraising

internal dew point.

In occupied spaces during the cooling season, a supply-air duct or chilled-water

pipe behind an interior wall may cool the wall surface below dew point. The

concrete beneath carpeted floors may be very cool compared with average

space temperature. During the heating season, indoor water vapor can easily

condense on cold windows, cold wall surfaces within the space, and inside the

exterior wall structure.

The mechanical equipment room offers many cold surfaces for the formation of

unplanned condensate, especially during the cooling season. The outsidesurfaces of air handlers and supply-air ducts, water chillers, chilled-water and

return-water pipes, and condensate drain pipes all operate at low temperatures.

If the equipment room is not conditioned, equipment-room dew point may be

very, very high, especially in hot, humid climates.

Inside the air handler, surface temperature approaches supply air temperature. If

design philosophy results in a high equipment-room dew point, any air leaks into

the air handler downstream of the cooling coil may cause significant

condensation and flooding inside the air handler.

To avoid unplanned condensation in any location, either raise the surface

temperature or lower the air dew point or both. No other alternative exists.

In summary, many sources of moisture in buildings must be considered. Some

can be eliminated, others can be minimized, but none can be ignored during

building design and operation. l

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Four major building elements must be designed and operated properly to

minimize the impact of moisture in the building

w Envelope

w Occupied spaces

w Equipment room

w HVAC equipment

Lets start with the building envelope.

Unwanted, unplanned condensate forms inside walls and ceilings if the dew

point of the internal air exceeds the coldest surface temperature. Since water

vapor can enter the wall cavity by evaporation, vapor-pressure diffusion or

infiltration, all three of these potential water-vapor sources must be managed toprevent unplanned condensation.

Prevent Liquid-Water IntrusionWater vapor inside the building envelope structure increases if evaporation from

liquid water occurs. Design and construct exterior walls to keep liquid water out.

Liquid water includes not only rainwater and melted snow, but also ground water,

water from leaky pipes and unplanned condensation. Use a water barrier near

the outside surface of exterior walls to keep the rain and snow out. Design the

roof to drain freely and to be watertight. Be sure to drain ground water away from

the building. Seal all envelope penetrations. Seal underground wall-floor systemsand wall-floor joints.

Minimize Vapor-Pressure DiffusionSince the indoor-to-outdoor temperature difference cannot be controlled,

condensation prevention relies on low moisture content (low dew point) within the

walls. In addition to sealing against liquid water, design to limit water-vapor

diffusion into the wallcondensation occurs if moist air can penetrate to a cold

surface.

Use a vapor retarder on the warm side. Warm side (as mentioned above) in

cooling and mixed climates means the outdoor side of the ceiling or the wallstructure; in heating climates, it means the indoor side. Do not use two vapor

retardersone on the warm side and one on the cold side. Moisture trapped

between the two vapor retarders cannot escape and condenses inside the ceiling

or the wall at the cold surface. Also, include a sealed vapor retarder in the ceiling

of the top floor: in heating climates use a vapor retarder on the space-side, and

in cooling climates use one on the attic side.

Moisture andBuilding Envelope


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Figure 10 Comparison of Humid Climate Walls

Avoid low-permeance wall finishes (e.g. vinyl wall coverings and latex paint) on

perimeter walls and ceilings in cooling climates; condensation and mold growth

are commonly found behind perimeter wall coverings in Floridas buildings.

Minimize InfiltrationSince all air contains water vapor, design the wall structure to limit the movement

of moist air into interstitial wall cavities. In cooling and mixed climates, limit

infiltration of outdoor air (the predominant source of moisture) into the building

structure using an airflow or weather barrier near the exterior surface of

perimeter walls.

Figure 10 shows both a poor design and a good design for exterior walls in

cooling or mixed climates. The poor design allows water vapor to penetrate to the

interior wall. The good design limits vapor-pressure diffusion with a warm-side

vapor retarder and outdoor air infiltration with an exterior weather barrier.

Design and operate buildings to assure that indoor static pressure exceeds

outdoor static pressure. Positive building pressure minimizes outdoor infiltration

air, while maximizing exfiltration of low-dew-point indoor air into wall cavities.



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Design the building so that the air-handling system can maintain a slight

positive pressure in all portions of the building, including not only the envelope

walls but also the occupied spaces and the equipment room. Positive building

pressure prevents infiltration due to mechanical system operation (powered

exhaust) and minimizes infiltration due to wind and stack effect. Remember,

makeup airflow must always exceed the airflow needed to replace exhausted

air and combustion air (for any open combustion processes) in order to

maintain positive building pressure. Trane Application Manual AM-CON-17

provides helpful recommendations related to building pressurization control.

Lower dew point and prevent condensation within the perimeter wall structure

by maximizing vapor-pressure diffusion out of the wall while minimizing moist

air infiltration into the wall.

