Quick answer
A power vent water heater that runs but does not ignite usually has a blower that works and a pressure switch that will not close, because the intake or exhaust is blocked, iced or nested at the sidewall, or the switch tubing is cracked. Next most likely are a failed hot-surface igniter or flame sensor. The status light on the gas control gives the fault code.
When a power vent water heater runs but does not ignite, the blower on top is doing its part and the sequence is stopping at a later step. Knowing the order tells you where: the thermostat calls, the blower starts, a pressure switch proves the vent is moving air, the igniter heats, the gas valve opens, and a sensor confirms flame. Blower noise followed by silence means the control never got past the pressure switch or the igniter.
Blower runs, no ignition: the pressure switch is not proving draft
The pressure switch is a small round diaphragm switch connected to the blower housing by a thin tube. It closes only when the blower is moving enough air through the vent, and the control will not power the igniter until it does. Anything that cuts airflow, or stops the switch from feeling it, holds the heater at this step with the blower running and nothing else happening.
- Blocked exhaust or intake: lint, a bird, leaves or a wasp nest in the pipe or at the termination hood.
- Ice at the sidewall termination: exhaust moisture freezes on the hood in a cold snap and closes the opening from the edges in.
- Snow: a termination set low on the wall gets buried by drifts or by shovelling.
- Cracked, split or disconnected switch tubing, or water sitting in the tube.
- A sagging horizontal vent run holding a pool of condensate.
- A vent that is too long or has too many elbows for the blower, which shows up on the coldest and windiest days first.
- A failed pressure switch or a weak blower, once everything else on this list is ruled out.
What to check yourself on a power-vent tank
- Go outside and look at the termination. Clear snow, ice and debris by hand; do not chip at plastic pipe with a tool.
- Check the plug and the outlet. A tripped GFCI or breaker gives a heater with no blower at all, which is a different problem.
- Look at the thin tube between the switch and the blower without pulling on it. It should be attached at both ends and dry.
- Unplug the heater for one minute and plug it back in. Watch one full attempt and note where it stops.
- Read the status light on the gas control and write down the flash count.
Stop there. Never bypass or jumper the pressure switch to get hot water: it is the only thing confirming that exhaust is leaving the house, and a heater that fires into a blocked vent puts carbon monoxide into the basement. If you smell gas or a carbon monoxide alarm sounds, leave and call from outside.
A failed hot-surface igniter or flame sensor
If the pressure switch closes, the next parts in line are the igniter and the flame sensor. Many power-vent tanks light the burner with a hot-surface igniter, a brittle element that glows orange behind the sight glass for several seconds before the gas valve clicks open.
- No glow, no click: a cracked igniter, or no power reaching it.
- Glow, click, flame for a few seconds, then out: the flame sensor is dirty or failed and the control is not seeing the flame it made.
- Glow, no click: the gas valve side of the control, or the gas is off at the appliance shut-off.
The status LED on the gas control narrows it down. Count the flashes and match them to the chart on the heater's label, which is specific to the control on your model; charts differ enough between makes that a list from another tank can mislead.
System 636 venting and vent length limits
Plastic venting on gas appliances in Ontario has to be certified to ULC S636, usually called System 636: PVC for lower exhaust temperatures and CPVC where the manufacturer calls for it, joined with the matching primer and cement. Older tanks vented in ABS or plumbing-grade PVC predate the requirement, and that pipe is replaced when the heater is.
Every power-vent model also has a maximum vent length in its manual, set by pipe diameter, with each elbow counted as extra length. A run at the limit works when new and starts tripping the pressure switch as the blower ages, so a nuisance fault on an older tank is often a venting problem and not a parts problem. Power-vent tanks are common in 2000s and newer builds that have no chimney, where the vent leaves through the side wall.
Who can repair it, and what it costs
Clearing a termination is a homeowner job. Everything on the gas train (igniter, flame sensor, gas valve, pressure switch, burner) and any change to the vent is work for a TSSA-licensed gas technician, who also checks combustion and draft once the heater is running again.
- Diagnostic visit: $90–$150, credited toward the repair.
- Most water heater repairs: $150–$800. A blower assembly or control on a power-vent tank can sit at the upper end or beyond, and is quoted before the work.
- Replacement, when a blower or control fails on a tank past 10 years: quoted in writing, with the vent brought up to System 636 where needed.
ZK Mechanical installs and repairs power vent water heaters from its Burlington shop, with a 24/7 emergency line and no after-hours surcharge. See power vent water heater repair and venting repair, compare vent types in power vent versus conventional vent, or call (647) 801-1252 or book a visit.