Quick answer
Confirm the pump is moving water and the filter is clean, then check the setpoint and mode. Record any error code and confirm the gas valve is on. Do not open the burner or gas piping; call a TSSA-registered gas technician if the heater does not fire or keeps locking out.
What you're seeing
- The heater fan runs but the water stays at air temperature.
- The heater fires briefly and shuts down.
- An error code appears on the display.
- The outlet pipe is warm for a moment, then cools.
What usually causes it
Cause 01
Insufficient water flow
Low flow trips the pressure switch or high-limit sensor. A dirty filter, closed valve or failing pump is often the real cause.
Cause 02
Gas or ignition fault
A closed gas valve, empty propane tank, faulty igniter or flame sensor will stop the burner. Gas repairs require a licensed technician.
Cause 03
Scale or a blocked heat exchanger
Hard water scale insulates the metal and reduces heat transfer. Poor chemistry can also corrode the exchanger.
Cause 04
Control or sensor failure
A thermostat, pressure switch or temperature sensor can report a fault even when the burner is capable of running.
How to fix it, in order
Work through these steps once, with the pump running and the filter clean. Do not stack a second chemical dose before the first has circulated and you have retested.
- 01
Check flow before the heater
Confirm the pump is primed, the filter is clean and all valves are open. Weak flow is the most common reason a heater refuses to fire.
- 02
Check the setpoint and mode
Make sure the heater is set above the current water temperature and is in pool mode, not spa-only or standby.
- 03
Record the error code
Write down the code and when it appears. That information helps a technician diagnose the heater without guessing.
- 04
Confirm the gas supply is on
Check the external valve position and the tank level if you use propane. Do not open the heater cabinet or attempt ignition.
- 05
Call a licensed technician
Gas, electrical and refrigerant work is regulated. In Ontario, gas work must be done by a TSSA-registered contractor.
What not to do
- Do not operate the heater with low flow or a closed bypass; the heat exchanger can overheat.
- Do not bypass a pressure switch or high-limit safety device.
- Do not attempt to light a gas heater manually or open the gas piping.
Keeping it from coming back
- Keep the filter clean and confirm flow before starting the heater.
- Balance pH and calcium to reduce scale and corrosion in the exchanger.
- Run the heater briefly each month during the season, even if you are not swimming.
- Book an annual service and keep the error-code history.
When to stop and call a pro
Do not force it
Call immediately if you smell gas, see soot or a yellow flame, or the heater repeatedly locks out. For a heater that runs but does not heat, a licensed technician should test flow, gas pressure and the ignition sequence.
Compare local repair helpGeneral guidance for residential pools in Ontario. Chemical doses depend on pool volume and test results; follow the product label and keep gas, electrical and major shell work with licensed trades.