Why Is My Pool Heater Not Working?

Why is my pool heater not turning on?

Work through five things in this order: water flow, power or gas supply, the thermostat setting, ignition or temperature, then the safety switches. Flow comes first because a heater physically refuses to fire when water is not moving through it fast enough, and a weak pump or a dirty filter is the single most common reason a heater sits dead. Only after flow is confirmed does it make sense to look at whether the heater has fuel and power, whether it is actually being told to heat, and whether a safety has latched off.

The checks split by heater type once you get past flow. A gas heater (natural gas or propane) has a burner, a gas valve, and an igniter, so its failures cluster around fuel and ignition. A heat pump has no flame at all; it pulls warmth out of the air with a refrigerant loop, so its failures cluster around air temperature, airflow, and the refrigerant charge. Knowing which one you have tells you which half of this guide applies. If you are not sure, the comparison in gas vs heat pump pool heater lays out how each works.

Before anything else, rule out the boring causes. Confirm the pump is running and primed, the heater's own power switch or breaker is on, and the thermostat is set above the current water temperature. A heater set to 82°F will not run on a day the water is already 84°F, and more than one "dead" heater is simply switched off at a panel nobody checked. For the basics of how heating works and what each type costs to run, see how to heat a pool.

How do I check water flow and the filter first?

Start at the filter, because low water flow trips a heater's pressure switch and locks out firing, and a dirty filter is the usual reason flow drops. Look at the filter pressure gauge: if it reads well above its clean baseline, the filter is clogged and is choking the whole system, the heater included. Clean or backwash it and see if the heater comes to life. The step by step for each filter type is in how to clean a pool filter. This one check fixes a large share of heaters that "stopped working" with no warning.

If the filter is clean, chase the rest of the flow path. Empty the pump basket and check the impeller for debris, make sure every valve between the pump and heater is fully open, confirm the water level is high enough that the skimmer is not sucking air, and bleed any air out of the filter. A pump that will not hold prime moves no water and no water means no heat, so if the pump itself is struggling, fix that first using how to prime a pool pump. A variable-speed pump set too low can also starve a heater; many heaters need a minimum flow, so run the pump on a higher speed when the heater is on.

One slower cause hides inside the heater. Water that runs chronically out of balance builds calcium scale inside the heat exchanger, which narrows the passages and restricts flow the same way a dirty filter does, and it corrodes the metal from the inside on the acidic end. If the heater short-cycles or trips on flow even with a clean filter and open valves, scale in the exchanger is a prime suspect. Keeping the water balanced is the fix and the prevention; check your balance with the pool saturation index calculator so the water is neither scaling nor corrosive.

Why does my gas pool heater run but not fire, or shut off?

On a gas heater, no flame almost always means a fuel problem or an ignition problem, and the heater usually tells you which with an error code like IF (ignition failure) or a lockout light. Check fuel first: a propane tank can be empty or its valve closed, and a natural-gas line can be shut off at a valve near the pad or starved if the line is undersized for the heater's demand. If other gas appliances in the house are fine but the pool heater starves, the line or regulator feeding the pad is the likely weak link.

If fuel is there but it will not light, the ignition system is next. Modern heaters use a hot-surface igniter or a spark to light the burner, and a dirty flame sensor, a failed igniter, or a gas valve that will not open all end the same way: the heater tries, fails to confirm a flame, and locks out. A classic seasonal cause is spiders or mud daubers nesting in the burner tubes or the orifices over the off-season, which blocks gas and causes ignition failure or a rough, sooty flame. Clearing the burner tray is a common first-fire-of-the-year fix.

Gas work has a hard safety line. You can clean or backwash a filter and open a valve yourself, but if you smell gas, shut the heater and the gas supply off and call a pro or the gas utility, and leave the gas valve, burners, and internal wiring to a licensed technician. A gas heater that ignites with a delay and lights with a small boom (delayed ignition) is unsafe to keep cycling. Error-code meanings vary by brand, so match the code on the display to your model's manual (Pentair, Hayward, and Raypak each use their own set) rather than guessing.

