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Combustion airflow faults: the failure signature

Technical review pending. This guide is based on the cited manufacturer documentation; check it against the manual for your exact model and firmware before acting.

The burner control proves combustion air with a pressure switch or sensor. An airflow trip means that proof dropped out, or appeared when it should not. The useful question is whether the air really fell, or the switch saw it wrong: trending air pressure, fan speed and damper position next to the switch state answers it.

Illustrative pattern · simulated, not plant data

Firing rateCombustion air pressureCombustion air proving switchAir not proven
Low-margin variant: as the burner modulates down, air pressure falls to the switch setpoint and the proving signal drops out.
Diagnose this with AI

Upload a trend export (CSV or Excel) or a screenshot of the trend around the event. InduQT lines up which signal moved first and compares it with these variants.

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Where it shows up

Signals to trend

SignalWhat it does in this failure
Combustion air proving switchDrops to 0 at the trip. Chatter (rapid 1/0) with steady air pressure points to the switch or its sensing line.
Combustion air pressureFalls toward the switch setpoint before a genuine loss. A small margin at low fire is the most common cause of intermittent trips.
Combustion fan speedA drop in speed or a VSD current limit just before the trip points to the fan drive.
Air damper positionTrips that only happen near minimum damper position suggest the switch margin at low fire.
Flue gas oxygenFalls as air is lost; steady O2 during a trip suggests the air did not actually fall.
Firing rateShows where in the firing range the trip happened.

Variants: what moves first, and what it points to

1Low switch margin at low fire

Points to it
Trips when the burner modulates down; air pressure at low fire sits just above the switch setpoint.
Check
Measure the differential at the switch across the firing range and compare with its setpoint and the burner maker’s settings.

2Restricted air path

Points to it
Air pressure at the same damper position falls slowly over weeks; O2 at low fire trends down.
Check
Inspect the intake, fan wheel, inlet damper and flue for restrictions.

3Fan drive problem

Points to it
Fan speed or current changes just before the trip; VSD fault or current-limit events.
Check
Check the fan VSD fault log, ramp settings, belts and motor.

4Switch or sensing line fault

Points to it
Proving signal chatters while air pressure is steady; water in the sensing tube.
Check
Clear and re-route the sensing tube; check switch contacts and wiring.

5Air proven with the fan off

Points to it
Proving signal is 1 in standby or minutes after shutdown.
Check
Check for a welded contact or jumper, a fan that keeps running, or stack draft through the boiler.

How to read the trend

  1. Read the state of the switchWas air lost when it was needed, or seen when it should not be? The fault code or subset says which.
  2. Plot the marginTrend air pressure (or the differential at the switch) against firing rate. Note the lowest value during normal running.
  3. Check the fanLook at fan speed, current and VSD events in the same window.
  4. Compare O2If O2 did not move at the trip, the air probably did not fall: look at the switch and its tubing.

Evidence to collect

Work on fuel trains, burners, pressure vessels and electrical equipment must be done by qualified personnel following the manufacturer’s documentation and local codes. Never bypass a safety device or interlock.

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