NIST APAR18 – Outdoor air fraction out of acceptable range (Mode 4 – Mech. Cooling w/min OA)

Applicable Systems

  • NISTAPAR: APAR18
  • Primary: AHU, RTU, MUA, ERV

Objective/Method/Outcome

  • Objective: Indicates that the fraction of outdoor air is too high or too low while the AHU is in Mode 4 (Mech. Cooling w/min OA).
  • Method: Calculate the OAF using the MAT, RAT, and OAT. Check if the OAF is outside of a 10% range of the specified minimum OAF during occupied hours. There must be a significant temperature difference between OAT and RAT for this rule to be effective.
  • Outcome: An insight is generated when the calculated OAF is not within the desired minimum OAF (30%) for more than 12 occupied hours per week.

Logic

If ((MAT-RAT)/(OAT-RAT)-Minimum Outdoor Air Fraction > Flow Error Threshold && CCV Open) { return true; }
NIST APAR18 – Outdoor air fraction out of acceptable range (Mode 4 – Mech. Cooling w/min OA)

Inputs

  • MAD {cmd, mixed, air, damper} (Real Trend Log)
  • MAT {sensor, mixed, air, temp} (Real Trend Log)
  • RAT {sensor, return, air, temp} (Real Trend Log)
  • AHU_MODE_CTL {ahu, mode} (Real Trend Log)
  • OAT {sensor, outside, air, temp} (Real Trend Log)

Parameters

Parameter Expression Block Series Number Description Default Value (Units)
Const- Minimum Outdoor Air Fraction 11 The desired minimum OAF for the rule to be effective. 30 (%)
Const- Flow Error Threshold 12 The acceptable deviation for the calculated OAF and Minimum OAF which is a constant value 30 (%)
Const- Damper Error Threshold 20 The movement tolerance of the CCV. This is used to define when the valve is open -everything above the formula (100 – CCV Error Threshold) is considered saturated. 2 (%)
Const- Min MAD Position 19 Minimum position for the damper to considered open. 20 (%)
Temperature Threshold 10 This is the minimum temperature difference between OAT and RAT needed to enable the rule. 5.6 (C)
Acceptable Hours per Week 31 The amount of time that this error is allowable for the unit per week. Anything more than that time is considered a fault. 12 (h)