Pool Heater Not Heating

Heaters are honest machines wrapped in safeties. Nearly every control in the chain exists to stop operation when conditions are wrong, which means a heater that refuses to run is often reporting a pool problem, classically a filter so loaded that flow through the heater dropped below its switch point. Owners who replace heater parts before reading the filter gauge pay tuition on that lesson every winter.

Symptoms

• Unit silent when heat is called for
• Error or service codes on the display
• Short runs that stop before temperature rises
• Heat far slower than the same time last year
• Ice forming on a heat pump coil in mild weather

Heat pumps add weather to the story: output falls as air cools, defrost cycles pause heating on frosty mornings, and a unit sized for October struggles in a January snap. Judging a heat pump requires comparing like with like, same air temperature, same run hours, which the heater repair page builds into its diagnosis sequence.

Heater faults reward callers who arrive with three facts: the filter pressure, the exact display message and whether the pump’s flow looks normal. Those facts let a technician separate the heater’s own failures from the pool’s refusal to feed it, often within the first minutes on site. In Southwest Florida the question is seasonal but sharp: the same cold snap that makes owners test the heater makes every heater in the county work its hardest. Gas owners should note one more witness: the meter. A heater that lights and dies in rhythm usually argues about flow or venting, while one that never attempts ignition argues about power, controls or gas supply, and that single observation halves the suspect list.

Common Causes

Cause	How likely	What a homeowner can check
Low flow from a loaded filter or valve position	Very common	Read filter pressure and confirm valves route water through the heater
Sensor or pressure switch failure	Common	Write down the exact error code before resetting anything
Coil, fan or burner condition on older units	Age-dependent	Look for debris and corrosion on the outdoor coil

Questions and Answers

Why does the heater show a flow code when the pump runs?

The switch measures flow through the heater branch, not pump motion. A loaded filter or a partly closed bypass starves that branch while returns still move.

How long should heating a pool take?

Heat pumps typically add a degree or two per hour of running in mild weather, less in cold air. Gas heaters raise temperature faster at higher fuel cost.

Is ice on the heat pump normal?

A light frost that clears in defrost is normal in cold, damp air. Ice that builds and stays signals airflow or defrost faults.

Sources

• University of Florida IFAS Extension publications