Quick answer
A pool heater that runs but doesn’t heat is almost always a flow, airflow, or fuel-supply problem, not a dead heater — a dirty filter or closed valve restricting water flow is the single most common cause pool-service technicians report, followed by low gas pressure on gas units and iced or blocked coils on heat pumps. Work through checks in order of likelihood before assuming the heat exchanger or compressor has failed: filter and flow first, then gas supply or airflow, then the thermostat sensor, and only then the heater’s internal components. Angi’s 2026 data puts the average professional diagnostic visit at $75-$200, often credited toward the repair if you go ahead with it.
Work through these 8 causes in order
| # | Cause | Applies to | How to check |
|---|---|---|---|
| 1 | Dirty filter / low water flow | All types | Check filter pressure gauge; clean if 8-10 PSI above clean baseline |
| 2 | Closed or bypassed valve | All types | Confirm valves feeding the heater are fully open |
| 3 | Low gas pressure / supply issue | Gas | Check gas meter isn't restricted; confirm propane tank isn't near empty |
| 4 | Restricted or iced coil / airflow | Heat pump | Look for clogged fins, debris, or ice that doesn't clear after defrost |
| 5 | Failed or drifted thermostat sensor | All types | Compare displayed temp to an independent thermometer; >3°F gap signals a bad sensor |
| 6 | Tripped high-limit switch | Gas, heat pump | Look for a reset button; repeated trips point to cause #1 or #2 |
| 7 | Heater undersized for the pool | All types | Compare rated BTU output to your pool's gallons using our sizing chart |
| 8 | Failed heat exchanger or compressor | Gas / heat pump | Usually needs a technician; often the point to weigh repair vs. replace |
Checking in this order matters because the first few causes are free or nearly free to rule out, while the last two require a technician and real money. Skipping straight to “the heater must be broken” is the single biggest reason pool owners overpay for repairs they didn’t need.
1-2. Flow: the most common cause by far
Every gas heater and heat pump needs a minimum water flow rate to transfer heat safely — Hayward’s own install specs put its Universal H-Series at a 40 GPM minimum, for example. When flow drops below that threshold, the heater’s high-limit safety switch shuts off heating output as a protective measure, even though the pump and heater both keep running normally, which is exactly why this cause gets mistaken for a “broken” heater so often. Check the filter pressure gauge first: a reading 8-10 PSI above the clean baseline means it’s time to backwash or clean the cartridge. Then confirm every valve in the heater’s plumbing run is fully open, including any bypass valve that may have been left partially closed after a repair — our bypass valve guide covers how those are supposed to be set. If flow is confirmed fine and the lockout persists, the water pressure switch itself may be worn or fouled — see our pressure switch replacement picks by brand.
3-4. Fuel supply and airflow
Gas heaters need adequate gas pressure to fire and stay lit. If the burner lights but the flame is weak or yellow instead of blue, or the heater short-cycles, a restricted natural gas meter or a low propane tank are common causes worth ruling out before assuming an internal gas-valve failure — if a technician does confirm the valve itself has failed, our gas valve guide covers the OEM part and price by brand so you can check the quote. Heat pumps have the equivalent problem on the air side: the outdoor coil needs unobstructed airflow to extract heat from the surrounding air, and a coil clogged with debris, grass clippings, or leaves — or heavy ice that doesn’t clear during the unit’s normal defrost cycle — will quietly tank heating output long before the unit throws an obvious fault code.
5. A drifted thermostat sensor
If the heater’s display claims it has reached the set temperature but the water still feels cold, the heater probably isn’t the problem — the temperature sensor reading it is. Pool techs use a gap of more than about 3°F between the displayed reading and an independent thermometer as the practical threshold for a bad or drifting sensor. It’s one of the more DIY-friendly fixes on this whole list; our thermostat guide covers the swap and typical sensor-kit pricing.
Pool water thermometer (for independent verification)
- A $10-$15 floating thermometer is the fastest way to confirm whether a "not heating" complaint is a real heat-output problem or just a sensor reading wrong.
- Keep one poolside year-round — it also flags a heat pump icing issue early, since output temperature swings are usually the first visible symptom.
Warm up your pool sooner — try Amazon Prime free for 30 days and get a replacement sensor or thermometer delivered in two days instead of waiting on a special order from a pool-supply store.
6. A tripped high-limit switch
Most gas heaters and heat pumps have a visible reset button near the control panel, and cutting power at the breaker for about 60 seconds accomplishes the same thing. A one-time reset that holds is nothing to worry about. A high-limit switch that trips again within a day or two almost always traces back to causes #1-#4 above — the switch is doing its job protecting the exchanger from a flow or airflow problem upstream, not failing on its own. If flow and scale are both confirmed fine and it keeps tripping, our high-limit switch replacement picks cover the OEM part number and price by brand.
7. The heater may simply be undersized
Sometimes “not heating” really means “heating too slowly to notice,” which points to a heater that was undersized for the pool from day one, or a pool that’s grown (a spa attached later, a larger liner on a refill) beyond what the original BTU rating supports. Compare your heater’s rated output against your pool’s actual gallons using our sizing chart before assuming a repair will fix a heater that was never big enough to begin with.
8. When it’s a real component failure
If flow, fuel supply, airflow, and the thermostat all check out and the heater still won’t produce heat, the remaining likely causes — a cracked heat exchanger, a failed compressor, a bad control board (not to be confused with a dead control panel or keypad, which is a separate and cheaper part on most brands), or (on gas heaters that run the full startup sequence but never actually fire) a dead igniter — need a technician’s diagnostic equipment to confirm, though a failed igniter is cheap and DIY-friendly enough to rule out yourself first with our igniter kit picks matched to your heater’s brand. A heat exchanger that’s failed early on a gas heater is also worth checking for corrosion from chronic condensation, not just wear — see our pool heater condensation guide for the return-water-temperature causes behind it. This is also the point to weigh repair against replacement: our repair cost guide breaks down typical prices by part and explains the 50% rule pool pros use to decide when a new heater is the better value than sinking more money into an aging one.
The bottom line
Work the free checks first — filter, valves, gas supply, coil airflow — before assuming the heater itself has failed; that’s where the large majority of “not heating” complaints actually resolve. If those all check out and the problem persists, a $75-$200 diagnostic visit is a reasonable next step, and it’s often credited toward whatever repair follows. For the maintenance habits that prevent most of these causes from happening in the first place, see our maintenance guide.
Sources: Hayward — Pool Heater Installation & Owner’s Manuals, Angi — Pool Heater Repair Cost 2026, TroubleFreePool.com — Heater Troubleshooting Discussions