Why Isn't My Pool Heating? (Heater Basics + the No-Heat Checklist)
Most 'it won't heat' problems aren't a broken heater. The #1 cause is a FLOW-SWITCH LOCKOUT: every modern heater refuses to fire until it senses enough water flow (roughly 10 GPM per 100,000 BTU), so a dirty filter, a closed valve, a clogged basket, low water, or a variable-speed pump set too slow will stop it cold — all free fixes. The #2 cause, if it's a heat pump, is that it's correctly refusing to run in cold air: heat pumps lose efficiency below about 50°F ambient and that's by design, not a fault. On picking a heater: in Florida it's almost always a HEAT PUMP (the Florida Solar Energy Center puts a typical year at about $500 to run a heat pump vs. $580 for gas and $1,450 for electric resistance). Choose GAS if you need heat fast/on-demand in any weather, SOLAR if you have a long sunny season and no screen cage. And whatever you heat with, a COVER is the single biggest money-saver — the DOE says it cuts heating cost 50–70%, because evaporation is by far the biggest heat loss.
Nine times out of ten, “my pool heater won’t heat” is one of two things: a free fix that has nothing to do with the heater, or a heat pump doing exactly what it’s supposed to do in cold air. Before you call a tech, work the checklist below. And if you’re still choosing a heater, the honest answer for most warm-climate pools is short — start there.
Which heater fits your situation
All three heat the same water; they differ in how they get the heat, which decides cost and speed.
- Heat pump — the warm-climate default. It moves heat from the outside air into the water using a little electricity (a fan pulls air over a coil; a compressor concentrates that heat and hands it to the water). Cheapest to run in mild weather. Its one weakness: it needs reasonably warm air — the DOE says heat pumps are most efficient above ~50°F and lose efficiency below the 45–50°F range. Best for: long seasons and holding a steady temperature in a mild climate.
- Gas (natural gas or propane) — fast, any weather. It burns fuel to heat the water directly, so it can hit any temperature regardless of the weather and heats the fastest. Best for: intermittently-used pools and spas, cold climates, or any time you need heat now. Highest running cost.
- Solar — near-free, but patient. Pool water is pumped through roof collectors and warmed by the sun, with a diverter valve that bypasses them when the sun isn’t giving. Best for: a long sunny season and no screen enclosure. Slow, and no heat on cold cloudy days.
The money view (Florida): the Florida Solar Energy Center puts a typical year at about $500 for a heat pump, $580 for gas, and $1,450 for electric resistance — which is why, in Florida, it’s almost always a heat pump. (For your exact pool and the specific models, see the pool heater showdown and its cost calculator — this guide is the concepts, that’s the picks.)
Why isn’t it heating? The checklist, most-likely first
1. Not enough water flow (the flow-switch lockout)
This is the #1 cause. Every modern heater has a flow (pressure) switch that refuses to let it fire until it senses enough water moving through — roughly 10 GPM per 100,000 BTU (so a 400,000 BTU heater wants ~40 GPM). And because the heater sits after the filter, a dirty filter chokes the flow and stops the heater (Trouble Free Pool’s point). Walk the flow:
- Clean or backwash the filter.
- Confirm every valve is open (a bumped valve is common).
- Empty the skimmer and pump baskets; top the water to mid-skimmer.
- Bleed air / re-prime the pump.
- Raise the pump speed — a variable-speed pump set to a low filtration speed may not make the flow the switch needs.
2. It’s a heat pump, and the air is cold (this is normal)
If it’s a heat pump and the ambient air is near or below ~45–50°F, it will run poorly, sit in defrost, or refuse to run — by design, not a fault (there’s simply little heat in cold air to harvest). Wait for warmer air, or if you routinely need heat in the cold, that’s the case for gas.
3. Thermostat, setpoint, or mode
The simplest miss: the setpoint is below the current water temp, or the unit is in the wrong mode (pool vs. spa, or a cooling-capable heat pump left in cool mode, or an economy/schedule holding it off).
4. The pump doesn’t run long enough
The heater only heats while the pump circulates. Too short a daily schedule — or a variable-speed pump only ever running low-RPM filtration — and the water never accumulates a temperature rise. It’s not broken; it never gets the runtime.
5. It’s undersized for the job
An undersized heater isn’t “broken” either — it runs constantly and never reaches setpoint, especially in wind, cold, or with a big temperature target. See sizing below.
6. Gas heaters: gas / pilot / ignition
If flow is fine and a gas heater’s burner won’t light: check for gas supply (empty propane tank, closed gas valve), and pilot/igniter or flame-sensor faults — often a fault-code lockout on the control board.
7. Solar: the controller isn’t diverting
If it’s solar and the collectors aren’t getting water: the controller is off, a sensor failed, the diverter valve is stuck, or it’s correctly bypassing because the collectors aren’t warmer than the pool (a cool or cloudy day).
8. A dirty heat-pump coil
A leaf-clogged evaporator coil starves a heat pump of the airflow it needs to harvest heat — slow heating and higher bills. Keep the coil and the area around the unit clear.
How big a heater do you need?
Heaters are sized by surface area and temperature rise, not gallons — a pool loses heat from its surface (mostly evaporation), not through its walls. The DOE formula:
Pool surface area (sq ft) × temperature rise (°F) × 12 — for about a 1–1.25°F/hour rise. Multiply by 1.5 for a 1.5°F rise, 2.0 for a 2°F rise.