As Figure 11 illustrates, without a warm-side vapor retarder, outdoor water

vapor diffuses into the wall structure. And, operating the building at a negative

pressure, with no exterior air barrier, encourages moist airflow through the wall

and into the space. On the other hand, using highly permeable materials on the

cold side and low permeability materials on the warm side maximizes vapor-

pressure diffusion out of the wall (into the space). Operating with positive building

pressure and using an external air barrier encourages dry indoor airflow through

the wall structure to outdoors.


Figure 11 Humid Climate Wall Operation


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During the winter in heating climates, indoor moisture sources predominate. It

seems logical to operate with negative building pressure to avoid forcing moist

indoor air into the walls. However, negative indoor pressure increases the

infiltration of very cold outdoor air, possibly resulting in drafts, cold spots,

temperature stratification and discomfort.

Operating with positive indoor pressure, on the other hand, results in exfiltration

of moist indoor air into the perimeter walls. Condensation within the wall structure

results. Very low outdoor-air vapor pressure eventually drives water vapor out for

the wall, but repeated freeze/thaw cycles can cause serious mechanical

damage, especially within masonry walls.

Proper air distribution plays an important role in uniform building pressurization.Air-distribution systems must be balanced after installation and should be

balanced periodically throughout the life of the building, especially after changes

in building use or after system alterations.

SummaryUse water barriers and internal drainage to minimize liquid-water penetration and

eliminate water accumulation inside wall structures. Use a vapor retarder on the

warm side of perimeter walls to minimize vapor-pressure diffusion. Do not use a

vapor retarder (either purposely or accidentally) on the cold side of perimeter

walls. Use an air (weather) barrier on the outside to minimize infiltration due to

wind and stack effect and liquid-water penetration. Maintain positive buildingpressure in cooling climates to eliminate infiltration due to mechanical system

operation.

In summary, buildings in humid and predominately cooling climates should be

designed and operated to be slightly positive. Buildings in heating climates are

comfortable if positive, but may suffer structural damage in winter. Some

designers and operators maintain slightly positive summertime pressure and

slightly negative pressure during transitional seasons and in the winter. l



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Now, lets turn from the building envelope to the occupied spaces. The

occupied spaces inside the building should also be designed and operated

properly to minimize the impact of moisture.

According to ASHRAE 62-89 requirements, relative humidity in occupied spaces

should be maintained below 60% RH at all load conditions. Moisture enters the

occupied space by a number of paths, increasing space dew point, relative

humidity and moisture load. Moisture entry can be minimized, but it cannot be

stopped. Therefore, spaces must be designed to minimize moisture load and

control relative humidity to 60% RH or less at all load conditions.

Minimize Liquid-Water SourcesUnplanned liquid water within the occupied space can damage furnishings and

promote microbial growth. It also evaporates, adding unplanned moisture load to

the indoor air and raising both dew point and relative humidity.

Design and install window systems that minimize leakage. Keep the windows

closed during rain showers. As mentioned above, fix leaky pipes or appliances

and dry accidental spills within 24 hours. If possible, avoid carpet shampooing; if

unavoidable, be sure to dry shampooed carpets quickly while maintaining control

of space relative humidity. This might mean adding heat to the space to assure

adequate latent cooling capacity while drying.

Prevent Unplanned CondensationWater vapor in the air cannot be eliminated, but keeping space dew point lower

than space surface temperature can prevent condensation within the space.

Minimize vapor-pressure diffusion using properly placed vapor retarders.

Minimize unnecessary wet surfaces (evaporation) within the space. Eliminate

unvented combustion processes. To minimize outdoor air infiltration, maintain

positive space pressure in cooling climates and design entryways (using entry

tunnels or airlock vestibules) to prevent excessive air-exchange. Do not

introduce untreated outdoor air for space ventilation.

Eliminate cold surfaces. Raise surface temperature by adding heat wherenecessary, or avoid cold-surface/moist-air contact by insulating surfaces from

potential heat sinks and applying a vapor retarder on the warm side. Finally,

Moisture andOccupied Spaces


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lower space dew point by removing water vapor. Account for all moisture

sources that cannot be eliminated and design the HVAC system with sufficient

dehumidification capacity for full-load operation and with proven control schemes

capable of maintaining relative humidity below 60% RH at all operating

conditions.

Dehumidify SpacesSince moisture enters the space continuously, it must be removed continuously

to maintain relative humidity below 60% RH. As Figure 12 illustrates, moisture

can be removed from the occupied space in two ways: direct dehumidification

within the space, or by replacing moist return air with dry supply air.

Direct dehumidification, commonly applied in residential basements, simply uses

a local dehumidifiera fan and cold and warm coils in seriesto remove water

vapor from space air.

