Pool heater guide: gas, electric, heat pumps, and solar

A pool heater is one of three types — gas, electric heat pump, or solar — and the right one depends entirely on your climate and how you use the pool. This guide compares the three, helps you find your exact heater model, and decodes the error codes that send most owners searching at the worst possible time.

Which heater type fits your situation?

Short version: gas heats fast but costs the most to run — it's the only real choice for an on-demand spa. Heat pumps cost almost nothing to run but heat slowly and quit when the air gets cold. Solar is free to run but only works when the sun's out.

The most-searched heater problems (and what they mean)

Pentair "ERR SFS" (stack flue sensor)

The single most-searched heater fault. Exhaust temp went over 480°F because the heat exchanger is scaled with calcium, which insulates it so the water can't absorb the heat. Fix is an acid descale of the heat exchanger — or replacing the sensor if chronic overheating shorted it.

Pentair "ERR AGS" (automatic gas shutoff)

Water leaving the heat exchanger went over 140°F — usually the same scaling/low-flow cause as SFS. Cool the unit, then power-cycle at the breaker to reset.

Hayward "IF" / Raypak "E-06" (ignition failure / lockout)

The heater tried to light and the flame sensor didn't confirm a flame within ~7 seconds, so it shut the gas off for safety. Usual cause: carbon buildup insulating the flame sensor rod, or spider webs blocking the pilot tube. Clean the sensor rod with steel wool and check the system ground — flame detection needs a good ground to work.

"ERR PS" / "FLO" (pressure switch / low flow) — the variable-speed trap

The heater won't fire because it doesn't sense enough water flow to avoid boiling. This is the modern variable-speed-pump paradox: your pump is running slow to save power, but a big gas heater needs ~30–40 GPM to fire. Fix: clean the filter and program a dedicated higher-RPM schedule on the pump whenever the heater runs.

Heat pump "LP" (low refrigerant pressure)

Suction-side pressure dropped below ~80 PSI. If it's cold out (below 40°F) that's a normal protective lockout — there's not enough heat in the air. If it's warm and the code sticks, you've got a refrigerant leak and need an EPA-certified tech.

Heat pump "HIGH REFRIG" / "HI/HP"

High discharge pressure — almost always a water-flow problem, not a refrigerant one. Restricted flow (dirty filter) stops the water from cooling the refrigerant, so head pressure spikes. Clean the filter and check flow.

Heater leaking from the bottom

Often misdiagnosed. High-efficiency gas heaters and heat pumps make gallons of condensation — that's normal and harmless. Test it: condensation has no chlorine or salt; pool water does. If it's pool water, a heat-exchanger tube has corroded through from bad chemistry, which is an expensive replacement.

Find your heater: full model reference (2016–2026)

Why your heater quit after you got a new variable-speed pump

This catches a lot of owners. Variable-speed pumps run slow to save electricity, but gas heaters need a minimum flow rate to fire safely — typically 30–40 GPM on a 400k BTU unit. Run the pump too slow and the heater throws a pressure-switch / low-flow code and won't light. The fix isn't a broken heater — it's programming a dedicated higher-RPM schedule on the pump for whenever the heater runs, and keeping the filter clean.