Supply Air Temperature Reset
Arik Cohen P.E.
Principal
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.
Course Description
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.
Learning Objectives
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
Supply/Discharge Air Temperate Control Basics
•1st Stage- economizer
•2nd Stage – Dx/Cooling Coil
SAT/DAT  to zone VAV Boxes
SAT/DAT – Supply/Discharge Air Temperature
Temperature Resets
Example Resets:
HHW OAT Reset
200
190
HHW Setpoint [°F]
• 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
180
170
160
150
140
130
120
20
40
60
OAT [°F]
80
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
Weather Bin Data from
San Jose CA
OSA
Temp
(F)
96
94
92
90
88
86
84
82
80
78
76
74
72
70
68
66
64
62
60
58
56
54
52
50
48
46
44
42
40
38
36
34
32
Total
Hrs/yr
2
5
4
7
19
11
39
96
93
155
157
226
239
320
170
339
686
719
1,040
887
781
629
487
304
392
326
169
195
104
70
72
15
2
8,760
Variable Air Volume with Reheat
Return
Air
Exhaust
Air
Outside
Air
VFD
COLD
Exterior Zone
DAMPER
DAMPER
REHEAT COIL
REHEAT COIL
Interior Zone
DAMPER
DAMPER
REHEAT COIL
REHEAT COIL
Interior Zone
DAMPER
DAMPER
REHEAT COIL
REHEAT COIL
DAMPER
DAMPER
Exterior Zone
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
Electricity Consumption Rate [kWh/day]
Maximum Demand [kW]
7,000
600
6,000
500
400
4,000
300
3,000
200
2,000
100
1,000
0
0
Jul
Aug
Sep
Oct
Nov
Dec
Jan
Feb
Mar
Apr
May
Mar
Apr
May
Jun
Natural Gas Consumption Rate [therms/day]
250
therms/day
200
150
100
50
0
Jul
Aug
Sep
Oct
Nov
Dec
Jan
Feb
Jun
kW
kWh/day
5,000
Benchmarking – Site 2
Electricity Benchmarking
PG&E CEUS - Large Office
SR1
-
10
20
30
40
50
kWh/sf
Gas Benchmarking
PG&E CEUS - Large Office
SR1
-
20
40
kBtu/sf
60
80
Screenshot of Typical Air Handler
General Site Recommendations
Fix Economizers
Take advantage of DDC at Zone level!
• Zone level scheduling
• Supply Air/Duct SP Reset
Estimated (Pre-Retrofit)
Annual Energy, Cost and GHG Savings
Measure
Number
Measure Name
Peak
Savings
(kW)
Energy
Savings
(kWh/yr)
Energy
Savings
(therm/yr)
Total Cost
Savings
Project Financials
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-2
Fix 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
Types of SA Temperature Control
Constant
•SA Temperature setpoint does not change
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)
Outside Air
Dry Bulb
Temperature
Supply Air
Temperature
Reset
[°F]
50
[°F]
70
70
55
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 = 4
Decrease 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
Supply Air Temp
AHU1 ‐ SAT Operation (Sept‐2010)
64
62
60
58
56
54
52
50
44
54
64
74
84
Outdoor Air Temperature
SAT SP
SAT
94
Site 1 – Post Programming
Site 1 – Post Programming
Issues
○ No variation in SA temperature setpoint observed
Solution
o Controls contractor elected to start conservative
o Max and Min CD DAT setpoint initially set to 60°F
Site 1 – Post Commissioning
DAT & DAT Setpoint on ACU‐1 (August 2011)
70
SAT
Cold Deck Temp
68
SAT SP
66
64
62
60
58
56
54
50
55
60
65
70
75
Outdoor Air Temperature
80
85
90
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 arrive
o 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
SAT Reset AC‐1, 2, 3, 4 (Dec‐2011)
AC‐1
70 AC‐2
AC‐3
AC‐4
Base Case
65 75 SAT Setpoint [°F]
68 66 64 62 60 58 56 54 52 50 35 45 55 Outdoor Air Temperature [°F]
Site 2 – Post Commissioning
SAT Reset AC‐5, 6, 7, 8 (Dec‐2011)
AC‐5
70 AC‐6
AC‐7
AC‐8
Base Case
SAT Setpoint [°F]
68 66 64 62 60 58 56 54 52 50 35 45 55 65 Outdoor Air Temperature [°F]
75 Site 1 Results
Site 1 - Verified Results
Verified (Post-Retrofit)
Energy Savings and Cost Savings
Measure
Number
Measure Name
RCx-1
Project Payback
Peak
Savings
(kW)
Energy
Savings
(kWh)
Energy
Savings
(therms)
Delamp Garage T8 Fixtures From 3 Lamps To 1 Lamp
Per Fixture
3.2
28,452
0
$
3,921 $
2,223 $
1,112
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-5
Recomission Economizers
(All Air Handlers)
20.3
120,613
6,761
$
21,788 $
5,749 $
2,875
0.1
RCx-6
Install 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
0
27,010
0
$
3,722 $
57,663 $
2,431
14.8
68.9
659,855
58,603
$
135,718 $
122,495 $
31,032
0.7
RCx-10
Install VFD on Chilled Water Pumps
(Not Recommended)
RCx-11
Schedule off AH-4 during nights and weekends by
installing two split system units
TOTALS (Implemented Measures)
Total Cost
Savings
Measure
Cost
Approved
PG&E
Incentive
Simple
Payback
(yr)
0.3
Site 2 Results
Site 2 - Verified Results
Verified (Post-Retrofit)
Annual Energy Savings and Cost Savings
Measure
Number
Measure Name
Peak
Savings
(kW)
Energy
Savings
(kWh)
Energy
Savings
(therms)
Total Cost
Savings
Project Financials
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-2
Fix 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
73.0
932,520
53,842 $
158,084 $
30,761 $
15,380
0.10
TOTALS (Implemented Measures)
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 setpoint
changes 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 Sequence
Currently 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