HVAC Distribution Systems

1 | WEATHERIZATION ASSISTANCE PROGRAM STANDARDIZED CURRICULUM – December 2012 eere.energy.gov

HVAC Distribution Systems

WEATHERIZATION ENERGY AUDITOR SINGLE FAMILY


Learning Objectives

By attending this session, participants will be able to:• Name functions of the components of forced warm air,

hot water, and steam distribution systems.• Demonstrate common diagnostic and assessment

methods for ducted distribution systems. • Describe common problems for each distribution

system type.• Explain solutions to these common problems.

HVAC DISTRIBUTION SYSTEMS


Good Ducted System Design

A well-designed duct system:• Provides conditioned air to meet all room heating loads.

• Provides thermal comfort evenly in all conditioned rooms.

• Is properly sized so that static pressure is within manufacturer specifications.

• Is sealed to provide proper airflow and prevent air from entering the house or duct system from polluted zones.

• Has balanced supply and return airflows to maintain a neutral pressure in the house.

• Minimizes duct air temperature losses between the air handler and supply registers.



Forced Air Distribution System

The parts include:• An air handler at the furnace.• A heat exchanger where the heat from combustion is

transferred to the distribution air.• A supply air plenum to which the supply trunks are attached.• Branches attached to the supply trunk.• Supply registers through which conditioned air flows to the

living space.• Return grilles through which air flows back to the furnace.• Return branches and trunks attached to the return plenum.



Ducted Distribution SystemHVAC DISTRIBUTION SYSTEMS


Ducted Distribution System, Cont’d



Open Returns

Photo Courtesy of PA Weatherization Training Center

Open return

Draft hood(open at bottom)



Atmospheric Gas Furnace

Distribution System Components

Find the:

1. Circulating fan

2. Air filter

3. Cold air return

4. Heat exchanger

5. Warm air to house

Source: Heating with Gas, Natural Resources Canada, 1998.



Function of Heat Exchanger

• The furnace heat exchanger is where the heat from combustion gases—usually between 70% and 95%— is transferred to the distribution air in the ductwork.

• The heat exchanger is an extremely important part of any furnace because it can have a significant impact on efficiency and health and safety.

• Combustion byproducts must not mix with distribution air.



Heat Exchanger Leakage Testing

Test methods:

1. Visual inspection

2. Inspection with small torch

3. CO reading

4. Wavering flame

5. Tracer gas

Photo courtesy of New River Center for Energy Research and Training (NRCERT)

Damaged area of heat exchanger



Ductwork Efficiency

• Specify duct sealing where ducts are located in unconditioned spaces.

• Ducts in unconditioned spaces should be insulated to recommended levels.

• Seal all returns in spaces where atmospheric fossil fuel appliances are located.

It is often necessary to remove duct insulation to properly seal ducts. Seal with mastic, then re-insulate.

Photos courtesy of NRCERT



Ductwork

This sheet metal ductworkis located within the building envelope, so it does not need to be insulated.Photos courtesy of R. Karg



Ducted System Controls

The primary controls are:

• Thermostat.

• Fan and limit control.

• Balancing dampers.

• Motorized dampers (these are not common).



Furnace Thermostat

The thermostat activates the burner on a furnace.

The fan and limit switch turns the air handler blower on and off.

Photo courtesy of R. Karg



Fan and Limit Control

This control turns the air handler blower on andoff at set temperatures. This is the fan control.

It also shuts down the blower if the heat exchanger area gets too hot (usually at about 200). This isthe limit control.

Recent versions areelectronic and cannot beadjusted in the field.

Graphic source: Heating with Gas, Natural Resources Canada, 1998.Photo courtesy of Honeywell.



Balancing damper

Balancing Dampers

Graphic source: Heating with Gas, Natural Resources Canada, 1998.

Dampers help control airflow to rooms.

Manual balancing dampers are not as commonas they should be. Sometimes they need to be added.



Motorized Dampers

Motorized dampers are used for zoning a ducted distribution system, rather than for balancing.

When a zone requires heat, the thermostat of that zone opens the zone damper and activates the furnace burner.

Motorized dampers are uncommon.




Common Ductwork Problems

Common problems include:• Duct leakage to/from the outdoors.

• Restricted return side.

• System not balanced.

• Temperature too high or too low at heat exchanger.

• Static pressure out of range.

• Airflow of air handler fan not matched to system.

• People live within the distribution system. Closing a bedroom door or covering a register or grille can significantly alter airflow.



Diagnostics for Ductwork

Diagnostics include:

• Pressure pan (duct leakage).

• Duct blower (duct leakage).

• Static pressure.

• Temperature rise.

• Room-to-room pressure differences (door restrictions).

• Air handler blower CFM flow.



