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Pool Heater Not Heating

Safety first

Pool heaters combine gas or high-voltage electricity with water. Never open burner compartments, adjust gas valves, bypass safety switches or work inside electrical panels. Those jobs belong to a licensed HVAC or pool technician or electrician. If you smell gas, do not operate switches; leave the area and call your gas utility or emergency number from a safe distance. Turn power off at the disconnect before cleaning around any unit, following the manual.

Homeowner checks

1. Is the pump running and is water flowing? Heaters need minimum flow and will not fire without it. Check the pump basket and skimmer baskets for debris.
2. Is the filter clean? A dirty filter cuts flow; many heaters have a pressure switch that stops the burner or compressor when flow is low. Clean or backwash as the filter maker directs.
3. Are valves set correctly? Make sure water is routed through the heater, not bypassing it, and in pool-spa systems that the right mode is selected.
4. Is the thermostat set above the water temperature? It sounds obvious, but many calls are settings issues, including timers and automation schedules.
5. Is power on? Check the breaker and disconnect; do not reset a breaker repeatedly if it keeps tripping, which calls for an electrician.
6. Is the vent or coil blocked? Leaves, nests, snow or mulch can block gas heater vents and heat pump airflow. Clear them with the power off, without opening panels.
7. What does the display say? Look up error codes in the manual; note them for the technician.

Gas heater symptoms

• Will not ignite or locks out: possible igniter, flame sensor, gas supply, pressure switch or control faults. Technician.
• Lights then shuts off: flow, high-limit, venting or flame-sensing issues. Technician.
• Yellow or sooty flame, soot at vent: combustion problem; stop using it and call a pro.
• Banging, rumbling or steam: low flow, scale or overheating. Shut it off and call a pro.
• Short runs, slow heating: low gas pressure, undersized line, scale or water chemistry.

Heat pump symptoms

• Fan runs, little heat: cool air, low refrigerant, dirty coil or low flow. Rinse the coil gently if the manual allows; refrigerant checks are for a licensed technician only.
• Ice on the coil: some frost is normal in cool, humid air; persistent ice suggests a fault or air too cold for the unit.
• Water under the unit: often condensate, normal on humid days; a leak test with the unit off can tell them apart.
• Unit will not start: power, flow switch, thermostat or control board.

Is it a heating problem or a loss problem?

Sometimes the heater works but the pool loses heat as fast as it gains. Compare the inlet and outlet water temperatures if the display or a thermometer shows them; a healthy gas heater often raises water by several degrees on a single pass, and a heat pump by a few degrees. If that rise is normal, look at losses: no cover, strong wind, cold nights, water features running. The pool cover heat loss page explains.

Was the heater too small to begin with?

If the heater was always slow, run the pool heater size calculator for your pool. An undersized heater can be healthy and still never reach target on cool nights. The heating time calculator shows what to expect for your gallons.

Water chemistry and the heater

Low pH and high sanitizer levels corrode heat exchangers; high calcium hardness and high pH leave scale, which reduces heat transfer and can cause overheating. Test the water and correct it according to chemical labels. Corrosion damage may not be reversible and can affect warranty coverage. See pool heater lifespan.

Worked example: checking the temperature rise

Figures below are illustrative examples (2026-10-29), not quotes or guarantees.

Assumptions: a gas heater with 200,000 BTU/h input at 84%, delivering about 168,000 BTU/h, with 50 US gallons per minute (3,000 gal/h) flowing. Expected rise across the heater ≈ 168,000 ÷ (3,000 × 8.34) ≈ 6.7°F. If you measure only 1 or 2°F, the heater may not be firing fully, which justifies a technician visit. At higher flow the expected rise is smaller, so note the flow rate.

When to call a professional

Call when you see error codes you cannot clear by fixing flow or settings, repeated lockouts, tripped breakers, soot, gas smell, refrigerant oil spots, cracked exchangers, leaks from the heater body or any noise that is new. Give the technician the model, age, error codes and what you have checked; it shortens the visit.

Repair or replace?

Age, repair cost and efficiency drive the decision. An older heater with a failed heat exchanger often costs enough to repair that replacement makes sense, while an igniter or sensor on a newer unit is usually worth fixing. Our installation cost guide and lifespan guide help frame that conversation.

Keeping a simple service log

A notebook or phone note with dates, water test results, error codes, filter cleanings and technician visits makes diagnosis faster and helps with warranty claims. Record water temperature at the same time each day for a few days when you suspect a problem; a steady pattern of losing ground on cold nights points to losses or sizing, while sudden drops point to a fault.

FAQ

Why does my heater turn on and off quickly?

Common causes include low flow, a dirty filter, scale or a faulty sensor; check flow first, then call a pro.

Can I reset the heater myself?

Following the manual's reset procedure is fine; repeated lockouts need a technician.

Is condensation from my heat pump a leak?

Often not. Condensate is normal in humid weather.

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