This small switch is what confirms enough air is actually moving through the vent before the gas valve is ever allowed to open.
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Quick answer
A power-vent or direct-vent gas water heater's pressure switch senses the pressure difference created by the blower motor and confirms adequate venting airflow before allowing the gas valve to open, acting as a safety interlock against firing the burner with inadequate exhaust removal. It fails mainly through a fouled sensing tube that gives an inaccurate reading, a worn diaphragm inside the switch itself, or corroded electrical contacts. Because this switch exists to prevent unsafe combustion, most failures show up as the unit refusing to fire rather than continuing to run in a degraded state.
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On a power-vent or direct-vent water heater, the blower motor creates a measurable pressure difference in the venting system whenever it runs. The pressure switch is connected to this airflow path by a small sensing tube and is designed to close an electrical contact only once that pressure reaches a safe, expected level, confirming the blower is actually moving enough air before the gas valve is permitted to open.
This is a deliberately simple, mechanical safety interlock. If the blower is weak, the vent is restricted, or the sensing tube itself is blocked, the switch will not close its contact and the unit will not fire, by design.
The small rubber sensing tube connecting the switch to the venting system can crack, disconnect, or become blocked with condensation or debris, giving the switch an inaccurate reading regardless of how well the blower itself is performing. Inside the switch, the diaphragm that responds to pressure can wear out or stiffen with age, making the switch less responsive or unable to close its contact even under normal airflow.
Electrical contacts within the switch can also corrode over time, particularly in a humid mechanical room, which can cause an intermittent fault that is harder to pin down than a clean failure.
| Symptom | Likely cause |
|---|---|
| Water heater will not ignite even though the blower runs normally | Cracked, disconnected, or blocked sensing tube giving a false reading |
| Unit fails to fire intermittently, sometimes working fine | Corroded contacts inside the switch, or a partially degraded diaphragm |
| Unit fails to fire and the blower also sounds weak | Airflow may genuinely be inadequate, which the switch is correctly detecting |
| Switch tests as open with a multimeter despite adequate airflow | Switch itself has failed and needs replacement |
A technician first confirms whether the blower itself is producing adequate airflow, since replacing the switch will not help if the real problem is a weak motor or a restricted vent. The sensing tube is inspected for cracks or blockage, and the switch is tested directly with a multimeter to confirm it opens and closes at the correct pressure threshold.
If the switch itself tests faulty with adequate airflow confirmed elsewhere, it is replaced rather than adjusted, since this is not a component with a field-serviceable calibration.
Safety note
The pressure switch is a safety interlock on a natural gas water heater's venting system and any fault should be diagnosed by a Technical Safety BC licensed gas fitter; CANRO holds gas licence LGA0043870 and boiler licence LBP0046450. If you ever smell gas near the water heater, leave the home immediately and call FortisBC at 1-800-663-9911 before doing anything else.
This often points to the pressure switch, either a blocked sensing tube giving a false reading or a switch that has failed internally despite normal blower operation.
The switch itself is typically replaced rather than repaired, though a blocked or disconnected sensing tube can sometimes be cleared or reconnected to resolve the issue.
No, the blower physically moves air, while the pressure switch is a separate safety device that confirms the blower is doing so adequately before allowing ignition.
An intermittent fault like this often points to corroded contacts inside the switch or a sensing tube with a partial blockage rather than a clean, complete failure.
No, this switch is a safety interlock and should never be bypassed; a licensed technician should diagnose and correct the actual fault.
It is not a routine maintenance item on its own, but a technician will check it as part of diagnosing any ignition failure on a power-vent or direct-vent unit.
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