Supply air temperature should be lower than mixed air temperature while cooling

Objective/Method/Outcome

Objective: Determines the supply air temperature should be lower than mixed air temperature while cooling

Method: Compares the SAT and the MAT, to determine if SAT is higher. Checks if the discharge air cooling command or the economizer command are active and the supply fan status is on. If the SAT is higher when CCV or Economizer commands are on and the Supply Fan is active, the system is determined to be at fault and the hours of fault is deetermined.

Outcome: Triggers an insight.

Description

Indicates an inconsistency between supply and mixed air temperatures, as SAT should be lower than MAT while cooling is active.

This rule is part of the CopperTree Standard rule-set, and is derived from NIST APAR11 and APAR16.

Default Priority

Low

Diagnosis

  1. Input sensor(s) faulty or miscalibration. Verify input sensor(s) operation.
  2. Cooling coil valve stuck. Verify valve/actuator operation.
  3. Heating coil valve stuck and/or leaking. Verify valve/actuator operation.

Message

Supply air temperature is not lower than mixed air temperature while cooling is active.

Rule Template

Insight Type: Fault Detection

Trigger Type: Active High

Logic

Logic for determining if supply air temperature is lower than mixed air temperature while cooling

Inputs

InputUnitTagsDescription
SATDischarge Air TemperatureThe recorded SAT
MATMixed Air TemperatureThe recorded MAT
CCVDischarge Air Cooling CommandThe recorded Discharge Air Cooling command
SF StatusDischarge Air Fan Command, Discharge Air Fan StatusThe recorded status of Supply Fan
Threshold – Weekly Hours of Faulty OperationThe minimum hours of faulty operation that can trigger an insight
Fan Temperature Gain
Threshold –  Temperature DeadbandThe minimum temperature deviation that can trigger an insight
EconomizerEconomizer Command

Notes

This rule compensates for the temperature rise across a supply fan between the MAT and SAT sensors. This value varies per system, and the parameter should be changed within the logic flow accordingly. The temperature rise over the supply fan is currently to 1.1 degrees in accordance with NIST recommendations.

Default Thresholds:

  • Fan Temperature Gain: 1.1°.
  • Temperature Error Threshold: 2°
  • Allowable Weekly Hours: 8
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