NIST APAR6 – Supply air temp should be less than return air temp (Mode 2 – Cooling w/OA)

Applicable Systems

  • NISTAPAR: APAR6
  • Primary: AHU, RTU, MUA, ERV

Objective/Method/Outcome

  • Objective: Indicates that the SAT is too high in relation to the RAT given the AHU is running in Mode 2 (Cooling w/ OA). This rule only applies when the AHU is in free cooling mode.
  • Method: Check if the SAT is greater than the RAT by more than 2 degrees for over 8 occupied hours per week while the unit is in Mode 2.
  • Outcome: An insight is generated when the SAT is greater than the RAT by more than 2 degrees for over 8 occupied hours per week while the unit is in Mode 2.

Logic

If SAT > (RAT + Temperature Threshold - Temperature Rise Over Fan) { return true; }</span>
NIST APAR6 – Supply air temp should be less than return air temp (Mode 2 – Cooling w/OA)

Inputs

  • RAT {return, air, temp, sensor} (Real Trend Log)
  • SAT {sensor, supply, air, temp} (Real Trend Log)
  • AHU_MODE_CTL {ahu, mode} (Real Trend Log)

Parameters

ParameterExpression Block Series NumberDescriptionDefault Value (Units)
Temperature Threshold74This is the minimum temperature difference between SAT and RAT needed to enable the rule.2 (C)
Const – Temperature Rise Over Fan73The estimated air temperature rise over the fan1.1 (C)
Acceptable Hours per Week77The amount of time that this error is allowable for the unit per week. Anything more than that time is considered a fault.8 (h)
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