Expression Rule Cookbook

About np in expressions: When you see np in a rule (e.g. np.maximum, np.abs, np.sqrt), it refers to NumPy — the Python computing library which Pandas uses under the hood for high-performance numerical math. open-fdd injects np into expression evaluation automatically, so you can use NumPy functions directly in your fault logic.

How to define your own expressions

Expressions are boolean (true/false) statements — when they evaluate to True, a fault is flagged. open-fdd evaluates expressions with pandas and NumPy under the hood: input names become pandas Series, and np gives you NumPy functions for vectorized math. Here’s a quick guide — AI can definitely help with this, and pandas and NumPy are popular open-source libraries in the modern data science world:

Choose your inputs — Each input maps a BRICK class (or rule name) to a DataFrame column. Use brick: for Brick model resolution and column: for the raw column name. Input names become pandas Series in the expression (one value per row).
Define params — Thresholds and constants go in params. Reference them by name in the expression (e.g. static_err_thres, vfd_max).
Write the expression — Use input names and param names as variables. The expression must evaluate to a boolean Series (True = fault). Use & for AND, | for OR, ~ for NOT. Use np for NumPy functions (np.maximum, np.abs, np.sqrt, etc.) — they work on Series and are vectorized (no Python loops). Comparisons and logical ops produce boolean Series. You can also use pandas Series methods like .diff(), .rolling(), .notna() on your inputs.
Example — A simple “sensor above threshold” rule:

name: my_custom_rule
type: expression
flag: my_fault_flag

inputs:
  Supply_Air_Temperature_Sensor:
    brick: Supply_Air_Temperature_Sensor
    column: sat

params:
  max_temp: 90.0

expression: |
  Supply_Air_Temperature_Sensor > max_temp

Save and run — Save as .yaml in your rules folder and run with RuleRunner(rules_path="my_rules").run(df).

AHU fault conditions for air handling units (ASHRAE Guideline 36, Fault Rules One–Fifteen)

Fault Rules One through Fifteen are defined by ASHRAE Guideline 36. open-fdd was originally based on G36 and has been expanded with BRICK terminology.

Fault Rule One — low_duct_static_at_max_fan (expression)

Duct static pressure too low with supply fan at max speed. VAV only.

name: low_duct_static_at_max_fan
description: Duct static pressure too low with supply fan at max speed
type: expression
flag: fc1_flag
equipment_type: [VAV_AHU]

inputs:
  Supply_Air_Static_Pressure_Sensor:
    brick: Supply_Air_Static_Pressure_Sensor
    column: duct_static
  Supply_Air_Static_Pressure_Setpoint:
    brick: Supply_Air_Static_Pressure_Setpoint
    column: duct_static_setpoint
  Supply_Fan_Speed_Command:
    brick: Supply_Fan_Speed_Command
    column: supply_vfd_speed

params:
  static_err_thres: 0.1
  vfd_max: 0.95
  vfd_err_thres: 0.05

expression: |
  (Supply_Air_Static_Pressure_Sensor < Supply_Air_Static_Pressure_Setpoint - static_err_thres) & (Supply_Fan_Speed_Command >= vfd_max - vfd_err_thres)

Fault Rule Two — mix_temp_too_low (expression)

Mix temperature too low; should be between outside and return air.

name: mix_temp_too_low
description: Mix temperature too low; should be between outside and return air
type: expression
flag: fc2_flag
equipment_type: [AHU, VAV_AHU]

inputs:
  Mixed_Air_Temperature_Sensor:
    brick: Mixed_Air_Temperature_Sensor
    column: mat
  Return_Air_Temperature_Sensor:
    brick: Return_Air_Temperature_Sensor
    column: rat
  Outside_Air_Temperature_Sensor:
    brick: Outside_Air_Temperature_Sensor
    column: oat
  Supply_Fan_Speed_Command:
    brick: Supply_Fan_Speed_Command
    column: supply_vfd_speed

params:
  mix_err_thres: 1.0
  return_err_thres: 1.0
  outdoor_err_thres: 1.0

expression: |
  (Mixed_Air_Temperature_Sensor - mix_err_thres < np.minimum(Return_Air_Temperature_Sensor - return_err_thres, Outside_Air_Temperature_Sensor - outdoor_err_thres)) & (Supply_Fan_Speed_Command > 0.01)

Fault Rule Three — mix_temp_too_high (expression)

