Supply Air Temperature and Pressure Setpoint Reset in VAV Systems Based on Zone Demand

Steven T. Taylor, PEReinhard Seidl, PE Taylor Engineering Alameda, CA

Outline

Supply Air Temperature and Static Pressure Setpoint Reset in VAV Systems •Control Strategies•Energy impacts•Control SequencesCommissioning•Tuning• “Rogue” Zones

Static Pressure Setpoint Reset

Fan Energy at Varying SP Setpoints

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20%

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120%

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Percent Fan CFM

Perc

ent F

an k

W SP setpoint = TSPSP setpoint = TSP*.75SP setpoint = TSP/2SP setpoint = TSP/3SP setpoint = 0

Surge Region

Real Fan Systems Many Hours At Low Loads

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0 20,000 40,000 60,000 80,000 100,000 120,000 140,000Total CFM (SF-1 + SF-2)

Pres

sure

Dro

p A

cros

s Fa

n ("

H2O

)

Actual DataPerfect System Curve (i.e. SP reset)Design Point (145,000 CFM at 4.0")Likely Actual System Curve (No SP Reset)

Static Pressure Setpoint Reset Control Logic Options

Option 1: Reset static pressure setpoint based on PID Loop on VAV damper positionOption 2: Reset static pressure setpoint using “Trim & Respond”based VAV damper position or airflow demand

Best Choice: Trim and Respond

Advantages•Easier to tune•Can “respond” more quickly than

“trim”•Does not require knowledge of

damper position•Easier to ignore “rogue” zones

Trim & Respond SP Setpoint Reset Logic

Static pressure setpoint shall be reset using trim and respond logic within the range 0.15 inches to 1.5 inches. When fan is off, freeze setpoint at the minimum value (0.15 inches). While fan is proven on, every 2 minutes, decrease the setpoint by 0.04 inches if there two (adjustable) or fewer pressure requests. If there are more than two (adjustable) pressure requests, increase the setpoint by 0.04. Where VAV zone damper position is known, a pressure request is generated when any VAV or underfloor damper served by the system is wide open. Where VAV zone damper position is unknown, a pressure request is made when the ratio of the zone’s actual supply airflow to supply airflow setpoint is less than 90%. Adjustable to limit

dominance of rogue zonesSet by TAB Contractor

Trim & Respond SP Setpoint Reset Logic

Example of heating fan on dual duct system, without calls for heating.

System runs close to minimum: 75°F for temperature SP and 0.24”wg for pressure SP.

Note that actual temperature is much higher than SP, this is because of the mechanics of the burner staging, not because of the reset logic.

Trim & Respond SP Setpoint Reset Logic

Example of heating fan on dual duct system, with calls for heating. Note that calls for heat (3) and calls for pressure (3) coincide here, but don’t necessarily do this.

System runs at maximum: 95°F for temperature SP and 1.5”wg for pressure SP.

Note that, again, actual temperature is much higher than SP, this is because of the mechanics of the burner staging, not because of the reset logic.

Variations on this Sequence

A damper position (valve position, loop output, etc.) of 80% counts as one request, while a position of 95% counts as two requests.The response rate is a function of the number of requests, e.g. response = 0.04” times the number of requests.

Tuning – Unstable Control

y = 0.6709x + 0.2656R2 = 0.5971

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Actual Static Pressure

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ic P

ress

ure

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oint

Tuning – Stable Control

Supply Air Temperature Reset

Supply Air Temperature Control Optimization

Analyzed for:• Mild California climates• Chilled water plant at building (not central)• Variable speed fans

Probably applicable to more extreme climates as well• Humid climates should use low end of

outdoor air temperature reset ranges

Impact of SAT on Energy Usage Sacramento Office Building

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rce

Ene

rgy,

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Out

door

Air

Tem

pera

ture

, F

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rce

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rgy,

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tu/s

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door

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pera

ture

, F

In very hot weather, increased fan energy more than eliminates savings in compressor energy (green line above blue line)

In cool weather, supply temperature reset saves significant energy (longer use of economizer and no excessive fan energy as result of low SAT)

Recommended SAT Reset Logic

Reheat Economizer SAT Logic

None None Constant SAT at design (~55ºF)

Low(e.g. DFDD,

FPU)Yes

Constant SAT at design. Allow to float up by locking out cooling until zone request indicates need for CHW

Medium to High

(e.g. VAVRH)

Yes

Reset by zone cooling demand during cool weather (<55ºF to 60ºF), then ramp down to design SAT in warm weather (>60ºF to 70ºF)