Pressure Pan

Test the duct system with a pressure pan and blower door to identify:

• Leakage to the outdoors when ducts are located outside the thermal boundary.

• Leakage pathways from duct-containing building cavities to the outdoors (e.g., floor-joist cavities adjacent to porch roofs, cantilevers).

Photo courtesy of NRCERT

Pressure pan testing of the duct

system



• Depressurize house to 50 Pa with blower door.

• Test each register and grille. Document readings.

• Registers too large or oddly shaped may be covered with plastic and taped on edges.

• Seal duct leaks and retest.

• Goal = readings lower than 1 Pa

Pressure Pan Procedure Summary


The handle allows for easy testing of

hard-to-reach ducts.



Pressure Pan Use

Sample mobile home duct pattern

Bedroom0.8 Pa

Bedroom1.2 Pa

Bath2.4 Pa

Furnace Closet (living room) 2.6 PaLiving Room

2.6 PaKitchen

1.2 PaKitchen

0.5 Pa

Total: 10.7 Pa

2.4 Pa at the bath register and 2.6 Pa at the living room register indicate a large leak between them, probably at the furnace plenum.




Duct Blower for Duct Leakage

Use duct pressurization testing to quantify:

• Total duct leakage (to indoors and outdoors).

• Duct leakage to outdoors.

Photo courtesy of The Energy Conservatory

Manometer

Duct blaster



Duct Blower Procedure Summary #1

To test for total duct leakage:1. Open a window or door to the outdoors.

2. Install duct blower to the air handler compartment.

3. Temporarily seal all supply registers and return grilles.

4. Insert manometer hose into a supply duct.

5. Open up rooms containing ducts.

6. Pressurize the ducts to 25 Pa and record the airflow.



Duct Blower Procedure Summary #2

To test for duct leakage to outdoors:

1.Close all exterior windows and doors.

2.Set up blower door to pressurize the house.

3.Connect duct blower to air handler compartment and manometer hose to air handler compartment.

4.Temporarily seal all supply registers and return grilles.

5.Pressurize the ducts to 25 Pa.

6.Pressurize the house until the pressure difference of the house and the ductwork is 0 Pa.

7. Record the airflow at the duct blower.



Room-to-Room Imbalances #1

Closed doors that prevent supply air from getting back to a return cause positive pressures in those rooms with supply vents. . .

…which starves the return for air, causing negative pressure in the zone where the return is located.

Return Supply

Closed door




MasterBedroom

Bedroom Bath

Utility Room Kitchen

Living Room

Whole-house return in hallway




Measure room-to-room pressure imbalancesRoom pressure imbalances over 3 Pa should be remedied by adding supply or return air. Then retest.

Photo courtesy of PA Weatherization Training Center




Graphic source: Air Distribution System Design, DOE, 2003.

Solutions1. Undercut door.2. Add jumper duct.3. Add door grille.4. Add wall grille.5. Add transfer grille.6. Install return in

affected room.

Find the size of the free vent area of your solution by opening the door while the air handler is running. When the manometer reading falls below 3 Pa, measure

the in2 of door opening. This is the in2 of free opening for your solution.



Point the tip into the air stream

Measuring Static Pressure #1

Static pressure tip

Photo courtesy of Rob deKieffer

Magnet




Measuring External Static Pressure• Check nameplate for External Static Pressure (ESP).

• Measure both return and supply sides of the air handler as the unit was shipped.o Measure at inlet and outlet of blower.o Have a clean filter in place (suggested).o Don’t measure beyond the A/C coil unless it shipped

with unit.

• Add return and supply pressures together, IGNORING the negative sign.

Measuring Static Pressure #2HVAC DISTRIBUTION SYSTEMS


External Static PressureIWC (Pa)

Air Handler Fan FlowCubic Feet per Minute

0.69 (173) 1,3500.62 (155) 1,4000.55 (138) 1,4500.47 (118) 1,5000.39 (98) 1,5500.31 (78) 1,600

If the static pressure is too high, the fan flow will drop.

General External Static Pressure and Fan Relationship



Measuring External Static Pressure• If ESP is too high, the airflow might be blocked or the ductwork might

be too small or restricted.

• If ESP is too low, the ductwork might be very leaky or the fan might be dirty or damaged.

• Typical ESP totals are around 0.5 IWC or 125 Pa with an air conditioning coil and filter.

• Typical ESP totals are around 0.25 IWC or 62 Pa without an air conditioning coil and filter.

• It is preferred to have the return and supply sides of similar magnitudes, for example, a return of -34 Pa and supply of +31 Pa.



• Place thermometer in supply side as close to furnace as possible but out of “sight” of the heat exchanger.

o Use manufacturer’s recommended measurement method, or

o Use the four-corner method (measure at each corner or supply plenum and average readings).

• Place thermometer in return side.

• Fire furnace.

