Arik Cohen P.E.Principal

Supply Air Temperature Reset

AIA Quality Assurance

The Building Commissioning Association is a Registered Provider with The American Institute of Architects Continuing Education Systems (AIA/CES). Credit(s) earned on completion of this program will be reported to AIA/CES for AIA members. Certificates of Completion for both AIA members and non-AIA members are available upon request.

This program is registered with AIA/CES for continuing professional education. As such, it does not include content that may be deemed or construed to be an approval or endorsement by the AIA of any material of construction or any method or manner of handling, using, distributing, or dealing in any material or product.

Questions related to specific materials, methods, and services will be addressed at the conclusion of this presentation.

The course will cover supply air temperature control as it pertains to variable air distribution systems with re-heat at the zone level. The discussion will focus on results from the installation of a trim and respond supply air temperature control strategy recently implemented at three sites on a high technology company campus in Northern California.

Course Description

At the end of this session, participants will be able to:

1) Understand the energy impacts as they relate to supply air temperature control in variable air volume systems that include reheat at the zone level. Constant and variable supply air temperature control schemes will be compared and discussed.

2) The session will include results and lessons learned from recent field installations of demand based supply air resets controls. Parameters to consider when recommending supply air temperature reset and barriers to successful implementation will be discussed

Learning Objectives

Supply/Discharge Air Temperate Control Basics

•1st Stage- economizer•2nd Stage – Dx/Cooling Coil

SAT/DAT – Supply/Discharge Air Temperature

SAT/DAT to zone VAV Boxes

Temperature Resets

Example Resets:• VAV: supply air temperature

(SAT) based on outside air temp or return air temp or zone level information

• Hot water supply temperature based on outside air or hw valve position

• Condenser water temperature based on ambient wet bulb

• Many others

HHW OAT Reset

120

130

140

150

160

170

180

190

200

20 40 60 80

OAT [°F]

HH

W S

etpo

int [

°F]

SAT Temperature Resets, Why Do We Care?

SAT control must be specified to• Maintain occupant comfort• Meet the conditioning requirements of

all zones

SAT Temperature Resets, Why Do We Care?○ Increasing SAT

- Increases the number of economizing hours thereby reducing coil load and cooling energy

» Level of savings dependent on climate and reset bounds

- Decreases reheat energy by increasing temperature at inlet to reheat coil

» Level of savings dependent on min VAV box cfm and reset bounds

- Increase fan consumption, VAV boxes open as warmer supply air is introduced resulting in high flows at the supply fan

OSATemp

(F) Hrs/yr96 2 94 5 92 4 90 7 88 19 86 11 84 39 82 96 80 93 78 155 76 157 74 226 72 239 70 320 68 170 66 339 64 686 62 719 60 1,040 58 887 56 781 54 629 52 487 50 304 48 392 46 326 44 169 42 195 40 104 38 70 36 72 34 15 32 2

Total 8,760

Weather Bin Data from San Jose CA

Variable Air Volume with ReheatReturn

Air

COLD

ExhaustAir

OutsideAir

Exterior Zone

Exterior Zone

Interior Zone

Interior Zone

DAMPERREHEAT COIL

DAMPERREHEAT COIL

DAMPERREHEAT COIL

DAMPERREHEAT COIL

DAMPER DAMPER

DAMPERREHEAT COIL

DAMPERREHEAT COIL

VFD

SAT Temperature Resets in Code (VAV with Reheat)

ASHRAE Standard 90.1 (Since 2007)• “The air temperature for cooling shall be reset

higher by 5°F (2.3°C) under the minimum cooling load conditions”

Title 24 (Since 2005?)• “shall include controls that automatically reset

supply-air temperatures: ○1. In response to representative building loads or to

outdoor air temperature; and ○2. By at least 25 percent of the difference between

the design supply-air temperature and the design room air temperature.

The Test Sites

• Site Characteristics○Mix of private and open

office areas○Some “labs”○DDC down to the zone level○80,000 – 120,000 ft2

• Study Details○Performed under PG&E RCx program○Completed over the past six months○Sites part of high-tech company

campus

Utility Data – Site 2

0

100

200

300

400

500

600

0

1,000

2,000

3,000

4,000

5,000

6,000

7,000

Jul Aug Sep Oct Nov Dec Jan Feb Mar Apr May Jun

kW

kWh/

day

Electricity Consumption Rate [kWh/day] Maximum Demand [kW]

0

50

100

150

200

250

Jul Aug Sep Oct Nov Dec Jan Feb Mar Apr May Jun

ther

ms/

day

Natural Gas Consumption Rate [therms/day]

Benchmarking – Site 2

- 10 20 30 40 50

SR1

PG&E CEUS - Large Office

kWh/sf

Electricity Benchmarking

- 20 40 60 80

SR1

PG&E CEUS - Large Office

kBtu/sf

Gas Benchmarking

Screenshot of Typical Air Handler

General Site Recommendations

Fix EconomizersTake advantage of DDC at Zone level!