Mix temperature too high; should be between outside and return air.

name: mix_temp_too_high
description: Mix temperature too high; should be between outside and return air
type: expression
flag: fc3_flag
equipment_type: [AHU, VAV_AHU]

inputs:
  Mixed_Air_Temperature_Sensor:
    brick: Mixed_Air_Temperature_Sensor
    column: mat
  Return_Air_Temperature_Sensor:
    brick: Return_Air_Temperature_Sensor
    column: rat
  Outside_Air_Temperature_Sensor:
    brick: Outside_Air_Temperature_Sensor
    column: oat
  Supply_Fan_Speed_Command:
    brick: Supply_Fan_Speed_Command
    column: supply_vfd_speed

params:
  mix_err_thres: 1.0
  return_err_thres: 1.0
  outdoor_err_thres: 1.0

expression: |
  (Mixed_Air_Temperature_Sensor - mix_err_thres > np.maximum(Return_Air_Temperature_Sensor + return_err_thres, Outside_Air_Temperature_Sensor + outdoor_err_thres)) & (Supply_Fan_Speed_Command > 0.01)

Fault Rule Four (hunting) — see Hunting Rule.

Fault Rule Five — sat_too_low_heating_mode (expression)

SAT too low in heating mode (broken heating valve).

name: sat_too_low_heating_mode
description: SAT too low; should be higher than MAT in heating mode (broken heating valve)
type: expression
flag: fc5_flag
equipment_type: [AHU, VAV_AHU]

inputs:
  Mixed_Air_Temperature_Sensor:
    brick: Mixed_Air_Temperature_Sensor
    column: mat
  Supply_Air_Temperature_Sensor:
    brick: Supply_Air_Temperature_Sensor
    column: sat
  Valve_Command:
    brick: Valve_Command
    column: heating_sig
  Supply_Fan_Speed_Command:
    brick: Supply_Fan_Speed_Command
    column: supply_vfd_speed

params:
  mix_err_thres: 1.0
  supply_err_thres: 1.0
  delta_t_supply_fan: 0.5

expression: |
  (Supply_Air_Temperature_Sensor + supply_err_thres <= Mixed_Air_Temperature_Sensor - mix_err_thres + delta_t_supply_fan) & (Valve_Command > 0.01) & (Supply_Fan_Speed_Command > 0.01)

Fault Rule Six (oa_fraction) — see OA Fraction Rule.

Fault Rule Seven — sat_too_low_full_heating (expression)

Supply air temp too low when heating valve fully open.

name: sat_too_low_full_heating
description: Supply air temperature too low in full heating mode with heating valve fully open
type: expression
flag: fc7_flag
equipment_type: [AHU, VAV_AHU]

inputs:
  Supply_Air_Temperature_Sensor:
    brick: Supply_Air_Temperature_Sensor
    column: sat
  Supply_Air_Temperature_Setpoint:
    brick: Supply_Air_Temperature_Setpoint
    column: sat_setpoint
  Valve_Command:
    brick: Valve_Command
    column: heating_sig
  Supply_Fan_Speed_Command:
    brick: Supply_Fan_Speed_Command
    column: supply_vfd_speed

params:
  supply_err_thres: 1.0

expression: |
  (Supply_Air_Temperature_Sensor < Supply_Air_Temperature_Setpoint - supply_err_thres) & (Valve_Command > 0.9) & (Supply_Fan_Speed_Command > 0)

Fault Rule Eight — sat_mat_mismatch_economizer (expression)

SAT and MAT should be approx equal in economizer mode.

name: sat_mat_mismatch_economizer
description: Supply air and mixed air temp should be approx equal in economizer mode
type: expression
flag: fc8_flag
equipment_type: [AHU, VAV_AHU]

inputs:
  Mixed_Air_Temperature_Sensor:
    brick: Mixed_Air_Temperature_Sensor
    column: mat
  Supply_Air_Temperature_Sensor:
    brick: Supply_Air_Temperature_Sensor
    column: sat
  Damper_Position_Command:
    brick: Damper_Position_Command
    column: economizer_sig
  Valve_Command:
    brick: Valve_Command
    column: cooling_sig

params:
  delta_t_supply_fan: 0.5
  mix_err_thres: 1.0
  supply_err_thres: 1.0
  ahu_min_oa_dpr: 0.1

expression: |
  (np.abs(Supply_Air_Temperature_Sensor - delta_t_supply_fan - Mixed_Air_Temperature_Sensor) > np.sqrt(supply_err_thres**2 + mix_err_thres**2)) & (Damper_Position_Command > ahu_min_oa_dpr) & (Valve_Command < 0.1)