Large buildings more commonly use return airflow to physically carry water vapor

away from the space. Air supplied to the space must be dry enough to absorb the

water vapor entering the spacesupply air dew point must be low. As local

moisture sources add water vapor to the space, average space dew point can be

maintained by adding sufficiently dry supply air. Space air absorbs the water

vapor produced within the space (the latent load) and return air carries it away.

Supply air is commonly dehumidified in one of two ways: by passing it through a

cold coil or by passing it over a desiccant material, usually mounted on a rotatingwheel. A cold coil dehumidifies as entering-air water vapor condenses on the coil

surface and flows down a drain. A desiccant material dehumidifies as entering-air

water vapor adsorbs onto the desiccant surface and is then rejected to a

separate regeneration air stream.


Figure 12 Two Space Dehumidification Methods


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Account for All Loads

High, local dew points can occur indoors. Near an exterior door in a Florida

school, for instance, water vapor enters whenever the door opens, and the door

may open quite often. Local dew point in the entryway rises considerably. This

moisture, and all locally introduced moisture, must be removed by the

dehumidification system. Account for all moisture loads, not just people loads,

when designing (sizing) the dehumidification system and the part-load control

system.

Select central dehumidification equipment with sufficient capacity to dehumidify

at worst-case conditions. For moisture load, worst-case conditions usually occur

on a cool rainy day. The 1997 ASHRAE Fundamentals Handbooknow includes

values for both design-dry-bulb/mean-coincident-wet-bulb and design-dew-point/

mean-coincident-dry-bulb to help designers size equipment properly.

Remember that local, ventilation-air moisture content may be even higher than

ambient conditions indicate. A sun-baked roof after a rain shower can add

significantly to moisture load.

Part-Load Control

An HVAC system sized for the sensible design load usually has sufficient

capacity to adequately remove moisture at both design and part load. However,

if not properly controlled at part load, the system may not maintain space

conditions below 60% RH.

For instance, a 15-ton packaged rooftop, modulated by a thermostat (a sensible

temperature controller), may maintain both sensible temperature and relative

humidity in a classroom very well on a summer day. On a cool, rainy day,

however, the thermostat modulates both the sensible and latent capacity of the

rooftop unit. Although it maintains sensible temperature, the thermostat allows

space relative humidity to rise unacceptably.

Some HVAC systems, VAV for instance, can provide central dehumidification and

control space relative humidity over a wide range of loads without actually

sensing it space thermostats control temperature by modulating the flow of

very dry air to the spaces. Other HVAC systems, like the constant-volume rooftop

system mentioned above, cannot assure indirect relative humidity control.

Rather, relative humidity must be sensed to allow active modulation of cooling

capacity for dehumidification as well as sensible temperature control.



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Systems that deliberately separate space temperature control from space

moisture control require both a thermostat and a humidistat. A classroom unit

ventilator, for instance, can use a thermostat to modulate the cooling coil and

control sensible temperature and a humidistat to sense relative humidity. If the

space exceeds 60% RH, cooling coil capacity increases to maximum and the

thermostat modulates a tempering or reheat coil to maintain sensible

temperature.

Note that part-load humidity control with cold coils usually requires supply-air

tempering to avoid overcooling the spaces. The boiler or some other source of

heat must be available at part-load operating conditions to add heat to the cool,

dry supply air. On the other hand, when using a desiccant-based dehumidifier,

the adsorption process heats the supply air; a chiller or some other source ofcooling must be available at all load conditions to remove heat from the warm,

dry supply air.

Some central system controls actually reset supply air temperature upward at

sensible part-load conditions, often resulting in very poor control of space relative

humidity. Warm supply air contains more moisture than cold supply air. It cannot

lower average space relative humidity nearly as well as supply air at design

temperature. Especially in cooling climates, be sure that supply-air-temperature-

reset schemes include high-humidity override techniques (return-air humidity

sensing, for instance) to limit the amount of reset operation.

Unoccupied ControlDesign and operate to keep indoor spaces dry at all times, not just during

occupied periods. Monitor building humidity during weekend and night setback

cycles, and automatically operate the dehumidification system to maintain space

humidity below 60% RH. Unoccupied dehumidification helps limit microbial

growth and dust mite populations. (Although the people leave the building on

weekends, the microorganisms do not!) It also helps avoid long dehumidification

pull-down times after the unoccupied period. Indoor relative humidity conditions

return to normal quickly. Carpet, furniture and other porous materials do not

store moisture during unoccupied hours if low vapor pressure is maintained.

Unoccupied dehumidification is particularly important for schools in hot, humid

climates. When the children leave the building at 3:00 p.m., significant water

vapor can enter the space via door-opening infiltration. If the dehumidification

system turns off at 3:00 p.m. too, space dew point rises and unplanned

condensation can easily occur. Allow continued dehumidification system

operation after the occupants leave.



