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; }

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)