• Zone level scheduling• Supply Air/Duct SP Reset

Annual Energy, Cost and GHG Savings Project Financials

Peak Savings

(kW)

Energy Savings (kWh/yr)

Energy Savings

(therm/yr)

Total Cost Savings

CO2 Emissions Avoided (tons/yr)

Estimated Measure

Cost

Potential 2011 PG&E Incentive

Simple Payback

(yr) NPV

RCx-1 Recommission All Economizers (AC1-8) 46.0 152,332 (22) 19,330$ 39.8 44,000$ 18,288$ 1.3 40,000$

RCx-2Fix Zone Temperature Setpoints, Implement SAT and DSP Resets (AC1-8) 23.0 553,991 44,868 103,402$ 446.8 51,600$ 25,800$ 0.2 396,000$

RCx-3 Schedule All Non-Essential VAV boxes and Air Handlers to Match Occupancy

0.0 282,840 9,325 42,789$ 136.8 12,000$ 6,000$ 0.1 173,000$

Estimated (Pre-Retrofit)

Measure Number Measure Name

Types of SA Temperature Control

Constant•SA Temperature setpoint does not change

Outside AirDry Bulb

Temperature

[°F]

Supply AirTemperature

Reset

[°F]50 70

70 55

Variable (zone information available to control system)

•SA Temperature setpoint changes in response to some variable

○Single variable (OAT or RAT)○Multiple variable (OAT or RAT plus additional variable such as solar gain or dewpoint)

Types of SA Temperature Control

Variable (zone level information available to Control System)

• Change supply air temperature to maintain worst case VAV box at 85% (adjustable) using PID control loop

• Zone temperature deviation algorithms (VAV boxes with limited info or pneumatics with DDC connected temps sensors

• Trim and Respond (T&R) from Taylor Engineering

SA Temperature Control T&R Sequence• Set maximum CD DAT setpoint to 68°F

(adjustable).• Set minimum CD DAT setpoint to 55°F

(adjustable).• Generate a cooling request when the cooling loop

output signal at any zone is 95% or above (adjustable).

NOTE: The cooling loop output signal is the output signal of the PI loop controlling the terminal box cold deck airflow. A value of 100% would provide the maximum cooling to the zone.

SA Temperature Control T&R Sequence (cont.)• If the CD fan is on, trim the setpoint every 3

minutes (adjustable) in the following way:○ If the number of cooling requests is zero or one

(adjustable), INCREASE the DAT setpoint by 0.5°F (adjustable)

○ If the number of cooling requests is greater than two (adjustable), DECREASE the DAT setpoint by 0.3°F (adjustable) times the number of cooling requests less 2 but no more than 1.2ºF (adjustable).

○ If the number of cooling requests is two, then no changes are made to the setpoint.

Example:Number of Cooling Requests = 4Decrease in CD DAT setpoint = (4 - 2) * 0.3°F = 0.6°F

SA Temperature Control T&R Sequence (cont.)• During warm-up mode, the CD DAT setpoint

shall be set to the maximum CD DAT setpoint.• For each terminal box, a new software point

(“cooling request count”) shall be added which counts the number of cooling requests produced by that terminal box. In other words, every time a cooling request is generated the “cooling request count” should increase by 1. This will allow the operator to easily determine which VAV boxes are driving the reset.

Site 1 – Pre Retrofit

5052545658606264

44 54 64 74 84 94

Supp

ly Air Tem

p

Outdoor Air Temperature

AHU1 ‐ SAT Operation (Sept‐2010)

SAT SP SAT

Site 1 – Post Programming

Site 1 – Post ProgrammingIssues

○No variation in SA temperature setpoint observedSolution

o Controls contractor elected to start conservativeo Max and Min CD DAT setpoint initially set to 60°F

Site 1 – Post Commissioning

54

56

58

60

62

64

66

68

70

50 55 60 65 70 75 80 85 90

Cold Deck Temp

Outdoor Air Temperature

DAT & DAT Setpoint on ACU‐1 (August 2011)

SAT

SAT SP

Site 2 – Pre Retrofit

○Eight Trane IntelliPak™ units serving VAV with reheat ○Supply air temperature set constant at 55°F

Site 2 – Post Initial Programming○The sequence was programmed and went live.

Same set points and programming as Site 1• Issues

○Next morning dozens of comfort complaints○Also Complaints about noisy boxes (too much

air flow)• What happened (we think)

o Supply air temperature 68 F before occupants arrive

o People arriveo VAVs to100% open, SAT from 68 °F to 55 °F in

11 min

Site 2 – Post Initial Programming

• Initial Solution○Adjusted maximum CD DAT setpoint back to 55°F

(adjustable) and increased by 1 °F per week while gauging complaint levels

• Got conservative:○Generate a cooling request when the cooling loop

output signal at any zone is 65% instead of 95% or above (adjustable).