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System Monitoring

Modern building control systems can monitor and log both equipment and

space conditions. Use trend-logging features to monitor changes in space

humidity, building pressure, outdoor airflow and temperatures throughout the

system. Observed changes in these variables may be useful when diagnosing

perceived or potential building problems. For instance, high relative humidity

might indicate negative building pressurization (a system malfunction),

deteriorated window or door seals, open loading-dock doors, pipe or roof

leaks, or other equipment failures.

SummaryDesign and operate the HVAC system, especially during periods of partial

sensible cooling load, to maintain indoor relative humidity below 60% RH, i.e.

pressurize and dehumidify. l



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In addition to the building envelope and the occupied spaces, proper design

and operation of another key building element, the equipment room, must be

considered.

Many building designers place the building HVAC components together in a

mechanical equipment room. Some equipment rooms include only chillers,

others include only air handling units, while still others include chillers, air

handlers and a variety of other equipment. Regardless of equipment-room

contents, improper design related to moisture can result in many costly

problems.

Equipment rusts, wood rots, insulation deteriorates, water pools on the floor and

may invade other parts of the building. Microorganisms bloom, resulting in odors,irritants and further deterioration of materials. To avoid or at least minimize these

problems, as with the envelope and the occupied spaces, certain basic design

principles apply.

Minimize Moisture SourcesDo not allow rain or snow to enter the equipment room. Minimize open water. Do

not allow condensate from air handlers to run freely across the floor. Fix leaky

pipes and valves. Design the equipment room to avoid unplanned condensation

on cold surfaces.

Minimize unvented combustion appliances or processes within the equipmentroom; combustion generates water vapor. Also, for all combustion processes

(vented or unvented), convey makeup air directly from outside to each process

using ducts; do not use the equipment room as a makeup-air plenum for

combustion processes. A negative-pressure equipment room leads to unplanned

condensation.

Minimize vapor-pressure diffusion into the equipment room by insulating

perimeter walls and using a low-permeability vapor retarder on the warm side, as

described for the building envelope.

Minimize infiltration due to wind and stack effect. Eliminate infiltration due to

mechanical exhaust (or leaky negative return ducts) by pressurizing the

equipment room, preferably using dry supply air. Especially in hot, humidclimates, outdoor-air dew point can be extremely highdo not allow untreated

outdoor air to enter the equipment room, either accidentally or by design.

Equipment-Room Moisture


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Prevent Unplanned CondensationCondensation occurs whenever air dew point exceeds surface temperature.

Design and operate the equipment as appropriate to raise exposed surface

temperatures and/or lower equipment-room dew point. Remember, the cold

surface location within the equipment room is irrelevantan air handler with a

warm, dry exterior, for example, may generate significant unplanned

condensation on interior airstream surfaces.

Raise Surface Temperature and Vapor Seal

Cold surfaces abound within the equipment room. Some can be heated, but

most must be insulated and vapor-sealed.

For instance, condensate drain pipes, chilled water pipes (both leaving and

return), even condenser water pipes may contain liquid at temperatures well

below room dew point. These pipes must be insulated and sealed with a vapor

retarder on the warm side. Water valves and other associated devices must also

be insulated and vapor-sealed. Insulation alone raises surface temperature, but

not interior temperaturesat some location within the insulation, a cold surface

still exists. To prevent condensation inside the insulation, vapor-pressure

diffusion into the insulation must be minimized using a vapor retarder.

Most equipment-room surfaces can be insulated and vapor-sealed adequately,

but some must be heated above dew point. The sections below discuss specific

solutions.

Lower Equipment-Room Dew Point

Process-side temperatures within the equipment room cannot be changed.

Supply-air, chilled-water and drain-line condensate temperatures all depend on

loads and design criteria, not condensation prevention. To avoid unplanned

condensation throughout the equipment room, lower the equipment-room dew

point. Insulation can then raise room-side surface temperature above the

equipment-room dew point and vapor retarders can keep most moisture away

from cold surfaces within the insulation.

Some insulation materials inherently form an effective vapor barrier without the

addition of a separate vapor-retarding material. One square foot of chillerinsulation, for instance, with a permeance factor of 0.30 gr/h ft2 in. Hg, allows

less than 0.23 gr/h to diffuse into the insulation more than adequate vapor

diffusion protection, provided the joints are well-sealed.



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Figure 13 Pressurize to Avoid Condensation

Figure 13 shows a cold condensate drain, first in an unconditioned equipment

room; then in an equipment room pressurized by a return air fan. In the first

room, open to outdoor air, a very high dew point results in unplanned condensate

formation on the uninsulated-pipe surface. Simply replacing the moisture-laden

outdoor air in the space with drier return air lowers dew point in the second room.

The dew point could have been lowered further using supply air rather than

return air to pressurize the room.

