The combustion fan manages both intake air and exhaust for a sealed-combustion tankless unit, and its failures often show up as an ignition or venting fault.
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Quick answer
A tankless gas water heater's combustion fan draws fresh air into the sealed combustion chamber and pushes exhaust gases out through the venting, a job that matters even more on tankless units than on standard tank water heaters because most tankless models use sealed, direct-vent combustion rather than drawing air from the room. It fails through worn bearings, a failed motor winding, debris or lint restricting the fan wheel, or a pressure switch downstream that senses inadequate airflow and prevents ignition as a safety measure. A licensed technician checks the fan alongside the pressure switch and venting together, since a restricted vent or intake can produce identical symptoms to a genuinely failing fan.
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Most residential tankless water heaters use sealed combustion, meaning they draw outside air directly through a dedicated intake pipe rather than pulling air from inside the mechanical room, and exhaust flue gases out through a separate dedicated vent pipe, often as a single concentric pipe assembly. The combustion fan actively drives both sides of that airflow, since sealed combustion cannot rely on natural draft the way some older, simpler gas appliances can. Before ignition, the control board typically confirms the fan is producing adequate airflow, verified by a pressure switch, before allowing gas to flow at all.
This means a fan or airflow problem on a tankless unit tends to prevent ignition entirely, producing a no-hot-water complaint, rather than allowing the unit to run poorly, which is a somewhat different pattern than component failures on a standard tank water heater.
A new grinding, whining, or rattling sound from the unit, or a hot water outage with an airflow-related fault indicator, are worth reporting. Homeowners can safely check that the outdoor vent termination is not visibly blocked by snow, leaves, or debris, since this is a common and easily fixed contributor to airflow faults in Metro Vancouver's wetter, leaf-heavy fall season, but should not open the unit's combustion chamber or attempt to service the fan itself.
| Situation | Typical outcome |
|---|---|
| Vent termination blocked by debris | Clearing the vent resolves it, no parts needed |
| Dust or lint on fan wheel, motor otherwise sound | Cleaning restores normal operation |
| Worn bearings or failed motor winding | Fan assembly replacement |
| Pressure switch reading incorrectly, fan itself healthy | Pressure switch replacement rather than the fan |
Safety note
Combustion fan and venting work involves a sealed gas combustion system; only a licensed gas fitter should inspect or service it. If you ever smell gas near the boiler or water heater, leave the home immediately and call FortisBC at 1-800-663-9911 before doing anything else.
An airflow-related fault, whether from the fan, a blocked vent, or a pressure switch, is one of the more common reasons a tankless unit will not ignite at all, producing an abrupt loss of hot water.
Yes, visually checking that the outdoor vent cap is not blocked by leaves, snow, or debris is a safe homeowner check, though servicing the fan or intake system itself should be left to a technician.
A new grinding or whining noise is worth having checked soon, since it often precedes a fan failure that will eventually prevent ignition entirely.
There is no fixed universal schedule, but checking the outdoor termination each fall, when falling leaves are more common in the region, is a reasonable practical habit.
It is a moderate repair on most tankless units, generally completed in a single visit once the correct part is confirmed and on hand.
The pressure switch, not the fan itself, can sometimes be the actual cause of an airflow-related lockout, so checking both avoids replacing a part that was not actually faulty.
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