Combustion fan (blower)

If a carbon-monoxide alarm sounds, leave the building and call 911 from outside. CO is odourless and invisible, so alarms, not senses, are the warning — treat an alarm as the emergency Technical Safety BC and FortisBC describe: get everyone, including pets, out of the building first, then call 911 from outside. Do not go back in to open windows, turn things off, or investigate until the building has been checked.

The combustion fan exists partly to keep flue gases from ever reaching the living space, and a fan fault is one of the failures a boiler detects and locks out against. The homeowner's part is to respond to alarms, keep vent terminals clear, and leave fan work to licensed gas contractors.

Modern condensing boilers are sealed appliances: combustion air comes in through a pipe, and flue gases are pushed out through the vent by a fan rather than drifting up a chimney. The combustion fan does three jobs at once. Before ignition it purges the combustion chamber — IBC's SL G3 sequence runs a 15-second fan pre-purge before the ignition trial — so no gas can accumulate; during firing it delivers the exact airflow the burner needs, with the control adjusting speed; and after shutdown it purges again. Fan performance is monitored continuously, which is how the boiler knows the vent is clear and the flame is stable.

On IBC's SL G3 the fan is an electrically commutated unit driven by the SIM+ control, which the manual describes as providing electronic commutating power to the fan windings to determine fan speed and power usage. The control's error table makes the safety logic visible: a low-RPM or air-flow error is raised when the fan is below 1150 RPM at the end of pre-purge, or below 100 RPM during heating, with blocked venting and fan wiring as the first checks, and a separate fan-pressure threshold error catches a vent that is obstructed while the fan still spins. Sealed combustion, correct fan speed and a clear vent are what keep combustion products outside the building.

Combustion fans fail mechanically and electrically. The bearing can wear until the fan growls, the motor can burn out, debris can foul the wheel, and condensation can find its way into electrical connections. Many fan faults are actually vent faults in disguise: a blocked, frozen or undersized vent makes the fan work against restriction until the control raises an air-flow or pressure error, so the first question is always whether the vent is clear.

The symptoms are usually audible or on the display: a fan that sounds rough, a boiler that refuses to start because pre-purge never completes, an air-flow, pressure or low-RPM error on the screen, or a boiler that locks out after retrying. A boiler that is firing with a failing fan can be producing incomplete combustion, which is why fan and vent faults are treated seriously rather than reset away.

If a CO alarm sounds, the first section applies and nothing else matters. Short of that, the checks are outside and at the display. Keep the vent terminals on the outside wall clear of snow, plants and debris, since a blocked terminal is a leading cause of fan and airflow errors. Note the boiler's exact error text and whether the fan sounds different from usual — louder, rougher, or slower at start-up — and pass both on when you call.

Do not open the boiler to inspect or free the fan, and do not reset a boiler repeatedly while it is raising air-flow or combustion errors. Sealed combustion appliances must not be run with their covers off or their vent paths compromised — that work is for licensed gas contractors.

A technician diagnoses a fan fault by elimination, starting with the cheapest cause: the vent. They verify the terminal and the full vent run are clear and within the manufacturer's length and configuration limits, then check the fan's behaviour against the control's expectations — the SL G3's error guidance directs checking for a blocked vent and the fan wiring before any part is replaced. Electrical checks cover the fan's connections and the control's drive signal, and a rough or failed fan is replaced as an assembly.

Afterwards the technician verifies the ignition sequence, measures combustion with an analyser, and confirms the boiler holds its flame across cycles. If your boiler shows an air-flow, fan or pressure error, or the fan sounds wrong, call ROMA Heating & Cooling — most fan faults are resolved with a clear vent, new wiring, or a fan assembly, and confirmed with a combustion test before the job is done.