NIST APAR12 – Supply air temp should be less than return air temp (Mode 3 – Mech. Cooling w/100% OA)
Applicable Systems
NISTAPAR: APAR12
Primary: AHU, RTU, MUA, ERV
Objective/Method/Outcome
Objective: Indicates that the supply air temperature is greater than the return air temperatures while the unit is in Mode 3 (Mechanical Cooling w/100% OA)
Method: Check if the SAT is more than the RAT for over 8 occupied hours per week while the unit is in Mode 3.
Outcome: An insight is generated when the SAT is more than the RAT for over 8 occupied hours per week while the unit is in Mode 3.
Logic
If (SAT > RAT + Temperature Threshold - Temperature Rise Over Fan) { return true; }
NIST APAR12 – Supply air temp should be less than return air temp (Mode 3 – Mech. Cooling w/100% OA)
Inputs
RAT {sensor, return, air, temp} (Real Trend Log)
SAT {sensor, discharge, air, temp} (Real Trend Log)
AHU_MODE_CTL {ahu, mode} (Real Trend Log)
Parameters
Parameter
Expression Block Series Number
Description
Default Value (Units)
Const- Temperature Rise Over Fan
31
The estimated air temperature rise over the fan
1.1 (C)
Temperature Threshold
30
This is the minimum temperature difference between OAT and RAT needed to enable the rule. This value is recommended by NIST.
2 (C)
Acceptable Hours per Week
33
The amount of time that this error is allowable for the unit per week. Anything more than that time is considered a fault.