Why is my heat pump running but blowing no heat?

A heat pump whose fan spins while the water stays cold is usually fighting one of four things: air that is too cold, a dirty or blocked coil, low refrigerant, or simple patience. Heat pumps pull warmth from the surrounding air, so their output falls as the air cools and many models stop heating entirely once the air drops below roughly 50°F. On a chilly morning a heat pump that worked fine in July can read as doing nothing, and that is the design, not a failure. Run it during the warm part of the day in shoulder season.

If the air is warm enough and it still will not heat, look at airflow and the coil. The large finned evaporator coil needs to breathe, so leaves, grass clippings, or a cover crammed against the unit starve it, and a coil caked with dirt or fuzz insulates it. Clear a few feet of space around the cabinet and gently rinse the coil. A reversing valve stuck in the wrong mode, a fan that will not turn, or a low refrigerant charge are the deeper causes, and refrigerant is not a DIY fix: it takes an EPA-certified tech, and a low charge means a leak that needs finding, not just topping off.

Manage your expectations on speed, too. A heat pump heats slowly and steadily, often only 1 to 2°F per hour, so a pool that feels cold after an hour may just need more time rather than a repair. A solar cover makes a large difference here by holding the heat the pump adds overnight instead of letting it radiate away, and running a heat pump without a cover wastes much of its output. If you are weighing a heat pump against free solar heat, solar vs heat pump pool heater covers the trade.

How do I reset a pool heater and clear an error code?

A reset clears a latched fault and is worth trying once the obvious causes are handled. Turn the thermostat all the way down, cut power to the heater at its breaker for a full minute, then restore power and set the thermostat back up. That reboots the control board and clears a warning that a passing condition latched. Some gas heaters also have a manual-reset button on a high-limit switch that pops after an overheat; press it once the heater has cooled, and look up the specific code on your display in the model's manual.

The key rule: a heater that keeps tripping the same safety switch is protecting itself, not malfunctioning. A high-limit switch trips when water gets too hot because it is not flowing, and a pressure or flow switch trips when water is not moving, so a repeat lockout points straight back to a flow problem (dirty filter, low pump speed, closed valve, scaled exchanger), not to a bad switch. Replacing the switch without fixing the flow just moves the failure down the line and can let the heater overheat.

Do not sit there resetting a heater over and over. Two or three resets that do not hold tell you the fault is real, and continually power-cycling a gas heater through failed ignition attempts is exactly the behavior that makes delayed ignition dangerous. If the unit locks out again after one clean reset and a flow check, write down the error code and the symptom, and that is the point to call a technician with real information in hand.

Should I repair or replace my pool heater?

Weigh the repair against the heater's age and what a new one costs. Gas heaters last roughly 5 to 10 years and heat pumps roughly 10 to 20 years, with water chemistry the biggest factor either way. A minor repair (a flow switch, an igniter, a flame sensor) on a heater that is only a few years old is almost always worth doing. A major repair, like a corroded heat exchanger or a failed control board on a gas heater near the end of its life, often is not, because you are putting a big bill into a unit about to fail somewhere else.

Rough costs frame the decision. A service call runs about $100 to $200, common parts repairs land around $200 to $600, and a major repair such as a heat exchanger can run $500 to $1,500 or more. A new gas heater installed is commonly $2,000 to $4,000 and a heat pump $3,000 to $6,000, so when a repair approaches half the price of a new unit on a heater past its expected life, replacement is the better money. The running-cost side of that choice is in how much does it cost to run a pool, and picking a replacement is covered in the best pool heaters.

Prevention is cheaper than either. The thing that kills heaters early is unbalanced water: low pH eats the heat exchanger from the inside, and high calcium and saturation scale it shut. Keep the water balanced, clean the filter on schedule so flow stays up, and clear debris away from a heat pump's coil, and a heater reaches the long end of its life instead of the short end. Run a balance check with the pool saturation index calculator and fix any drift before it reaches the metal.

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