OA Fraction Rule

Type: oa_fraction — Built-in check; no custom expression.

What the rules engine does

The oa_fraction rule which is defined by ASHRAE Guideline 36 Fault Rule Six flags when the calculated outdoor air (OA) fraction deviates too much from the minimum design OA fraction in non-economizer modes. The engine computes OA fraction from MAT, RAT, and OAT using the standard enthalpy/temperature-based formula. It compares that to the design minimum (based on ahu_min_oa_cfm_design and supply airflow). When |OA_frac_calc - OA_min| > airflow_err_thres and the AHU is in heating (OS1) or mechanical-only cooling (OS4), a fault is flagged. Catches OA fraction calculation errors or AHU not maintaining design airflow.

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oa_fraction_airflow_error (oa_fraction)

OA fraction calc error or AHU not maintaining design airflow.

name: oa_fraction_airflow_error
description: OA fraction calc error or AHU not maintaining design airflow in non-economizer modes
type: oa_fraction
flag: fc6_flag
equipment_type: [AHU, VAV_AHU]

inputs:
  Supply_Fan_Air_Flow_Sensor:
    brick: Supply_Fan_Air_Flow_Sensor
    column: supply_fan_air_volume
  Mixed_Air_Temperature_Sensor:
    brick: Mixed_Air_Temperature_Sensor
    column: mat
  Outside_Air_Temperature_Sensor:
    brick: Outside_Air_Temperature_Sensor
    column: oat
  Return_Air_Temperature_Sensor:
    brick: Return_Air_Temperature_Sensor
    column: rat
  Supply_Fan_Speed_Command:
    brick: Supply_Fan_Speed_Command
    column: supply_vfd_speed
  Damper_Position_Command:
    brick: Damper_Position_Command
    column: economizer_sig
  Heating_Valve_Command:
    brick: Valve_Command
    column: heating_sig
  Cooling_Valve_Command:
    brick: Valve_Command
    column: cooling_sig

params:
  airflow_err_thres: 0.1
  ahu_min_oa_cfm_design: 1000.0
  oat_rat_delta_min: 5.0
  ahu_min_oa_dpr: 0.1

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Next: ERV Efficiency Rule