Most “my heater is broken” calls resolve in the first 15 minutes of the visit. The trick is having a repeatable diagnostic sequence. This is the one we work through on every no-heat call before we start pulling parts.
The universal first five checks
- Is the pump running and delivering flow?Every heater has a pressure or flow switch that refuses to fire without circulation. The most common “heater fault” is actually a dirty filter, a closed valve, or a skimmer basket full of leaves.
- Is the thermostat set above pool temp? A laughable number of calls end here. Guests dial it down; shared controls get bumped; the setting resets after a power blip on some models.
- Is the bypass open on solar or a heat-recovery loop? If flow is being diverted away from the heater you get cold water and a confused heater.
- Fault code on the display? Read it before you touch anything. Manufacturers publish code tables; most techs have them bookmarked on their phone.
- Fireman switch or remote enable?If the heater is time-clocked or controller-driven and the schedule isn't calling for heat, the heater won't fire no matter how hot the thermostat wants it.
Gas heater: specific failures by symptom
| Symptom | Likely cause | First check |
|---|
| Clicks, no ignition | No gas, pilot out, blown igniter | Confirm gas valve open; inspect igniter visually for cracks. |
| Lights, runs briefly, faults out | Pressure switch, flame sensor, air switch | Check flame sensor cleanliness; measure switch continuity. |
| Sooty flame or yellow color | Dirty burners, wasp nest, low gas pressure | Vacuum burner tray; test inlet pressure. |
| Loud rumble or boom on ignition | Delayed ignition from dirty burners or weak spark | Shut down and service immediately — safety risk. |
| Runs but heats slowly | Scaled heat exchanger, undersized gas line | Inlet gas-pressure test; schedule descale. |
Heat pump: specific failures by symptom
| Symptom | Likely cause | First check |
|---|
| Fan runs, no heat | Low refrigerant, bad reversing valve, compressor off | Listen for compressor; amp-clamp the compressor leads. |
| Nothing runs | Blown capacitor, tripped breaker, bad contactor | Check breaker; test capacitor with meter. |
| Ice on the coil | Low ambient, low refrigerant, stuck reversing valve | If ambient <50°F this may be normal defrost cycle. |
| Water puddle under unit | Normal condensate or heat-exchanger leak | Sniff for chlorine; if pool-smelling, the exchanger has a leak. |
| Loud compressor rattle | Worn bearings or grounding-out | Shut down; refrigerant work is licensed-tech territory. |
When to stop troubleshooting and call a specialist
- Gas leak. Evacuate, shut off the gas at the meter, call the utility. Never diagnose a gas leak with a flame or an energized ignition source.
- Refrigerant work. Heat-pump refrigerant is a licensed-technician task (EPA Section 608). Beyond a capacitor or contactor swap, call it.
- Cracked heat exchanger. Pool water getting into the combustion side of a gas heater eats the exchanger fast. Diagnose by pressure test; replace, not repair.
Ninety percent of heater calls are dirty filters, tripped breakers, and bumped thermostats. Work the simple list first. The hard problems are real, but they're rare.