• When the supply-side temperature reaches steady state, subtract return-side from supply-side temperature to get temperature rise.

• Check specified temperature rise on furnace name plate. Actual should be in the middle of the nameplate range.

Temperature Rise Test #1HVAC DISTRIBUTION SYSTEMS


Measuring Temperature Rise &

Calculating CFM

Temperature Rise Test #2

The temperature rise for this example

is 70:

145 supply side -75 return side 70 temperature rise



Excessive temperature rise can result from:• Low fan output.

o Wrong fan speed, bad motor bearings, low voltage to motor, dirty blower wheel, wrong motor rotation, slipping blower belt.

• Low airflow from restrictions in system.o Undersized or restricted ducts, dirty filter, dirty cooling

coil.

• Overfired burner (gas pressure or oil nozzle).



Low temperature rise can result from:

• Excessive fan speed.

• Excessive duct leakage.

• Underfired burner.o Low gas pressure.o Oil nozzle not matched with airflow rate.



Temperature rise that is too high can:

• Damage the heat exchanger.

• Cause rocking on the high limit.

Temperature rise that is too low can:

• Lead to condensation.

• Cause excessive soot buildup.

• Lead to occupant discomfort.



TrueFlow® Air Handler Flow Meter

Measures airflowin residentialair handlers

#20#14

Measuring Air Flow at Air Handler #1

Photos courtesy of The Energy Conservatory



Airflow in CFM Furnace BTUH Input

500 Less than 60,000

700 60,000 to 79,999

900 80,000 to 99,999

1,200 More than 100,000

Measuring Air Flow at Air Handler #2

General Minimum Airflow Values



Analysis of Existing Ductwork - 1Interview occupants about the thermal comfort of the existing system.

Ask about:• Uncomfortable rooms.

• Excessive noise.

• Frequent cycling of furnace.

Assessing Ductwork #1

Plumbing through return duct!Photo courtesy of R. Karg



Analysis of Existing Ductwork - 2Inspect air handler and ductwork for such things as:

• Disconnected ducts.

• Duct leakage.

• Restricted returns.

• Panned floor joists.

• Ducts in unconditioned spaces.

• Balancing dampers.


Disconnected duct!





Do technical testing and appraisal of the duct system and equipment.

• Duct leakageo Pressure pan testingo Duct blower testing

• Room-to-room pressure imbalances

• Static pressure

• Temperature rise

• Blower CFM


Very dirty blower vanes!




Analysis of Existing Ductwork - 5Determine strategies for duct repair:• Write down possible problems.

• Determine required alterations to furnace and ductwork.

• Decide on consumer education strategies.

Assessing Ductwork #4HVAC DISTRIBUTION SYSTEMS


Good Hot Water Distribution Design

Good design:

• Provides conditioning to meet all room heating loads.


• Heats the dwelling quietly.

Hot water or steam distribution is often referred to as “hydronic.”



Hot Water Distribution System

The parts include:• Thermostat(s) that activate the circulator pump.

• Circulator pump(s). Might include zone valves rather than two or more circulator pumps.

• Aquastat control.

• A heat exchanger where the heat from combustion is transferred to the distribution water.

• Supply and return piping at boiler.

• The expansion tank.

• Hot water baseboard units (convector) where the thermal energy is transferred to the conditioned rooms.



Series Loop Hot Water System

Based on graphic from the International Association of Certified Home Inspectors, Inc.

A series loop hot water distribution system is probably the most common system layout because it is the least expensive.



Series Loop Hot Water Baseboard

Typical hot water baseboard distribution

Photos courtesy of Slant/Fin

Damper fin



Expansion Tanks

Old-style tanks (above) and newer tanks (right) allow for expansion of heated water and contraction of cool distribution water.

Photos courtesy of R. Karg



Hot Water Distribution Controls

Thermostat

Circulator pump(s)

Zone valves

Aquastat


Basic controls include:


Boiler Thermostat

The thermostat activatesthe boiler circulator orzone valve and circulator.

The aquastat controlsthe burner.




Boiler Aquastat

An aquastat:1. Maintains boiler water

temperature.2. Provides high-limit

temperature protection.3. Will not allow circulator

to operate if boiler water temperature is too low.

Also assists with DHWtemperature control if the

water heater is tankless or indirect-fired.

Normally, the aquastat control is covered.




Zone Valves

Zone valves are controlled by thermostats in each zone.

This house has 3 zones with one thermostat for each.

The 4th zone valve is for domestic hot water from the boiler.Zone valves take the place of circulators.




Potential Problems with Hot Water #1

Poor maintenance:• If a hot water distribution

system is maintained properly, there is little that can go wrong.o Oil-fired boilers should be

cleaned and tuned every year.

o Gas-fired boilers should be cleaned and tuned once every three years.