Reheat System SAT Reset Logic

55 65

55

70

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65

Outdoor Air Temperature

Supp

ly Ai

r Tem

pera

ture

Set

poin

t

Tmin = 53F

SAT reset by demandwithin this range

VAV Reheat System SAT Reset Logic

During occupied mode, the setpoint is reset from T-min (53°F) when the outdoor air temperature is 70°F and above, proportionally up to T-max when the outdoor air temperature is 60°F and below. T-max shall be reset using trim and respond logic within the range 55°F to 65°F. When fan is off, freeze T-max at the maximum value (65ºF). While fan is proven on, every 2 minutes, increase the setpoint by 0.2ºF if there are two (adjustable) or fewer zone cooling requests. If there are more than two (adjustable) cooling requests, decrease the setpoint by 0.3ºF. A cooling request is generated when the cooling loop of any zone served by the system is >99%. Lower if reheat reduced

using “dual maximum” logic and central plant

Adjustable to limit dominance of rogue zones

Performance – Oversized Zones

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Outdoor Air Temperature (oF)

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ply

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ture

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poin

t (oF

)

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pera

ture

(oF)

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of C

oolin

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eque

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SAT SetpointOutdoor Air TemperatureCooling Requests

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Outdoor Air Temperature (oF)

Sup

ply

Air

Tem

pera

ture

Set

poin

t (oF

)

Performance – Rogue Zones

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Outdoor Air Temperature (oF)

Supp

ly A

ir T

empe

ratu

re S

etpo

int (

oF)

Rogue Zone

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6/27/06 3:00 6/27/06 6:00 6/27/06 9:00 6/27/06 12:00 6/27/06 15:00 6/27/06 18:00 6/27/06 21:00

Cool

ing

Loop

Out

put (

%)

VAV2-20 VAV2-21 VAV2-11 VAV2-22 VAV2-23 VAV2-24 VAV2-25VAV2-27 VAV2-30 VAV2-31 VAV2-6 VAV2-7 VAV2-28 VAV2-9

Rogue Zones

Causes• Undersized zone/unexpected loads• Thermostat setting too low• Duct design problem – high pressure drop

fitting or duct section

Handling Rogue Zones

Exclude them from reset sequence• May result in temperature problems in

these zones• Not advised for boss’ office

Limit thermostat setpoint adjustmentsFix duct restrictions/sizing issuesAdd auxiliary cooling to augment VAV

Supply Air SP vs Duct Static SPHighest temp. reset occurs at warmest point of the day. Note that pressure remains high almost all day, possibly rogue zone

Pressure setpoint (blue)

Temp. setpoint (red)

Outside Air Temp. (green)

Supply Air SP vs Duct Static SP

Same building, next day – completely different pictureboth resets now working – might look like they are running in “counter-phase”, but incidental

Pressure setpoint (blue)

Temp. setpoint (red)

Outside Air Temp. (green)

Supply Air SP vs Duct Static SP

Same building, next day – now both resets almost run “in phase”

Pressure setpoint (blue)

Temp. setpoint (red)

Outside Air Temp. (green)

Poor Supply Air Temp controlActual temperature does not adhere to setpoint

Actual temperature (green)

Temperature setpoint (orange)

Poor Supply Air Temp controlTemp. setpoint does not appear to follow prescribed logic, and actual temperature does not adhere to setpoint)

Actual temperature (green)

Temperature setpoint (orange)

Temperature setpoint per sequence of operations (red)

Poor Supply Air Temp controlPressure setpoint (blue)

Temp. setpoint (red)

Actual Temp. (orange)

Outside Air Temp. (green)

Pressure control

Number of pressure requests – when these drop below 5 requests, dP setpoint is lowered

Pressure setpoint (signal stays high even when requests drop, because more than 5 requests)

Pressure control

VFD speed drops to maintain pressure after initial system start, then builds again as VAV’s open after 9AM, then drops as pressure setpoint is reduced.

Pressure setpoint signal stays high (>5 requests)

After initial morning start, VFD speed and pressure setpoint go hand in hand

Summary

Static pressure setpoint reset• Use Trim & Respond SP setpoint reset logic based on

damper position or airflow demandSupply air temperature setpoint reset• Not effective for all systems and climates• Use combination of Trim and Respond SAT reset and

outdoor air temperature reset to minimize fan/cooling/reheat energy use

Commissioning• Tuning is essential for stable operation• Review trend data of normal operation to identify (and fix)

rogue zones