Why Is My Pool Heater Not Working?
Short answer
The most common causes are inadequate water flow from a dirty filter or failing pump, a thermostat set below current water temperature, interrupted gas or electrical supply, and safety lockouts triggered by high limit or pressure switches. Check flow and settings first, since those account for most no-heat calls.
Most pool heaters that stop producing heat are not broken. They are locked out or starved. The usual causes are inadequate water flow from a dirty filter or a struggling pump, a thermostat set below the current water temperature, an interrupted gas or electrical supply, and safety switches doing their job.
Work through the simple checks in order before assuming the worst. A large share of no-heat service calls end at the filter.
Start with water flow
Every pool heater has a pressure or flow switch that prevents it firing without adequate water moving through it. This protects the heat exchanger, and it is the most frequent reason a heater refuses to run.
Check these in sequence:
- Is the pump running and primed, with no air in the basket lid?
- Is the filter dirty? A rise of about 8 to 10 psi over the clean starting pressure means it is time to clean or backwash.
- Are the pump and skimmer baskets clear?
- Are all valves in their normal positions? A valve left closed after service is a classic.
- Is the water level high enough that the skimmer is not drawing air?
Restore proper flow and a surprising number of heaters simply start working again.
Then check the settings
Confirm the thermostat is set above the current water temperature. A heater set to 80F sitting in 82F water is behaving correctly by doing nothing.
Confirm the unit is in heat mode rather than off, or a pool/spa selection pointing at the wrong body of water. If an automation panel or timer controls the heater, check that its schedule overlaps pump run time. A heater cannot run when the pump is off.
Supply problems
Gas heaters. Verify the gas valve at the heater is open and that other gas appliances work. If you run propane, check the tank level, because an empty tank looks exactly like a failed igniter from the deck. Failure to ignite usually locks the unit out until reset.
Heat pumps. Check the breaker. Heat pumps need unobstructed airflow, so clear vegetation, furniture or covers away from the intake and fan discharge. A heat pump wrapped in a hedge will run poorly and eventually fault. Heat pumps also work efficiently above roughly 50F ambient, so on an unusually cold morning a unit may produce little heat without being faulty.
Reading the fault codes
Modern heaters display codes rather than just failing silently. The common families are:
- Flow or pressure switch faults, which point back to the circulation checks above.
- High limit faults, meaning excessive temperature was sensed. Often low flow again, or a failed sensor.
- Ignition failure on gas units, pointing at gas supply, the igniter, or the gas valve.
- Sensor faults, where a thermistor has failed or its wiring is damaged.
Codes are not standardized across brands, so check the label or manual for your model.
When to stop and call a professional
Anything involving gas connections, combustion components, refrigerant, or opening electrical panels belongs with a licensed technician.
Also call if the heater keeps locking out after you have confirmed good flow, if you see soot, scorching or corrosion at the heat exchanger, if it trips a breaker repeatedly, or if you smell gas. Repeated high limit faults can mean a scaled heat exchanger, which gets more expensive the longer it runs.
While diagnosing, keep testing on schedule. Warmer water accelerates chlorine loss and algae growth, so chemistry should not slide during a heater problem.
Heater faults in the Florida Keys
Salt air is the local factor that changes the odds. It corrodes cabinets, contactors, control boards, fasteners and heat exchanger components, and it genuinely shortens heater lifespan here compared to inland installations. A Keys heater that fails at eight years is not necessarily a lemon.
Practical local habits: rinse the exterior and heat pump coils with fresh water periodically, keep the unit out of direct salt spray where the site allows, and inspect electrical connections for corrosion during storm season prep. Since it never freezes here, heat pumps are the common local choice, and they mostly fail from corrosion and airflow restriction rather than cold weather stress.
Check the filter pressure and the thermostat setting before anything else, because those two account for most of the heaters we get called out to look at.
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