Pool Heater Not Heating? Causes & How to Fix It

A pool heater that won't fire — or runs but never warms the water — almost always comes down to a handful of causes. If it's the first cold night of the season, start with why Houston heaters fail after five idle months. Here's how to tell them apart, what you can safely check yourself, and when it's time for a pro. A practical guide from the Redzone Pools team in Greater Houston.

First, which problem do you actually have?

Heater trouble splits into two very different situations, and knowing which one you're in narrows the cause fast:

It won't turn on or fire at all — no ignition, no rumble, maybe an error code. Usually a flow, gas, ignition or control issue.

It runs but the water stays cold — the unit fires but you never feel the heat. Usually low flow, a scaled heat exchanger, or a thermostat/sensor problem.

And before anything else, check the one thing that trips more heaters than any other: water flow. A heater needs strong, steady flow to fire — a dirty filter or a weak pump will keep it from ever starting, or shut it down after a minute.

Five things to check before you call

These are all safe for a homeowner — no gas or electrical panels involved:

Is the pump running and the filter clean? Low flow is the #1 cause. If your filter is dirty or the pump is weak or noisy, the heater may never fire. Clean or backwash the filter and try again.

Is the thermostat set above the current water temp, and in the right mode (pool vs. spa)? It sounds obvious, but it's a common one.

Is the gas actually on? For gas heaters, check that other gas appliances (stove, water heater) work. If the whole house is out of gas, that's the problem.

Is the breaker on? For heat pumps and for the heater's controls, a tripped breaker stops everything.

Are the bypass/valves set correctly, and is there an error code on the display? Write the code down — it points straight at the cause and saves diagnosis time. On a Pentair MasterTemp or Max-E-Therm, our MasterTemp error code guide decodes every one of them.

If all of that checks out and the heater still won't heat, the cause is inside the unit — and that's where it gets technical.

What's actually failing inside

When the basics are fine, here's what we most often find on Houston-area heaters:

Scaled heat exchanger — our hard water leaves calcium inside the exchanger, choking flow and heat. One of the top reasons a heater "runs but won't warm up," and it fails units early.

Dirty flame sensor or bad igniter — the heater tries to light but can't confirm a flame, so it locks out. Classic "won't ignite."

Failing pressure switch or high-limit — safety switches that shut the heater down on low flow or overheating; often the real story behind short-cycling.

Faulty thermostat or temperature sensor — the heater misreads the water temp and won't call for heat.

Gas valve or control board — the electronics that run the whole sequence; when they fail, nothing fires.

Cracked or corroded heat exchanger — if it's leaking water, the exchanger is usually done, and that's a repair-vs-replace conversation.

Anything involving gas, electrical or refrigerant is not a DIY job — it's a safety and code issue. That's where our pool heater repair service comes in: we read the code, test the whole system, and fix the real cause on gas heaters and heat pumps of every major brand.

Past the basics? Let us take it from here

If flow is good, the settings are right, and it still won't heat — or you're seeing an error code you can't clear, a leak, or a burner that won't light — it's time for a technician. Heater internals mean gas lines, high voltage and sealed refrigerant systems, none of which are worth the risk to poke at yourself.

Redzone Pools diagnoses and repairs gas heaters and heat pumps from Pentair, Raypak, Jandy, Hayward and Sta-Rite. We're family-owned, licensed and insured, and fully mobile — we come to you, find the real cause fast, and give you a straight answer on repair versus replacement.

Get a Free Quote

Heater FAQ

Common questions

Why does my pool heater turn on but not heat the water?

Usually low water flow (a dirty filter or weak pump), a heat exchanger scaled up with calcium, or a faulty thermostat or sensor. Because several of these look identical from the deck, the whole system needs testing to find the real one.

Why won't my pool heater turn on or ignite?

Common causes are no gas or a tripped breaker, low flow shutting it down on the pressure switch, a dirty flame sensor or bad igniter, or a failed control board. Check the gas, breaker and filter first; the rest is an internal repair.

Why does my heater shut off after a few minutes?

Short-cycling like that is almost always a flow or safety-switch issue — a clogged filter, a weak pump, a bad pressure switch, or the high-limit tripping on a scaled heat exchanger. We trace it to the root so it doesn't come right back.

Can I fix a pool heater myself?

You can safely check flow, filter, settings, gas supply and the breaker, and note any error code. But anything inside — gas valves, igniters, control boards, heat exchangers, refrigerant on heat pumps — is a licensed, code-regulated repair for safety reasons.

Is it worth repairing my pool heater or should I replace it?

If the unit is sound and the failed part is straightforward — igniter, sensor, pressure switch, control board — a repair is usually the better value. If the heat exchanger is corroded through or the heater is well past its life, replacement makes more sense. See our heater repair page for how we weigh it.