○ Added a release cooling request at 45% cooling loop output

○Slowed update rate from 3 to 5 minutes

Site 2 – Post Commissioning

50 

52 

54 

56 

58 

60 

62 

64 

66 

68 

70 

35  45  55  65  75 

SAT S

etpo

int [°F]

Outdoor Air Temperature [°F]

SAT Reset AC‐1, 2, 3, 4 (Dec‐2011)AC‐1 AC‐2 AC‐3 AC‐4 Base Case

Site 2 – Post Commissioning

50 

52 

54 

56 

58 

60 

62 

64 

66 

68 

70 

35  45  55  65  75 

SAT S

etpo

int [°F]

Outdoor Air Temperature [°F]

SAT Reset AC‐5, 6, 7, 8 (Dec‐2011)AC‐5 AC‐6 AC‐7 AC‐8 Base Case

Site 1 ResultsSite 1 - Verified Results

Energy Savings and Cost Savings Project Payback

Peak Savings

(kW)

Energy Savings (kWh)

Energy Savings (therms)

Total Cost Savings

Measure Cost

Approved PG&E

Incentive

Simple Payback

(yr)

RCx-1Delamp Garage T8 Fixtures From 3 Lamps To 1 Lamp Per Fixture 3.2 28,452 0 3,921$ 2,223$ 1,112$ 0.3

RCx-2 Schedule Half Garage Lights Off During Daytime 0.0 1,222 0 168$ 2,774$ 110$ 15.8

RCx-3 Photocell Control for Lobby Lights 0.0 1,860 0 256$ 134$ 67$ 0.3

RCx-4 Repair Broken Chilled Water Valve on AHU-2 7.4 43,113 16,605 18,632$ 5,314$ 2,657$ 0.1

RCx-5Recomission Economizers(All Air Handlers) 20.3 120,613 6,761 21,788$ 5,749$ 2,875$ 0.1

RCx-6Install VFD on Heating Furnace Fans(HF-1 and HF-2) 14.4 66,903 0 9,219$ 17,117$ 6,021$ 1.2

RCx-7 Implement Supply Air Reset On All Airhandlers 23.6 179,553 33,705 50,503$ 7,193$ 3,596$ 0.1

RCx-8 Schedule Air Handlers and Chilled Water System 0 191,127 1,532 27,508$ 24,327$ 12,164$ 0.4

RCx-9 Implement Chilled Water Setpoint Reset

RCx-10Install VFD on Chilled Water Pumps(Not Recommended)

RCx-11Schedule off AH-4 during nights and weekends by installing two split system units 0 27,010 0 3,722$ 57,663$ 2,431$ 14.8

TOTALS (Implemented Measures) 68.9 659,855 58,603 135,718$ 122,495$ 31,032$ 0.7

Verified (Post-Retrofit)

Measure Number Measure Name

Site 2 ResultsSite 2 - Verified Results

Annual Energy Savings and Cost Savings Project Financials

Peak Savings

(kW)

Energy Savings (kWh)

Energy Savings (therms)

Total Cost Savings

Measure Cost*

Approved Incentive

Simple Payback

(yr)

RCx-1 Recommission All Economizers (AC1-8) 17.0 118,417 11 15,047$ 12,579$ 6,289$ 0.42

RCx-2Fix Zone Temperature Setpoints, Implement SAT and DSP Resets (AC1-8) 56.0 564,029 34,833 97,286$ 14,751$ 7,376$ 0.08

RCx-3 Schedule All Non-Essential VAV boxes and Air Handlers to Match Occupancy

0.0 250,074 18,998 45,751$ 3,431$ 1,715$ 0.04

TOTALS (Implemented Measures) 73.0 932,520 53,842 158,084$ 30,761$ 15,380$ 0.10

Verified (Post-Retrofit)

Measure Number Measure Name

Site 3 – Currently being commissioned• Findings

○Multiple “rogue” VAV boxes are always generating cooling requests and driving resets, not much variation in DAT currently observed

○Rogue VAV boxes classified into two categories- Low cost: Changes to zone temperature SP and/or maximum

CFM setpoint are required so that VAV box is not always generating a request for cooling

- High cost: VAV box replacement required: Software setpointchanges alone will not alleviate continued call for cooling; VAV box is undersized

Conclusions

• T&R control works and can be very cost effective! – performance issues are site specific

• Project not complete after programming is finished – Cx very important

• Run VAV box report pre implementation to check for rogue zones

• T&R very sensitive, alarming, periodic trend reviews and training recommended to insure persistence

• Only use when less complicated resets won’t work

Modified Trim and Respond SequenceCurrently testing modified T&R sequence

• Reduce SAT when there are multiple cooling requests

• Calculates total energy cost associated with increasing or decreasing SAT before deciding which way to go when there are no cooling requests

Thanks

Arik Cohen, PE

kW Engineering