When a pool heater won’t fire, the safe homeowner checks come down to four things: power to the unit, gas supply, adequate water flow, and the thermostat set correctly above the water temperature. A heater needs all four before it will even attempt to ignite, and a tripped breaker, a closed gas valve, a dirty filter starving the flow, or a thermostat set too low will each keep it cold. Work through these below. But the burner, ignitor, gas valve, and pressure switch are where DIY stops — combustion and gas components are strictly a licensed pro’s job.
What you'll need
- A garden hose
- A flashlight
- A pool thermometer
Recommended parts & supplies
- Filter cleaner (for a dirty filter starving flow) — low flow from a dirty filter stops a heater firing
- Pool cartridge filter element — a clogged, worn cartridge restricts the flow a heater needs
- Pool thermometer — confirm the real water temp against the thermostat setting
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Step by step
- 1
Confirm the heater has power and gas
Check that the heater’s breaker hasn’t tripped and its power switch is on — most gas heaters still need electricity for the control board and ignitor. Then confirm the gas is on: check that the gas shutoff valve at the heater is open, and that other gas appliances in the house (stove, water heater) are working, which tells you gas is reaching the property. On propane, check the tank level.
- 2
Set the thermostat above the current water temp
A heater only fires when it’s being asked to. Make sure the mode is set to Pool (or Spa) heat and turn the thermostat up several degrees above the actual water temperature — drop a thermometer in the pool to confirm the real number. If the set point is at or below the current temp, the heater is satisfied and won’t light. Also make sure it isn’t in a "spa" mode expecting a valve that isn’t turned.
- 3
Restore proper water flow
Heaters have a pressure/flow switch that blocks ignition until enough water is moving through — a safety feature so it never fires dry. The most common cause of a "won’t fire" heater is low flow from a dirty filter. Check your filter pressure gauge; if it’s well above the clean starting pressure, clean or backwash the filter. Also confirm the pump is running strongly, the baskets are clear, and all valves to the heater are open.
- 4
Clear the area around the heater
Look at the heater’s exhaust and intake. Wasp nests, leaves, spider webs, or a rodent nest inside the unit can block airflow or the flue and stop ignition on a safety lockout. Clear any obvious debris and nests from around and inside the vent openings (power off). Houston’s mud daubers love the warm interior of a pool heater.
- 5
Cycle the power and read the error code
Turn the heater off at the switch, wait a minute, and turn it back on to reset a soft lockout. Many modern heaters flash a diagnostic code on the display — note it and check your manual. Codes for low flow, ignition lockout, or high limit each point somewhere specific and tell you (and any pro you call) exactly where to look.
When to call a pro
Call a licensed pro for anything past power, flow, gas-on, and the thermostat. The burner, ignitor, flame sensor, gas valve, pressure switch, and heat exchanger all involve combustion or gas, and a mistake there risks a gas leak, carbon-monoxide exposure, or fire. Gas heater repair legally and practically belongs to a qualified technician. The same goes for heat pumps: the refrigerant side of a pool heat pump requires an EPA-certified tech, and topping off or opening a sealed refrigerant system is not a DIY job. If your heater keeps locking out on an ignition or flame code, stop resetting it and call.
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Pool Heater Won’t Fire — FAQ
Why won’t my pool heater turn on?
Why does my pool heater turn on then shut off?
Can I fix a pool heater myself?
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