Fault Rule Nine — oat_too_high_free_cooling (expression)

OAT too high in free cooling without mechanical cooling.

name: oat_too_high_free_cooling
description: Outside air temp too high in free cooling without mechanical cooling
type: expression
flag: fc9_flag
equipment_type: [AHU, VAV_AHU]

inputs:
  Outside_Air_Temperature_Sensor:
    brick: Outside_Air_Temperature_Sensor
    column: oat
  Supply_Air_Temperature_Setpoint:
    brick: Supply_Air_Temperature_Setpoint
    column: sat_setpoint
  Damper_Position_Command:
    brick: Damper_Position_Command
    column: economizer_sig
  Valve_Command:
    brick: Valve_Command
    column: cooling_sig

params:
  outdoor_err_thres: 1.0
  delta_t_supply_fan: 0.5
  supply_err_thres: 1.0
  ahu_min_oa_dpr: 0.1

expression: |
  (Outside_Air_Temperature_Sensor - outdoor_err_thres > Supply_Air_Temperature_Setpoint - delta_t_supply_fan + supply_err_thres) & (Damper_Position_Command > ahu_min_oa_dpr) & (Valve_Command < 0.1)

Fault Rule Ten — oat_mat_mismatch_econ_mech (expression)

OAT and MAT approx equal in economizer + mechanical cooling mode.

name: oat_mat_mismatch_econ_mech
description: OAT and MAT should be approx equal in economizer + mechanical cooling mode
type: expression
flag: fc10_flag
equipment_type: [AHU, VAV_AHU]

inputs:
  Outside_Air_Temperature_Sensor:
    brick: Outside_Air_Temperature_Sensor
    column: oat
  Mixed_Air_Temperature_Sensor:
    brick: Mixed_Air_Temperature_Sensor
    column: mat
  Valve_Command:
    brick: Valve_Command
    column: cooling_sig
  Damper_Position_Command:
    brick: Damper_Position_Command
    column: economizer_sig

params:
  outdoor_err_thres: 1.0
  mix_err_thres: 1.0

expression: |
  (np.abs(Mixed_Air_Temperature_Sensor - Outside_Air_Temperature_Sensor) > np.sqrt(mix_err_thres**2 + outdoor_err_thres**2)) & (Valve_Command > 0.01) & (Damper_Position_Command > 0.9)

Fault Rule Eleven — oat_mat_mismatch_economizer (expression)

OAT and MAT approx equal in economizer mode.

name: oat_mat_mismatch_economizer
description: OAT and MAT should be approx equal in economizer mode
type: expression
flag: fc11_flag
equipment_type: [AHU, VAV_AHU]

inputs:
  Outside_Air_Temperature_Sensor:
    brick: Outside_Air_Temperature_Sensor
    column: oat
  Mixed_Air_Temperature_Sensor:
    brick: Mixed_Air_Temperature_Sensor
    column: mat
  Damper_Position_Command:
    brick: Damper_Position_Command
    column: economizer_sig

params:
  outdoor_err_thres: 1.0
  mix_err_thres: 1.0

expression: |
  (np.abs(Mixed_Air_Temperature_Sensor - Outside_Air_Temperature_Sensor) > np.sqrt(mix_err_thres**2 + outdoor_err_thres**2)) & (Damper_Position_Command > 0.9)

Fault Rule Twelve — sat_too_high_cooling_modes (expression)

SAT too high in econ+mech or mech-only cooling.

name: sat_too_high_cooling_modes
description: SAT too high; should be less than MAT in econ+mech or mech-only cooling
type: expression
flag: fc12_flag
equipment_type: [AHU, VAV_AHU]

inputs:
  Supply_Air_Temperature_Sensor:
    brick: Supply_Air_Temperature_Sensor
    column: sat
  Mixed_Air_Temperature_Sensor:
    brick: Mixed_Air_Temperature_Sensor
    column: mat
  Valve_Command:
    brick: Valve_Command
    column: cooling_sig
  Damper_Position_Command:
    brick: Damper_Position_Command
    column: economizer_sig

params:
  delta_t_supply_fan: 0.5
  mix_err_thres: 1.0
  supply_err_thres: 1.0
  ahu_min_oa_dpr: 0.1

expression: |
  (Supply_Air_Temperature_Sensor > Mixed_Air_Temperature_Sensor + np.sqrt(supply_err_thres**2 + mix_err_thres**2) + delta_t_supply_fan) & (((Damper_Position_Command > 0.9) & (Valve_Command > 0)) | ((Damper_Position_Command <= ahu_min_oa_dpr) & (Valve_Command > 0.9)))