Some designers refer to this measure as conditioning the equipment room.However, conditioning usually implies controlling one or more variables to

maintain a set point. Simply pressurizing the room slightly with either return or

supply air can lower equipment-room dew point significantly. Both sensible

temperature and dew point float as supply (or return) air conditions change and

room loads vary. No variables are directly controlled, so strictly speaking, the

room is not conditioned. Pressurized more accurately describes an equipment

room with low dew point. During construction and normal operation, equipment-

room pressurization solves most moisture problems, including internal

condensation. (See side-bar.)

Dehumidify Ventilation AirLets examine dehumidification of ventilation air in more detail. Only two air-dehumidification methods need to be considered: desiccant dehumidification and

cold-coil dehumidification.

A desiccant dehumidification unit (Figure 14) incorporates a desiccant material

mounted on a rotating wheel. Warm, moist outdoor air enters the rotating wheel.

Some portion of the water vapor adsorbs onto the desiccant surface. As the

rotating wheel turns, the moisture-laden portion moves into the regeneration air


During Construction

Air handlers are often operated at

full capacity (wild coils with

very cold, chilled water) providing

cooling and dehumidification for

the construction crew. If the

equipment room is open to

outside, high-dew-point

conditions, and chilled water is

supplied at low-surface, design

temperatures; wet insulation,

internal flooding and generalwater damage due to unwanted

condensation often results.

If at all possible, close the

equipment-room envelope prior to

operating the air handler

properly pressurizing the room to

lower its dew point. Reset the

chilled-water temperature as high

as possible during construction

(i.e., 55F) to raise equipment-

room surface tempertures.

Dont allow construction-phase

operation to ruin a well-designed

equipment room before the

building is even finished.


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SYS-AM-15 29


stream. Hot air (either heated outdoor air or heated return air) drives the

adsorbed water vapor from the desiccant material, drying or regenerating it.

The wheel rotates the regenerated material into the outdoor air stream again and

the process repeats. Since the hot desiccant raises the temperature of the

leaving dehumidified outdoor air, it must be cooled (usually by mechanical and/or

evaporative cooling) before it can be delivered to the occupied spaces.

Figure 14 Desiccant Dehumidification

A cold coil dehumidifies simply by presenting a cold surface for controlled

condensation. As warm, moist outdoor air (or a mixture of outdoor and

recirculated return air) passes through the coil, water vapor condenses on the

cold coil surface and flows down to the drain pan. Since the cold coil removes

heat, the dry supply air may be too cold for the space sensible load; the air must

be tempered (reheated) to avoid overcooling the occupied spaces.

When using direct refrigerant expansion (DX) to cool the dehumidifying coil,

simple sensible-temperature-based cycling for capacity control may lead to

humidity control problems. Coil cycling can lead to wide variations in supply-air

dew point and loss of dehumidifying capacity during the off cycle. DX

dehumidifiers should use a thermostat to control sensible temperature (by

cycling coil capacity or by modulating reheat capacity) and a humidistat toactivate full coil capacity when required for proper dehumidification.

Some designers dehumidify the outdoor air before it mixes with recirculated

return air in the air-handler mixing section, removing only ventilation moisture.

Other designers dehumidify mixed air, removing both ventilation and internally

generated moisture. Either way, the outdoor moisture load must be removed

before delivering the ventilation air to the space.


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Equipment-Room Design ExamplesEquipment-room designs vary widely. Depending on climate and operation,

design decisions may differ. The following examples illustrate poor, better, and

best design practices, especially for equipment rooms in predominantly cooling

climates.

A Poor Design: Negative Pressurization

Equipment rooms designed to operate at a negative pressure with respect to

outdoor pressure (unconditioned rooms) commonly encounter unplanned

condensation on all cold surfaces. As a result of infiltration, the dew point simply

rises too high. Avoid designing negative-pressure equipment rooms that use theroom as an intake plenum for outdoor air. Also, avoid other poor, though

common, design practices illustrated in Figure 15.

Key Elements Of Poor Design

Liquid-Water Sources

D Rain water easily penetrates the outdoor air louver.

D No trapping allows flooding inside the air handler and leakage onto the floor.

D Condensate from drain pan runs across the floor.

Water-Vapor Sources

D Exterior walls with no vapor retarder allow unimpeded flow of water vapor

from outside.

D Without an outdoor air duct, the equipment room operates at a negative

pressure to induce ventilation airflow, resulting in significant infiltration.

D By design, outdoor air for ventilation enters the equipment room through

louvers in the equipment-room wall, bringing moist outdoor air with it.

Unplanned Condensation

D Condensate forms on uninsulated pipes and puddles on the floor.

D Condensate forms on the air handler exterior.

D Condensate forms on the interior insulation of poorly-sealed supply ducts.

D Condensate forms inside the air handler.

Unplanned condensation occurs in a negatively pressurized equipment room

despite surface insulation and sealing. Any small air leak in air-handling units or

ducts, or any small break in piping insulation or vapor retarders, results in

condensate. Do not design equipment rooms that can become negatively

pressurized.



