Commissioning Instructions

Supply Air Temperature Reset and Zone Demand

Poor Indicators of Zone Load

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A6V12006922

3 Supply Air Temperature Reset and Zone Demand

Keeping the Supply Air Temperature (SAT) at a constant temperature can be effective in certain situations

but can also lead to wasted energy and increased costs in other scenarios. A common practice, even

required by some guidelines and standards, is to reset the SAT regularly to a temperature that can reduce

the amount of heating and cooling to the air, as well as reducing fan energy. By implementing SAT Reset,

the control system has more flexibility to efficiently satisfy the zone, regardless of the load required.

By using central functions, it is possible to collect data from each VAV box and use it to automatically reset

supply air temperature if necessary. ASHRAE recommends choosing from available strategies that reset the

supply air temperature setpoint based on building loads or outside air temperature.

This document focuses on the simple and efficient demand signal method to achieve supply air

temperature reset.

3.1 Poor Indicators of Zone Load

1. Outside air temperature does not consider internal loads into the reset. If the temperature is reset

when external loads are low but internal loads are high, the space could become over-heated.

2. Return air temperature provides overall average but does not consider individual zones that may have

internal loads. Some zones may be well above the average, while others may be well below.

3. Zone temperature (room temperature) does not consider building inhabitant factors. If a zone is

chosen to represent a hot or cold section of the building, that could change depending on the load. For

example, if a zone is chosen to represent the coldest area, people in the zone may add things like local

space heaters for comfort, thus making the zone reading much higher than normal.

3.2 Good Indicators of Zone Load

The cooling or heating demand signal (PID controller) from an air terminal provides a good demand

indicator for supply air temperature. When an air terminal is in cooling and meets air-flow conditions but

fails to meet zone comfort conditions in the space served, it requires the colder supply air. The same is true

in reverse for heating. Therefore, using the output of the cooling or heating PID controller for an air

terminal provides a good demand indicator for supply air temperature.

In every Supply Air VAV (VavSuXX) application function, BACnet objects send the Heating and Cooling

demand out to be read by Central Functions or the AHU. These points send a heating and cooling demand

signal (from 0-100%) that the Supply Air Central Function collects for every terminal.

The central function averages the highest 10 values to determine how to increase or decrease the supply

air temperature setpoint. By evaluating the number of zones that call for cooling or heating as well as the

average demand of the zones, the central function can continuously optimize the supply air to be at an

appropriate temperature.

When the terminal DXRs are assigned to the central function group the calculations for Supply Air Setpoint

will occur, and varying setpoints will be sent to the AHU as loads change.

Group Master Object

The Group Master object in the Central Function application for supply air coordination collects data from

supply air Group Members to support supply air temperature reset.