Fault Rule Thirteen — sat_too_high_full_cooling (expression)

SAT too high vs setpoint in full cooling mode.

name: sat_too_high_full_cooling
description: SAT too high vs setpoint in OS3/OS4 full cooling mode
type: expression
flag: fc13_flag
equipment_type: [AHU, VAV_AHU]

inputs:
  Supply_Air_Temperature_Sensor:
    brick: Supply_Air_Temperature_Sensor
    column: sat
  Supply_Air_Temperature_Setpoint:
    brick: Supply_Air_Temperature_Setpoint
    column: sat_setpoint
  Valve_Command:
    brick: Valve_Command
    column: cooling_sig
  Damper_Position_Command:
    brick: Damper_Position_Command
    column: economizer_sig

params:
  supply_err_thres: 1.0
  ahu_min_oa_dpr: 0.1

expression: |
  (Supply_Air_Temperature_Sensor > Supply_Air_Temperature_Setpoint + supply_err_thres) & (((Damper_Position_Command > 0.9) & (Valve_Command > 0.9)) | ((Damper_Position_Command <= ahu_min_oa_dpr) & (Valve_Command > 0.9)))

Fault Rule Fourteen — cooling_coil_drop_when_inactive (expression)

Temperature drop across inactive cooling coil. Requires coil entering/leaving sensors.

name: cooling_coil_drop_when_inactive
description: Temperature drop across inactive cooling coil in heating/economizer modes
type: expression
flag: fc14_flag
equipment_type: [AHU, VAV_AHU]

inputs:
  Cooling_Coil_Entering_Air_Temperature_Sensor:
    brick: Cooling_Coil_Entering_Air_Temperature_Sensor
    column: clg_coil_enter_temp
  Cooling_Coil_Leaving_Air_Temperature_Sensor:
    brick: Cooling_Coil_Leaving_Air_Temperature_Sensor
    column: clg_coil_leave_temp
  Heating_Valve_Command:
    brick: Valve_Command
    column: heating_sig
  Cooling_Valve_Command:
    brick: Valve_Command
    column: cooling_sig
  Damper_Position_Command:
    brick: Damper_Position_Command
    column: economizer_sig

params:
  coil_enter_err_thres: 1.0
  coil_leave_err_thres: 1.0
  ahu_min_oa_dpr: 0.1

expression: |
  ((Cooling_Coil_Entering_Air_Temperature_Sensor - Cooling_Coil_Leaving_Air_Temperature_Sensor) > np.sqrt(coil_enter_err_thres**2 + coil_leave_err_thres**2)) & (((Heating_Valve_Command > 0) & (Cooling_Valve_Command == 0) & (Damper_Position_Command <= ahu_min_oa_dpr)) | ((Heating_Valve_Command == 0) & (Cooling_Valve_Command == 0) & (Damper_Position_Command > ahu_min_oa_dpr)))

Fault Rule Fifteen — heating_coil_rise_when_inactive (expression)

Temperature rise across inactive heating coil. Requires coil entering/leaving sensors.

name: heating_coil_rise_when_inactive
description: Temperature rise across inactive heating coil in econ/mech cooling modes
type: expression
flag: fc15_flag
equipment_type: [AHU, VAV_AHU]

inputs:
  Heating_Coil_Entering_Air_Temperature_Sensor:
    brick: Heating_Coil_Entering_Air_Temperature_Sensor
    column: htg_coil_enter_temp
  Heating_Coil_Leaving_Air_Temperature_Sensor:
    brick: Heating_Coil_Leaving_Air_Temperature_Sensor
    column: htg_coil_leave_temp
  Heating_Valve_Command:
    brick: Valve_Command
    column: heating_sig
  Cooling_Valve_Command:
    brick: Valve_Command
    column: cooling_sig
  Damper_Position_Command:
    brick: Damper_Position_Command
    column: economizer_sig