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Figure 15 Poor Equipment-Room Design


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A Better Design: Positive Return-Air Pressurization

Pressurizing the equipment room goes a long way toward reducing unplanned

condensation. One pressurization technique uses return air dumped into the

equipment room by a return fan. This, of course, can only be accomplished in

systems that use a return-air fan.

The dew point in such an equipment room roughly matches the return-air dew

point. Some designers refer to this design as a conditioned equipment room,

since air returns from conditioned spaces. This level of moisture effectively

avoids unplanned condensation on external surfaces. However, local moisture

sources may still elevate equipment-room dew point to an unacceptably high

level during many operating hours. Although the better design illustrated in

Figure 16 resolves some problems, it fails to address others.

Key Elements Of Better Design

Diminished Liquid-Water Sources

C A rain hood protects the intake from rain intrusion.

C No open water tanks in the room.

D Poor trapping allows flooding inside the air handler and leakage onto the floor.

C Condensate drain-line guides condensate to a sanitary drain.

Diminished Water-Vapor Sources

D Exterior walls with no vapor retarder allow unimpeded flow of water vapor

from outside.C Return air fan assures positive room pressure (return-air positive), to

eliminate the infiltration moisture load, but return-air moisture content

contributes to the equipment-room dew point.

C Ventilation air, ducted directly to unit, adds no equipment-room moisture load.

Unplanned Condensation

C Well-insulated, sealed pipes prevent condensation.

C Well-insulated air handler limits surface condensation.

C Well-sealed supply ducts prevent condensation inside insulation.

D Condensate still forms inside the air handler since high-dew-point air can leak

into the unit.

When using powered exhaust or relief fans in the equipment room, duct returnair to the air handler and use the best design approach described below. Do not

use the equipment room a negative return plenum. Again, an equipment room

under negative pressure can be subject to extensive infiltration of untreated,

moist outdoor air. Using the return fan to pressurize an equipment room

eliminates outdoor-air infiltration.



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Figure 16 Better Equipment-Room Design


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Best Design: Positive Supply-Air Pressurization

An equipment room pressurized with supply air (supply-air positive) offers the

best solution. Although conditioned by cool, dry supply air, equipment-room

temperature can float. No controls (thermostats or humidistats) are needed to

modulate the pressurizing supply airflow. Dry airflow into the equipment room

(see side-bar) limits infiltration and ventilation moisture load by simply filling the

space with low-dew-point air. Therefore, this design avoids the problems of

unplanned condensate formation.

Key Elements Of Best Design

No Liquid-Water SourcesC A rain hood protects the intake from rain intrusion.

C Proper trapping prevents internal flooding.

C Condensate drain line guides condensate to a sanitary floor drain.

Minimum Water-Vapor Sources

C A vapor retarder on the warm side of exterior walls minimizes moisture load

due to vapor-pressure diffusion from outside.

C Low-volume supply airflow assures positive room pressure (supply air

positive), eliminating infiltration moisture load and decreasing equipment-

room dew point.

C Ventilation air ducted to the unit adds no moisture load to the equipment room.

No Unplanned Condensation

C Well-insulated, sealed pipes prevent condensation.

C Well-insulated air handler prevents surface condensation.

C Well-sealed supply ducts prevent condensation inside the insulation.

C A low equipment-room dew point combined with well-sealed air-handler joints

prevent condensate formation inside the air handler.

The supply airflow needed to pressurize the equipment room varies greatly. Very

low airflow may be enough to maintain sufficient static pressure (0.10 to 0.20

in. wc), especially if good, exterior wall construction limits exfiltration through

cracks and openings.

Some designers actually control temperature and/or humidity to truly condition

interior equipment rooms. However, environmentally controlled equipment rooms

must be designed carefully. Avoid sensible-temperature control schemes that

allow dew point to float. Maintaining sensible temperature cannot assure proper

dew-point control. Also, remember that a low dry-bulb temperature in the


How Much Air?

Based on wall construction, the

volume of dry supply air needed

to pressurize an equipment room

can be estimated using Figure 8.

Assuming that an average wall

and a positive pressure of 0.10

in. wg negates wind effects, we

can estimate leakage [Q = 0.66

(0.10)0.65= 0.15 cfm/ft2]. If the

exterior wall area is 1800 ft2

, theleakage flow rate is only 270 cfm.

Of course, depending upon

equipment room heat sources,

high dry bulb temperatures may

result. In other words, additional

airflow may be needed to lower

equipment room dry bulb

temperatures to an acceptable

value.


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equipment room means lower surface temperatures. Ideally, a controlled

equipment room would sense and maintain dew point, while maintaining sensible

temperature below a high limit.

