Determine if outdoor air fraction is suspect

Applicable Systems

  • NIST: APAR2, APAR18
  • Primary: AHU, RTU

Objective/Method/Outcome

  • Objective: Determine if the actual minimum outdoor air fraction (OAF) is not within the desired minimum, which could be an indication of control damper issues. Based on NIST APAR 2 and 18.
  • 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.
  • Outcome: An insight is generated if the actual OAF is not within the desired minimum OAF for more than 12 hours per week.

Logic

OAF = (MAT-RAT) / (OAT-RAT)</span>
If(MAD == minimum position) { If(OAF - minimum_OAF > Flow_Threshold) { return true; } }
Determine if outdoor air fraction is suspect

Notes

Within the Logic Flow, the desired minimum air fraction is an important parameter – usually unique to a site. This needs to be changed within the logic flow for this rule to be effective.

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)
  • OAT {sensor, outside, air, temp} (Real Trend Log)
  • Occ {occupied} (Binary Trend Log)

Parameters

ParameterExpression Block Series NumberDescriptionDefault Value (Units)
Minimum Outdoor Air Fraction (in percent)11The desired percent of Outdoor Air, when the unit is set to draw minimum Outdoor Air.5 (%)
Flow Error Threshold12The acceptable air flow deviation tolerance.10 (%)
Minimum Temperature Difference to Enable10This is the minimum temperature difference between OAT and RAT needed to enable the rule. Anything less has a chance of creating disproportionately high OAFs. This value is recommended by NIST.5.6 (C)
Min MAD Position19This defines the minimum MAD position. Anything less than this value (while the damper is still open) is considered to be “at minimum position”30 (%)
Damper Error Threshold20The movement tolerance of the MAD. This is used to define when the MAD is closed and open -everything above this value is considered open.5 (%)
Acceptable Hours per Week31The amount of time that is allowable for the unit to have a minimum OAF different than the desired amount. Anything more than that time is considered a fault.12 (h)
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