A pool heater that won’t fire is usually protecting itself from low water flow rather than failing. Check the baskets, the filter pressure, the valve positions, and the pump speed first, in that order.
Those checks come first because they are safe, and they take about ten minutes. The heater is the one piece of pool equipment where guessing can hurt you, because it involves gas, high voltage, and combustion. Under Texas licensing rules, most of the work past those checks belongs to a licensed installer.
Stop reading and act if you smell gas
If you smell gas near the equipment pad: don’t operate any switch, don’t try to light anything, leave the area, and call your gas utility from a safe distance. This isn’t a troubleshooting scenario. Nothing below applies until that is resolved by a professional.
Start here: it’s probably flow
Every pool heater has a flow-related safety interlock, usually a pressure switch, that confirms enough water is moving through the heat exchanger before the burner fires. Without it, the exchanger would heat with little or no water in it and destroy itself. So a heater that won’t fire is often correctly protecting itself from a flow problem somewhere else in the system. Check, in this order:
- Skimmer and pump baskets. Full baskets starve the pump. This is free to check and fixes a real share of no-heat calls.
- Filter pressure. A filter well past its cleaning point chokes flow. Compare against your clean baseline; if it’s 8 to 10 psi above, clean it and try again. Our filter guide covers how to establish that baseline.
- Valve positions. Someone may have left a valve turned after vacuuming or backwashing, diverting flow away from the heater.
- Pump speed. This one catches a lot of people with newer equipment. A variable-speed pump running at a low, efficient filtration speed may not move enough water to satisfy the heater’s pressure switch. Many systems are programmed to step the pump up when heating is called for; if that programming is wrong or was lost after a power outage, the heater won’t fire. Try running the pump at high speed and see if the heater lights.
How fast a healthy heater warms the pool
A real share of no-heat calls turn out to be heaters working as designed while the owner expects something the equipment wasn’t sized to do. The math settles the question in about a minute.
One BTU raises one pound of water one degree, and a gallon of water weighs 8.34 pounds. So a 15,000-gallon pool is about 125,000 pounds of water, and raising it one degree takes roughly 125,000 BTUs. A 400,000 BTU gas heater, at an assumed 84 percent thermal efficiency, delivers something like 336,000 BTUs an hour into the water, which works out to about 2.7 degrees per hour before losses. Count the losses from an uncovered surface on a breezy night and one and a half to two degrees an hour is a realistic figure.
| Equipment | Output into the water | Degrees per hour, before losses |
|---|---|---|
| 250,000 BTU gas heater | About 210,000 BTU/hr | About 1.7 |
| 400,000 BTU gas heater | About 336,000 BTU/hr | About 2.7 |
| 125,000 BTU heat pump, mild day | About 125,000 BTU/hr | About 1.0 |
| 125,000 BTU heat pump, cold morning | Lower, and falling with air temperature | Well under 1.0 |
Taking a cool pool from 68 to 88 degrees is not an afternoon job on any residential equipment. It’s most of a day on a strong gas heater and potentially several days on a heat pump. A heater climbing two degrees an hour is healthy, and no amount of service will make it faster; only a cover, which cuts the losses, will. A heater climbing a quarter of a degree an hour, or fired for an hour with no measurable change at all, has a real fault worth diagnosing.
What the symptom pattern tells you
A pool heater’s failure pattern narrows the cause, and knowing which pattern you have shortens the diagnosis, whoever does it.
| What you observe | Where it usually points |
|---|---|
| Fires, runs a few minutes, shuts off, tries again | Flow or overheat cycling. Filter, baskets, valves, or pump speed, in that order |
| Never fires at all, no error code, no sound | Power, gas supply, set point, or the heater not being in a mode that calls for heat |
| Fires and runs steadily, water barely warms | Usually correct behavior for the equipment size. Run the math above before assuming a fault |
| Heats the spa fine but not the pool | Usually valve position or automation programming, not the heater |
| Worked yesterday, dead today, after a storm or outage | Lost programming or a tripped breaker. Check the pump speed schedule before anything else |
| Soot, scorching, or an odd flame | Stop. Shut it down and call a professional |
The other safe checks
- Is it calling for heat? Set point above current water temperature, heater in the right mode, and if you have pool and spa set points, confirm you’re adjusting the one you think you are.
- Power and gas supply. Check the breaker for the heater, and check that the gas supply valve is open. If other gas appliances in the house are also misbehaving, the problem is upstream of your pool.