SummaryEquipment rooms need special design and operational attention to avoid

problems due to unplanned condensate. Do not use the equipment room as an

outdoor air plenum. Duct outdoor air directly to the air handler.

Consider pressurizing the equipment room with return air if the system uses a

return fan. Better yet, consider pressurizing and dehumidifying the equipmentroom with a small volume of supply air. The simplest, most effective method to

resolve all equipment-room condensate problems is to pressurize the equipment

room with supply air. l


Figure 17 Best Equipment-Room Design


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HVAC equipment, both water chillers and air-handling units, must be selected,

applied and operated properly with regard to moisture. First, lets discuss the

water chillers. An operating water chiller develops cold surfaces. Uninsulated or

insulated-but-unsealed chiller surfaces exposed to moist air lead to unplanned

condensate. Chiller condensation problems can be attacked on three fronts:

surface heating, surface insulating and sealing, and lowering equipment-room

dew point.

The dew point in a ventilated, unconditioned mechanical room cannot be

decreased; therefore, cold surfaces must be insulated or heated. Some cold

surfaces, valve handles for instance, cannot be insulated. Increased air motion

may add sufficient heat by convection to raise the temperature of a cold chiller

surface above surrounding dew point (see Figure 18). In some cases air motionalone is insufficient for very cold spots. A small heater can be used to raise

surface temperature above ambient dew point and prevent condensate

formation.

Cold chiller surfaces are often insulated to increase efficiency. Insulation can

adequately raise exposed surface temperatures above the dew point of the

equipment room, but merely adding insulation between warm, moist air and a

cold surface cannot prevent condensation.

If significant moisture penetrates (permeates) the insulation, it eventually

encounters a surface temperature lower than dew point. Without a vapor

retarder, moisture permeates the insulation until it encounters a cold enough

temperature, then condenses. The insulation must be sealed with a low-permeability vapor retarder applied on the warm side. Alternatively, insulation

with low permeability (0.30 gr/h ft2 in. Hg), if well sealed, eliminates harmful

condensation within the insulation.

Of course, lowering equipment-room dew point significantly reduces all

unplanned chiller condensation. If possible, pressurize the equipment room with

supply air. l

Figure 18 Raise Surface Temperature

Moisture and Chillers


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To prevent carryover, do not exceed the maximum allowable coil-face velocity.

Coil geometry (fin type, shape and spacing) and allowable face velocity

determine required drain pan length. Below the velocity limit, water droplets on

the coil surface flow down the coil into the drain panthe air stream can blow

droplets from the coil, but it cannot carry them beyond the drain pan. Above the

velocity limit, however, the air stream carries water droplets beyond the drain pan

and deposits it on downstream air-handler and duct surfaces.

Specify coil size to assure that air stream velocity falls below the maximum

allowable coil face velocity. Clean the coils regularly: dirty coils cause local high-

velocity jets and high-velocity jets lead to carryover beyond the drain pan. Re-

evaluate coil performance if airflow requirements change. Increased airflow

across the coil face may increase face velocity above the carryover limit. Consultmanufacturers literature for maximum coil-face velocity limits.

Slope to Prevent Standing Water

Collected condensate must not be allowed to accumulate in the air-handler drain

pan. Standing water results in microbial growth (microbial slime). The drain pan

must be properly sloped and properly connected to a drain system to assure

speedy, complete drainage with the fan on or off.

Design or specify drain pans that slope in two planes, as shown in Figure 20. A

flat pan retains water. A single-plane sloped pan with one drain connection limits

the volume of water retained; some water hangs up in the low corners. A dual-

plane sloped pan assures that all water drains out. Be sure to place the drain

connection at lowest point of the pan. Even a very small slope assures complete

drainage when the pan slopes in two planes. Remember, proper drain pan slope

depends not only on design but also on installation: the unit must be level. Be

sure to install the air-handling unit according to the manufacturers

recommended instructions and within the tolerance specified for levelness.

Moisture andAir-Handling Units

Figure 20 Drain Pan Designs


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Drain to Prevent Flooding

If collected condensate cannot drain through the drain connection, it spills over

the top of the drain pan and floods the air-handling unit and duct system.

Microbial growth as well as equipment and building damage can result from

internal flooding. Proper condensate drainage depends on drain-pan slope,

drain-connection size and maintenance, and, as explained below, drain-seal

design and maintenance.

Condensate forms at a high rate when very moist air enters the coil (as much as

5 gpm on a 100-ft2 coil). To assure speedy drainage without drain-pan overflow,

do not reduce the drain-pipe diameter to be smaller than the drain connection

provided by the manufacturer. The equipment manufacturer selects the drain

connection size to accommodate the maximum condensate flow at extreme

conditions. Reducing its diameter may result in slower drainage and drain-pan

overflow.

