AHU1 and AHU2 serve equivalent gallery loads on the same sequence of operations. Only one of them followed it. Finding out why took a functional performance test, not a guess.
THE SITUATION
AHU1 and AHU2 condition effectively identical gallery spaces on the same sequence of operations, so their behaviour should be close to indistinguishable. Compared side by side they correlate well on humidification and economising — and diverge completely on dehumidification.
AHU1 had not entered dehumidification since mid-November 2022, and never entered neutral mode either. It sat in humidification almost permanently while zones above 53 % relative humidity went unanswered.
No dehumidification mode since mid-November 2022.
Roughly a 20 °F oscillation around setpoint.
Every downstream VAV inherits the air handler’s instability.
AHU1 stayed in one mode No dehumidification since mid-November 2022 and no neutral mode at all, against an explicit SOO.
The setpoint itself was wrong AHU1’s dewpoint setpoint violated the SOO while AHU2, on identical rules, reset correctly.
The loops hunted, they did not track Supply dewpoint oscillating roughly 20 °F as the cooling and heating coil PI loops chased setpoint.
The equipment could not keep up Humidifier refill faults, static pressure below setpoint at full fan, and unstable fan control.
FAILURE DOMAIN 01
FAILURE MODES
Bottom line: the SOO never reached the code
The setpoint itself was wrong
Mode is only half the sequence; the dewpoint setpoint the air handler resets to is the other half. AHU1’s dewpoint setpoint was in open violation of the sequence of operations while AHU2’s followed it. In full humidification the supply dewpoint setpoint should be 63 °F, and in dehumidification 48 °F. AHU1 was reaching neither at the right time — so even a perfectly tuned coil loop would have been tracking the wrong target.
FAILURE MODES
Bottom line: right loop, wrong target
FAILURE DOMAIN 03
The loops hunted, they did not track
Supply air dewpoint on both AHU1 and AHU2 oscillated across roughly a 20 °F band. The cooling and heating coil PI loops that throttle to hold supply dewpoint at setpoint were not tracking it — they were hunting around it. This is a controls problem rather than a design problem, and the data proves the distinction: the supply dewpoint does reach 63 °F at times, so the capacity is present. By design the air handlers are the primary conditioning equipment and the VAVs only trim, which means an unstable supply dewpoint is inherited by every box downstream rather than corrected there.
FAILURE MODES
Bottom line:a controls problem, not design
FAILURE DOMAIN 04
The equipment could not keep up
The AHU1 humidifier carried a water-level refill fault and its actual humidity output collapsed the moment it faulted. The AHU3 humidifier showed the same status 3 refill fault. On the air side, AHU1 could not reach its supply static pressure setpoint even with the supply fan at 100 %, as downstream VAVs opened and the setpoint reset from 0.4 to 0.7 in. w.c. AHU2’s supply fan speed was highly unstable, its static pressure swinging between zero and setpoint as downstream dampers short-cycled. AHU1’s return fan status also did not read as continuously on.
FAILURE MODES
Bottom line: hardware faults surfaced late
THE SOLUTION
Verify the air handler against its own sequence — mode, setpoint and loop — every single day.
Dehumidification, humidification and neutral criteria are evaluated continuously against live zone relative humidity, and every mode the air handler should have entered but did not is flagged.
The dewpoint setpoint the air handler actually resets to is checked against the value the sequence calls for — so a wrong target is caught before the loop gets blamed for it.
Supply dewpoint is scored against its setpoint continuously, separating a tuning problem from a capacity problem and proving which one you actually have.
Humidifier faults, static pressure shortfalls and fan instability surface directly — and fault windows are excluded from analysis so the diagnosis is not contaminated by them.
ROOT-CAUSE ISOLATION
AHU1 would not dehumidify.
Max-zone RH was overridden to 60 % at the BAS. AHU1 entered dehumidification mode immediately and correctly.
Min-zone RH was overridden to 40 %. AHU1 entered humidification mode immediately and correctly.
With both inputs returned inside the band, AHU1 held neutral mode — neither humidifying nor dehumidifying.
Result: the mode logic was correct. The fault was in the calculated max-zone / min-zone relative humidity point feeding it — handed to the controls contractor as a specific, testable action item rather than a symptom.
WHAT CONTINUOUS COMMISSIONING CAUGHT
Five months without entering dehumidification mode, on an air handler whose twin followed the same sequence correctly. Caught by mode-criteria comparison, not by any built-in alarm.
The coil loops were hunting rather than tracking. Because the capacity was demonstrably present, the finding could be handed over as a tuning task rather than an equipment question.
The AHU1 humidifier’s refill fault was caught in the water-level data — the same day ambient relative humidity fell to an extreme and the galleries dropped with it.
Each of these was visible in the air handler data well before it was visible in the galleries.
THE RESULT
The controls contractor implemented dewpoint setpoint changes.
BEFORE
of the two gallery air handlers was resetting its dewpoint setpoint according to the sequence. AHU1 had not dehumidified.
AFTER
air handlers resetting dewpoint setpoint correctly — AHU2 holding 48 °F in dehumidification, AHU1 sitting inside the neutral-zone range.
From corrected setpoint logic to verified conformance measured in the field.
Entered correctly by AHU1 for the first time in the analysis record.
Humidifiers clear across the verification week apart from one transient event.
THE REAL VALUE
The air handlers are the primary conditioning equipment. Correcting them corrected the input every downstream box depends on, instead of trimming symptoms zone by zone.
An air handler locked in permanent humidification, and coil loops hunting across a 20 °F band, are both energy spent to produce an out-of-spec result.
A functional performance test turned "AHU1 will not dehumidify" into a named point to fix — the difference between a work order and an investigation.
THE OBJECTIVE, DELIVERED
Continuous, design-intent-based diagnostics and monitoring-based commissioning — engineer-backed, with high-resolution IoT data and an AI-powered cloud on one platform. Monthly commissioning reviews and on-demand reporting included.