Air-side AHU faults
Air handlers, including the Guideline 36 conditions.
Thirty-two rules on the air side: sixteen aligned with ASHRAE Guideline 36 fault conditions FC01–FC16, and sixteen more for the problems Guideline 36 does not cover — fighting coils, missing resets, schedule overrides, frozen sensors and hunting loops. Air handlers are where most of a building’s fan and coil energy is spent, so this is the largest family.
The sixteen AHU-FC rules are aligned with the fault conditions FC01–FC16 described in ASHRAE Guideline 36. The logic is expressed in Pelorus Engineering’s own engineering terms; Guideline 36 text is not reproduced here.
How to read it. Blue is the outdoor air temperature, the dashed line is the supply-air setpoint the unit is trying to reach. The bar at the bottom shows when the cooling valve is open. In the red band the outdoor air is already colder than the setpoint, so the economizer could do the whole job, yet the coil is running.
The rules
- AHU-FC01
Duct static pressure cannot be met
The supply fan is at full speed but duct static pressure still isn't reaching its setpoint.
Capacity-basedBAS trendsGuideline 36 - AHU-FC02
Mixed-air temperature below both OAT and RAT
MAT is colder than both the outdoor air and the return air — physically impossible without a sensor or damper fault.
Diagnostic · data qualityBAS trendsGuideline 36 - AHU-FC02/03/06
Mixed-air temperature doesn't match OA/RA mix
The measured mixed-air temperature is inconsistent with what the OA damper position and the OAT/RAT temperatures predict it should be.
Diagnostic · data qualityBAS trends - AHU-FC03
Mixed-air temperature above both OAT and RAT
MAT is hotter than both the outdoor air and the return air — physically impossible without a fault.
Diagnostic · data qualityBAS trendsGuideline 36 - AHU-FC04
Operating state hunting
The AHU is flipping between heating, free cooling, and mechanical cooling many times an hour.
Capacity-basedBAS trendsGuideline 36 - AHU-FC05
Supply air not warming across active heating
Heating valve is open but SAT isn't rising above MAT by more than the fan-heat tolerance — the heating coil isn't delivering heat.
Capacity-basedBAS trendsGuideline 36 - AHU-FC06
OA fraction wrong for current operating state
The fraction of outside air being introduced doesn't match what the operating state calls for.
Diagnostic · reliabilityBAS trendsGuideline 36 - AHU-FC07
SAT low while heating valve is full open
Heating valve is saturated open but SAT still hasn't reached setpoint.
Capacity-basedBAS trendsGuideline 36 - AHU-FC08
SAT and MAT not tracking in economizer
In pure economizer mode (no mechanical heating or cooling), SAT should equal MAT plus the fan-heat rise — but it doesn't.
Capacity-basedBAS trendsGuideline 36 - AHU-FC09
OAT too high for SAT setpoint with free cooling alone
It's too hot outside to hit SAT setpoint without mechanical cooling, but the AHU is still in economizer mode.
Capacity-basedBAS trendsGuideline 36 - AHU-FC10
Mixed air not tracking outdoor air at 100% OA
With the outdoor-air damper fully open, the mixed-air temperature should equal the outdoor-air temperature — it doesn't.
Diagnostic · data qualityBAS trendsGuideline 36 - AHU-FC11
Mechanical cooling with outdoor air cold enough to do the job
The unit is at 100% outdoor air and still running the cooling coil, although the outdoor air alone is colder than the supply setpoint.
Capacity-basedBAS trendsGuideline 36 - AHU-FC12
SAT too high relative to MAT in cooling mode
When cooling is active, SAT should drop below MAT.
Capacity-basedBAS trendsGuideline 36 - AHU-FC13
Supply air too warm with the cooling valve full open
The cooling coil is doing everything it can and the supply air is still above setpoint.
Capacity-basedBAS trendsGuideline 36 - AHU-FC14
Temperature drop across inactive cooling coil
Cooling valve is commanded closed but air is still being cooled as it crosses the coil.
Capacity-basedBAS trendsGuideline 36 - AHU-FC15
Temperature rise across inactive heating coil
Heating valve is commanded closed but air is still being warmed as it crosses the coil.
Capacity-basedBAS trendsGuideline 36 - AHU-X01
Fighting coils — heating and cooling on at the same time
The heating valve and the cooling valve are both driving open simultaneously on the same AHU.
Physics-basedBAS trends - AHU-X02
Reheat clawing back overcooled air
Zone temperature dropped below setpoint and reheat then activated to recover — the AHU overcooled and is paying to heat back up.
Physics-basedBAS trends - AHU-X03
Fan power deviates from fan-curve expectation
The fan is drawing more (or less) power than the affinity laws predict for its current speed.
Capacity-basedBAS trends - AHU-X04
SAT setpoint is constant — no reset strategy
The supply-air-temp setpoint never moves across the analysis window, indicating no trim-and-reset or load-based reset is happening.
Capacity-basedBAS trends - AHU-X05
Equipment running outside its occupancy schedule
A channel is showing active output during hours the building schedule says it should be off.
Physics-basedBAS trends - AHU-X06
Control loop oscillating
A control output is oscillating quickly enough that the loop is unstable.
Capacity-basedBAS trends - AHU-X07
Sensor value frozen
A sensor's value hasn't changed beyond its noise floor for a long period when it should have.
Diagnostic · data qualityBAS trends - AHU-X08
Valve pegged open or closed
A valve has been at 100% (or 0%) for sustained periods, suggesting it can't meet load.
Capacity-basedBAS trends - AHU-X09
Supply fan riding high with no cooling demand
The supply fan is running above its minimum speed but the cooling load is near zero.
Capacity-basedBAS trends - AHU-X10
Hot-water supply not following OAT reset
The hot-water supply temperature is too high for the current outdoor temperature — the reset schedule isn't being followed.
Capacity-basedBAS trends - AHU-X11
Supply air temperature missing setpoint
Supply air temperature is drifting above or below its setpoint by more than the tolerance.
Capacity-basedBAS trends - AHU-X12
Reheat active during unoccupied hours
Hot-water reheat is running at night or on weekends when the building should be unoccupied and unconditioned.
Capacity-basedBAS trends - AHU-X13
Outside-air damper open past minimum without economizer reason
The OA damper is open well past its minimum position even though OAT is too hot or too cold for free cooling to be useful.
Capacity-basedBAS trends - AHU-X14
Mechanical cooling while free cooling was available
Outdoor air is cool enough to provide free cooling, but the AHU is at minimum OA and running mechanical cooling instead.
Physics-basedBAS trends - AHU-X15
Static-pressure setpoint never resets
The supply-duct static-pressure setpoint sits at one value whenever the unit runs — there is no trim-and-respond reset from the VAV box dampers.
Capacity-basedBAS trends - ERV-FC16
Energy recovery not recovering
The recovery wheel or core is commanded on but the supply air isn't being pre-conditioned much by the exhaust stream.
Capacity-basedBAS trendsGuideline 36
What should the economizer be saving?
Estimate the free cooling a working economizer delivers in your climate.
Enter positive numbers to see an estimate.
Cooling capacity, times the hours a year outdoor air can do the job in your climate zone, times an effectiveness factor, priced at your rate. Energy Agent counts the actual hours the coil ran when it did not need to.
Screening estimate. Energy Agent measures the real fault hours and load from your trends and prices them at your tariff.
See these rules run on your building.
Send twelve months of interval data and a BAS trend export. Energy Agent returns a ranked list of what is wasting money, what each item is worth per year at your tariff, and what to do about it.