Commissioning Instructions

Supply Air Temperature Reset using Central Functions

Good Indicators of Zone Load

A6V12006922

Restricted

7 | 21

4 Supply Air Temperature Reset using Central

Functions

DXR controllers can accomplish SAT Reset using central functions by using the second strategy described

above. The central function (SplyAir11) collects the number of terminals calling for cooling and heating, and

determines which mode to use (Heating, Cooling, Neither, or Neutral). Based on this mode, the SAT can be

set to a configured value and ramped up or down based on what type of air is needed.

In every Supply Air VAV (VavSuXX) application function, there are objects that send the Heating and

Cooling demand out to be read by Central Functions or the AHU. These points (VavSuCDmd and

VavSuHDmd) send a demand signal (from 0-100%) that the Supply Air Central Function collects for every

terminal.

The central function then averages the highest 10 values to determine how to ramp the SAT up or down

(see picture above). The average of the highest cooling and heating values (EvlTSuCDmdMax,

EvlTSuHDmdMax) determine the setpoint. The high and low limits for Cooling and Heating can be

configured (TSuMax, TSuMin), as well as the starting temperatures that determine cooling and heating

(SttValC, SttValH).

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.

These two calculations determine the actual SAT setpoint (SpHiTSu, SpLoTSu) sent to the AHU.

As long as 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.

NOTICE! In general, if VAV dampers are used for cooling only, the Changeover mode will be in Cooling

and the SAT will only fluctuate between SttValC and TSuMin.

There will be no control in Heating. Thus, it is recommended to configure SttValC as the Max SAT and

TSuMin as the Minimum SAT. See Configuring VAV Terminal with Default Values for more information.

NOTICE! VAV box airflows MUST be configured to support heating & cooling airflow.

If the VAV terminal box support heating from the AHU, make sure the VAV terminal box airflows are

configured to modulate VAV supply air damper for heating.