Wet coils help clean particles from the air stream. Condensate flow keeps the

coils relatively clean, but dirt accumulates in the drain pan. Accumulated dirt

increases the likelihood of restricted or blocked condensate flow and subsequent

flooding. Be sure to inspect drain pans on a regular basis and clean if necessary.

Drain-Line SealsAfter collecting in the drain pan, condensate exits the air handler via the drain

connection, passes through a drain seal, then follows the drain line to a sanitaryfloor drain. Faulty drain seals result in more moisture-related problems than any

other air handler element. Improper design, installation or maintenance of the

drain seal can cause water droplet carryover or internal flooding within the air-

handling unit. Although some designers use other sealing devices, simple

P-traps seal the vast majority of air-handler drain pans.



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Figure 21 Blow-Thru Trapping

Blow-thru designs positively pressurize the drain-pan surface with respect to the

equipment room (Figure 21). Supply air pushes condensate through the trap,

eliminating any concern for water droplet carryover due to trap operation, but

raising a concern for maintaining the internal-to-external air seal. Without an air

seal, supply air can flow out of the unit into the equipment room, creating a small

supply-air leak. Inadequate trap depth (D) allows casing pressure to force all

water out of the trap destroying the seal.

Specify a trap depth no less than the casing static pressure (CP) plus a safety

margin to allow for final, as-installed balancing (D = d + CP + 1 in. wg). Specify a

total trap height (T) no less than the trap depth plus an installation safety margin(T = D + 1 in. wg).


Figure 22 Draw-Thru Trapping


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Draw-thru designs negatively pressurize the drain-pan surface (Figure 22).

Equipment-room air pushes condensate through the trap toward the drain pan.

With insufficient total trap height (T), the trap contains adequate water to

maintain the air seal but not enough to allow water flow. Consequently, the drain

pan overflows and floods the inside of the air handler. With insufficient trap depth

(D), equipment-room pressure pushes trap water into the drain pan. High velocity

air flows into the drain pan throwing water droplets into the supply air stream,

causing carryover, due to trap spitting or geysering. Also, the high-velocity

airflow prevents water flow through the drain opening, causing drain-pan overflow

and internal flooding.

Each time the draw-thru fan starts, the trap must be primed (contain water).

Without a water seal, a properly designed trap operates as though it hasinsufficient trap depth: it spits and causes internal flooding. Specify the trap head

(H) no less than the negative casing pressure (CP) plus a 1 in. wg safety margin

(H= 1+CP). Specify trap depth (D) no less than half the trap head (D = H + d).

Total trap height (T) is the sum of installed head and depth (T = H+D). Inspect

traps regularly and prime when necessary, especially just before the cooling

season begins.


Plan for proper trapping: the unit must be mounted high enough above the floor

to allow for the recommended total trap height (T) and trap depth (D). Countless

air-handler installations include crudely chopped holes in concrete floors; proper

trap dimensions were considered as an afterthought, after flooding or carryover

problems resulted in service expense and building damage. Follow the trap

design guidelines discussed above.

Remember, even a well-designed trap, if plugged, causes drain-pan overflow and

internal flooding. Inspect traps regularly. Clean and prime if necessary. Do not

gang drain pans at different surface pressures on the same trap. Use shutoff

values, not traps, to seal maintenance-only drains.

External CondensationSupply air downstream of the cooling coil and in the supply ducts can be very

cold. The coil itself can be even colder; chilled-water temperature may be 45F or

lower. Therefore, external air-handler surfaces and supply-duct surface

temperatures may also be very low unless adequately insulated. If high dewpoint air can contact these cold surfaces, unplanned external condensation

(sweating) occurs, leading to equipment-room flooding, unit discoloration and

damage (corrosion and deterioration), possible building damage, and increased

potential for microbial growth.


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Seal Penetrations and Joints

A supply air leak from the air-handling unit or supply duct causes a very cold

local surface, a cold spot. In a high-dew-point equipment room (negative

pressure, unconditioned), water vapor from the air condenses readily at the cold

spot. Unplanned, liquid water results. Design the pressurized sections of the air

handler and the supply ducts to minimize leakage of cold supply air.

Each penetration of the air handler represents a potential supply air leak. Specify

airtight seals (Figure 23) at all air handler penetration points, including pipe

penetrations at the coils (both heating and cooling), unused maintenance-only

drain openings, and any penetrations for electrical wiring.

Figure 23 Seal Pipe Penetrations

All air-handling units include access doors and panels. Poorly sealed doors and

panels result in supply air leakage. Some units use individual sections, joined at

the factory or in the field, to form the air handler. Each joint downstream of the

fan represents a potential supply air leak.

Gasket all access panels, door openings and inspection windows in positive-

pressure sections. Seal all joints between sections (Figure 24). Note that draw-

thru air handlers usually include fewer pressurized sections than blow-thru

configurations, so fewer joints need sea

Managing Building Moisture

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