A bigger heater reaches temperature in fewer hours, which for heat-then-swim use can burn less fuel per session (less time bleeding heat during a slow ramp) — but it’s not a license to buy the biggest unit, since it burns more per hour and oversizing has diminishing returns. For your exact pool plus a running-cost estimate, use the calculator on the heater showdown.
The cheapest degree you’ll ever buy: a cover
Whatever you heat with, a cover is the single biggest money-saver. Evaporation is by far the largest way a pool loses heat — the DOE notes each pound of 80°F water that evaporates carries off about 1,048 BTU — so covering the pool when it’s not in use saves 50–70% of heating cost (DOE). (It’s the same evaporation physics that a fountain uses to cool a pool, working for you instead of against you.) After the cover: set a realistic temperature (78–82°F is comfortable; every degree up costs more), and heat during the warm part of the day.
One upkeep note: your heater’s heat exchanger is the first thing bad water chemistry kills — low pH corrodes it, high pH and calcium scale it — so keep the water in balance.
The Florida version
- Heat pump is the default, and the FSEC dollar figures above are why — Florida winters mostly stay in the heat pump’s efficient (>50°F) band, so its one weakness rarely bites.
- The screen cage changes solar. A cage blocks a big share of the sun (FSEC’s collector-sizing tables jump roughly 40–50% for a screened vs. unscreened pool — the data behind the widely-cited ~30–40% clear-day heating reduction), so under a cage, solar has to be oversized substantially — another reason caged pools default to heat pumps. More on the cage factor.
- You mostly need heat in the shoulder seasons, not summer (some heat pumps even reverse to cool the pool in August — see pool coolers).
Bottom line: if it won’t heat, check flow before anything else — a clean filter and open valves fix most cases for free — and remember a heat pump refusing to run in cold air is behaving correctly, not broken. If you’re buying, a heat pump is the running-cost winner for a warm climate; gas is for fast, any-weather heat; solar is for a long sunny season without a cage. Then put a cover on it and heat the smart hours — that’s where the real savings are.
Frequently asked
Why isn't my pool heater heating the water?
Start with flow, because a heater won't fire without it. Every modern heater has a flow (pressure) switch that blocks it from lighting until it senses enough water moving through — roughly 10 GPM for every 100,000 BTU of heater. So the most common no-heat cause is anything that starves that flow: a dirty filter (the heater sits after the filter, so a clogged filter chokes it), a closed or half-closed valve, a full skimmer or pump basket, low water below the skimmer, or a variable-speed pump running too slow. All of those are free fixes. If it's a heat pump and the air is cold, it may be refusing to run on purpose (see below). After that, check the thermostat setpoint and mode, and whether the pump runs long enough each day for the water to actually warm up.
What's the best type of pool heater — heat pump, gas, or solar?
It depends on how you use the pool, but in a warm climate the heat pump usually wins on running cost. A heat pump moves heat from the air into the water using a little electricity, so it's the cheapest to run in mild weather — the Florida Solar Energy Center pegs a typical Florida year around $500 for a heat pump versus $580 for natural gas and $1,450 for electric resistance. Choose GAS instead if you need heat fast and on-demand in any weather (an intermittently-used pool or spa, or a cold climate) — it heats quickest but costs the most to run. Choose SOLAR if you have a long sunny season and no screen enclosure — near-zero operating cost, but slow and no heat on cold cloudy days.
Do heat pumps work in cold weather?
Only down to a point, and that's by design, not a defect. A heat pump harvests heat from the outside air, so as the air cools there's less heat to grab. The Department of Energy notes heat pumps are most efficient above about 50°F and start losing efficiency below the 45–50°F range; near their low limit they run slowly, can ice the coil and go into defrost, and make little net heat. So a heat pump that 'won't heat' on a cold morning usually isn't broken — it's correctly waiting for warmer air. This is exactly why heat pumps are the default in Florida (winters mostly stay in their efficient band) and why cold-climate pools lean on gas.
What size pool heater do I need?
Heaters are sized by the pool's surface area and the temperature rise you want, not by gallons — because a pool loses heat from its surface (mostly to evaporation), not through its walls. The Department of Energy's formula is Pool Surface Area (sq ft) × Temperature Rise (°F) × 12 for a roughly 1–1.25°F/hour rise; multiply by 1.5 for a 1.5°F rise or 2.0 for a 2°F rise. A bigger heater reaches temperature in fewer hours, which for heat-then-swim use can actually burn less fuel per session — but it's not a reason to buy the biggest unit (it burns more per hour and oversizing has diminishing returns). For your exact pool and a running-cost estimate, use the calculator on our heater showdown.
What's the cheapest way to lower my pool heating bill?
A cover, by a wide margin. Evaporation is by far the biggest way a pool loses heat — the DOE notes each pound of 80°F water that evaporates carries off about 1,048 BTU — so simply covering the pool when it's not in use is the single most effective thing you can do, saving 50–70% of heating cost per the DOE. After that: set a realistic temperature (a comfortable pool is about 78–82°F, and every degree higher costs more), run the heater during the warm part of the day, and in Florida let the heat pump do the shoulder-season work it's efficient at rather than forcing heat on the coldest nights.