params:
  coil_enter_err_thres: 1.0
  coil_leave_err_thres: 1.0
  delta_t_supply_fan: 0.5
  ahu_min_oa_dpr: 0.1

expression: |
  ((Heating_Coil_Leaving_Air_Temperature_Sensor - Heating_Coil_Entering_Air_Temperature_Sensor) > np.sqrt(coil_enter_err_thres**2 + coil_leave_err_thres**2) + delta_t_supply_fan) & (((Heating_Valve_Command == 0) & (Cooling_Valve_Command == 0) & (Damper_Position_Command > ahu_min_oa_dpr)) | ((Heating_Valve_Command == 0) & (Cooling_Valve_Command > 0) & (Damper_Position_Command > 0.9)) | ((Heating_Valve_Command == 0) & (Cooling_Valve_Command > 0) & (Damper_Position_Command <= ahu_min_oa_dpr)))

Fault Rule Sixteen (erv_efficiency, custom) — see ERV Efficiency Rule.

Chiller plant

pump_diff_pressure_low (expression)

Variable pump does not meet differential pressure setpoint at full speed.

name: pump_diff_pressure_low
description: Variable pump does not meet differential pressure setpoint at full speed
type: expression
flag: fc_pump_flag

inputs:
  Differential_Pressure_Sensor:
    brick: Differential_Pressure_Sensor
    column: diff_pressure
  Differential_Pressure_Setpoint:
    brick: Differential_Pressure_Setpoint
    column: diff_pressure_setpoint
  Pump_Speed_Command:
    brick: Pump_Speed_Command
    column: pump_speed

params:
  diff_pressure_err_thres: 2.0
  pump_speed_max: 0.95
  pump_speed_err_thres: 0.05

expression: |
  (Differential_Pressure_Sensor < Differential_Pressure_Setpoint - diff_pressure_err_thres) & (Pump_Speed_Command >= pump_speed_max - pump_speed_err_thres)

chw_flow_high_at_max_pump (expression)

Primary chilled water flow too high with pump at high speed.

name: chw_flow_high_at_max_pump
description: Primary chilled water flow too high with pump at high speed
type: expression
flag: fc_chiller_flow_flag

inputs:
  Water_Flow_Sensor:
    brick: Water_Flow_Sensor
    column: flow
  Pump_Speed_Command:
    brick: Pump_Speed_Command
    column: pump_speed

params:
  flow_error_threshold: 1000.0
  pump_speed_max: 0.95
  pump_speed_err_thres: 0.05

expression: |
  (Water_Flow_Sensor > flow_error_threshold) & (Pump_Speed_Command >= pump_speed_max - pump_speed_err_thres)

Weather station

weather_temp_stuck (flatline) — see Flatline Rule.

weather_temp_spike (expression)

Unrealistic temperature change between consecutive readings.

name: weather_temp_spike
description: Unrealistic temperature change between consecutive readings
type: expression
flag: fault_temp_spike

inputs:
  Outside_Air_Temperature_Sensor:
    brick: Outside_Air_Temperature_Sensor
    column: temp_f

params:
  temp_spike_f_per_hour: 15.0

expression: |
  Outside_Air_Temperature_Sensor.diff().abs() > temp_spike_f_per_hour

weather_rh_out_of_range (expression)

Relative humidity outside valid range.

name: weather_rh_out_of_range
description: Relative humidity outside valid range (sensor error or bad data)
type: expression
flag: fault_rh_out_of_range

inputs:
  Humidity_Sensor:
    brick: Humidity_Sensor
    column: rh_pct

params:
  rh_min: 0.0
  rh_max: 100.0

expression: |
  (Humidity_Sensor < rh_min) | (Humidity_Sensor > rh_max)

weather_gust_lt_wind (expression)

Wind gust reported lower than sustained wind (sensor error).

name: weather_gust_lt_wind
description: Wind gust reported lower than sustained wind (sensor error)
type: expression
flag: fault_gust_lt_wind

inputs:
  Wind_Gust_Speed_Sensor:
    brick: Wind_Gust_Speed_Sensor
    column: gust_mph
  Wind_Speed_Sensor:
    brick: Wind_Speed_Sensor
    column: wind_mph

expression: |
  Wind_Gust_Speed_Sensor.notna() & Wind_Speed_Sensor.notna() & (Wind_Gust_Speed_Sensor < Wind_Speed_Sensor)

Next: Flat Line Sensor Tutorial