- Read the error code. Modern heaters display one, and it’s the most useful piece of information available. Look it up in your manual. It’s also the first thing we’ll ask for, and it can turn a diagnostic visit into a single-trip repair.
- Look at the airflow. Leaves, nesting material, or debris blocking the heater’s vents or exhaust will prevent firing. Clearing debris from around the outside of the unit is safe. Opening it up isn’t.
Where to stop
The pool heater jobs in the table below aren’t DIY, regardless of what a video suggests. Gas appliances fail dangerously when they’re worked on by people without the training and tools to test them properly.
This is a licensing line as well as a safety one. Under Texas Department of Licensing and Regulation rules, installing or replacing pool heaters and other pool-related electrical devices requires at minimum a Residential Appliance Installer License. For a new or upgraded circuit, TDLR says “only a licensed electrical contractor with a designated master electrician may offer to perform that work.” Anyone offering to do it without the right license is telling you something useful about the rest of their work.
| Symptom or task | Who should handle it |
|---|---|
| Full baskets, dirty filter, wrong valve position, pump speed too low | You, safely |
| Reading the error code, checking breaker and gas valve, clearing exterior debris | You, safely |
| Any gas smell | Gas utility, immediately |
| Ignitor, burner, gas valve, or flame sensor | Licensed professional |
| Internal wiring, control board, or high-voltage components | Licensed professional |
| Corroded or leaking heat exchanger | Licensed professional |
| Soot, scorching, or an unusual flame appearance | Shut it down, call a professional |
Why scale shows up here too
Heating water across metal surfaces encourages calcium to come out of solution. In our area, where NTMWD describes its treated water as moderately hard and evaporation concentrates that calcium further with every top-off, an unbalanced pool will scale the inside of a heat exchanger over time. Scale insulates, so the heater works harder for less result, and eventually the restriction damages the unit.
Heater scale is one of the quieter reasons to keep calcium and pH in range properly rather than approximately: the chemistry that protects your tile also protects the most expensive component on your pad. The ranges are in our water chemistry guide.
The failure runs in the other direction too, and it has a visible tell. Water held aggressive for weeks, which is a common outcome of chasing pH on a salt pool, dissolves the copper heat exchanger itself. The first thing most owners notice is a blue-green stain spreading under the returns rather than a heater fault. If that is what you are looking at, start with metal stains in pool water, because the heater is the source rather than the victim.
Gas heater or heat pump, in a North Texas winter
Gas heaters and heat pumps fail differently in a North Texas winter, because they get their heat from different places. A gas heater burns fuel, so it works however cold it is outside and heats quickly, at a higher cost per hour that suits someone who heats the pool occasionally or a spa on demand. A heat pump moves heat from the air into the water, which is more efficient but depends on there being usable heat in the outside air. As the air cools, its output falls and its run times get longer, and below a certain point it effectively stops being useful. A heat pump that “stopped working in winter” is often behaving as designed on a cold Texas morning.
In our shoulder seasons, March, April, October, and November, the water is a few degrees too cool to enjoy while the afternoons are pleasant. Those are the months when a heat pump works hardest for the least return, because it’s pulling from cool morning air. On a damp morning in the 40s a heat pump may also stop periodically to defrost its coil, which owners reasonably read as the unit cutting out. It isn’t. If you’re choosing between the two, the trade-offs are laid out in gas heater versus heat pump in North Texas.
Sizing is the other quiet culprit. Pools with an attached spa are frequently fitted with a heater chosen for the spa volume and the builder’s budget rather than for the pool, which heats the spa in half an hour and the pool over two days. No service fixes that, so it’s worth knowing before you pay anyone to look for a fault.
A cover does more for heat retention than most upgrades
Evaporation is the largest single route by which a pool loses heat, and it runs all night. If you’re heating regularly without a cover, you’re paying to warm the air above the pool, and no heater upgrade fixes that as cheaply as a cover.
If you would rather someone else sort it out
Heater repairs sit far apart on price. A flow problem is usually free and takes an afternoon. A sensor or pressure switch at the end of its life on a sound heater is a part and an hour. A corroded heat exchanger lands close enough to the price of a new heater that the age of the unit decides it for you. What pool repairs cost in DFW sets out what drives each of those, and how long equipment lasts here gives a heater a service life you can hold a replacement quote against.
Our equipment inspection covers diagnosing heater problems. We find out what is wrong and tell you the cheaper fix before the expensive one. If your heater needs gas work beyond what we should touch, we’ll say so and send you to the right trade rather than guess with a combustion appliance in your backyard.