Older tanks (above) shouldbe drained and refilled annually.

Newer expansion tanks (right)require very little maintenance.

Poor expansion tank maintenance

Photos courtesy of R. Karg




If the air bleeder valve malfunctions, air will not be purged from the distribution system.

This air (oxygen) will create sludge and make the system noisy as the air is pumped with the water.

Air bleeder vent

Expansion tank

Air in the distribution system



Good Steam Distribution Design

A good design:

• Provides conditioning to meet all room heating loads.




Steam Distribution System

The parts include:• A thermostat(s) that activates the circulator pump.

• Pressure control (Pressuretrol).

• A heat exchanger where the heat from combustion is transferred to the distribution water/vapor.

• Supply and return piping at boiler.

o For one-pipe distribution, the supply and return pipes are the same.

o For two-pipe distribution, there are separate supply and return pipes.

• Steam radiators that transfer thermal energy to the conditioned rooms.



Steam Distribution Controls and Gauges

Basic controls include:

• Thermostat

• Pressure control (Pressuretrol)

• Sight or gauge glass

• Low-water cutoff



Steam Boiler Thermostat

The thermostat activates the steam boiler burner.

The Pressuretrol turns the burner off when the set pressure is reached.




Steam Boiler

Pressuretrol(pressure control)

Sight glass

Low-water cutoff

Oil burner




Steam Distribution Controls

Sight (gauge) glass

Low-water cutoff

The low-water cutoff will shut off the burner if the water falls to an unsafe level. This is required by code.

The sight or gauge glass provides an easy way to determine the water level in a steam boiler.




Pressure Control for Steam

Photo courtesy of Honeywell Controls

This device determines the operating range of the boiler during the heating cycle.

When the thermostat calls for heat, the burner will cycle up to the cut-out pressure setting of the Pressuretrol. The burner will then shut off.



One-Pipe Steam Distribution

Graphic based on Basic Steam Heating Systems, Hoffman Specialty, ITT Industries, 1999, p. 2, www.hoffmanspecialty.com.

One-Pipe Steam Distribution

Both steam and condensate use the same pipe.

Steam travels to each radiator, condenses (giving off heat), and flows back to the boiler through the same pipe as condensed water.


http://www.hoffmanspecialty.com/


Two-Pipe Steam Distribution

Steam moves to theradiators in one pipe and the condensate flows back to the boiler through the other pipe.

These pipes are usually a smaller diameter thanone-pipe systems.

Graphic based on Basic Steam Heating Systems, Hoffman Specialty, ITT Industries, 1999, p. 2, www.hoffmanspecialty.com.


http://www.hoffmanspecialty.com/


Potential Problems with Steam #1

Steam distribution pipesare sometimes coveredwith asbestos insulation.

If this material is friable,be careful; it might bebest to avoid blower doortesting.





Air valveSupply valve

If one- or two-piperadiators don’t heat up, the supply valve may be closed or the airvalve may be blocked.

Two-pipe radiator

Photo source: The Open Fire Centre Ltd., Yorkshire St., Oldham, Lancashire, UK. www.fireplaces-oldham.co.uk.


http://www.fireplaces-oldham.co.uk/



Steam pressure is often set too high. This can cause distribution problems and wastes energy.

For most residential low-pressure, one-pipe systems, 2 psi cut-out pressure or less will work fine and maximize efficiency.

High pressure can cause distribution problems and waste energy.

Photo courtesy of Bill Van der Meer




When replacing a steamboiler, the new unit mustbe sized to match the installed radiation, rather than the heat load of the house.




Summary #1

• The function of all distribution systems is to provide even thermal comfort in all rooms of the home.

• Major components of forced air distribution systems include an air handler, heat exchanger, supply air plenum, supply and return registers, grilles, branches, and ducts.

• Forced air system diagnostic procedures include duct leakage testing, measuring static pressure, temperature rise, room-to-room pressure imbalances, and airflow across the heat exchanger.

• Common problems associated with ducted systems include room pressure imbalances, improper temperature rise, and energy wasted through duct leakage to the outdoors.



Summary #2

• Major components of hot water distribution systems include the thermostat, circulator pump, aquastat control, heat exchanger, supply and return piping, expansion tank, and hot water baseboard units.

• Major components of steam distribution systems include the thermostat, pressure control (Pressuretrol), heat exchanger, supply and return piping at the boiler, steam radiators, sight glass or gauge glass, and low-water cutoff.

• Some common problems associated with hot water or steam distribution include expansion tank degradation, low water levels (steam), or improper sizing once the home is weatherized.

• Compared with ductwork, hot water and steam distribution are relatively trouble-free, and diagnostic procedures are easy. We merely ask the occupants if there are problems with the thermal comfort of the home and conduct a simple inspection of the distribution components.


HVAC Distribution Systems

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