The first question isn't which heater. It's how you plan to use the pool.
Somebody who wants the water warm on a whim next Saturday needs a completely different machine to somebody who wants 82°F from April to October. Get that backward and you'll be unhappy with a perfectly good heater.
Key takeaways
- Choose on use pattern, not on price. Occasional weekend heat and season-long warmth are different problems.
- Gas heater sizing: roughly 1.5 BTU per hour per square foot per degree of rise, then add margin.
- Heat pump output is quoted at 80°F air. Hayward and Pentair also rate theirs at 50°F, where it is roughly 60 to 70 percent of that.
- In salt pools, specify a cupronickel heat exchanger. Standard copper wears faster.
- Undersizing a heater is worse than oversizing. It'll run constantly and never quite get there.
| Gas | Heat pumpMy default | Solar | Electric resistance | |
|---|---|---|---|---|
| Install cost | $3,500-$7,000 | $4,500-$9,000 | $3,000-$8,000 | $1,500-$4,000 |
| Running cost | High | Low | Almost none | Very high |
| Speed to heat | Fast, 8-14 hrs for 10°F | Slow, 24-72 hrs | Slow, weather dependent | Slow |
| Cold weather | Works at any temperature | Loses output; defrost stops near 50°F | No | Works, expensively |
| Lifespan | 8-12 yrs | 10-15 yrs | 10-20 yrs | 10-15 yrs |
| Best for | On-demand, spas, cold climates | Season-long warmth | Cheap season extension | Small pools and spas only |
Gas
Burns natural gas or propane in a combustion chamber, transferring heat through an exchanger into the water.
Buy it if you want heat on demand, you have an attached spa, or you're in a climate where a heat pump won't cope. Nothing else heats a spa from cold in half an hour. The Pentair MasterTemp 400 and Raypak 406A are the common 400k BTU choices at residential scale; the Raypak is actually rated at 399,000, and its page explains the dealer clause in its warranty. The MasterTemp's own page has the full parts list and the one spec that catches people out — every model has a minimum flow rate, 40 GPM on the 400, below which it will not fire at all. Hayward's Universal H-Series is the direct alternative, and the comparison is less obvious than price suggests: it asks only 30 GPM at the same size, and it puts a cupro-nickel heat exchanger in every model where Pentair charges for one and gives up two points of efficiency to fit it. The third is the Jandy JXi, which is the compact one and the only one of the three whose warranty is void entirely if you buy it online.
Sizing: roughly 1.5 BTU/hr per square foot of surface, per degree of rise, then add margin. Take a 16×32 pool, which is 512 sq ft. Want a 20°F rise? That's 512 × 20 × 1.5 = 15,360 BTU/hr as a bare theoretical minimum.
In the real world you're buying 250,000–400,000 BTU. The theoretical number assumes no heat is escaping, and heat is always escaping.
At the bottom of that range, Pentair's condensing ETi 250 is worth knowing about, because the efficiency figure below is not the one it runs at: Pentair rates it 94% on natural gas and 96% on propane against the 84% a conventional heater manages. That changes the gas arithmetic materially. It also brings conditions — altitude limits that differ between natural gas and propane, and acidic condensate that has to be neutralized before it goes anywhere.
Run your own pool through the heater sizing calculator — it does the sizing rule and the heat-up time together, which matters because they come from different numbers. Size comes from surface area, and how long it takes comes from volume.
Add margin for wind exposure, no cover, and cool nights.
Watch for: propane costs a lot more than natural gas per BTU. Check the gas meter and line size — a big heater often needs the supply upsized, which is a separate cost people find out about late. If you might change fuels later, Hayward's Universal HC Series ships set for natural gas with the propane mixer in the box, and a qualified service agency converts it on site.
Heat pump
Pulls heat out of the air and moves it into the water using a refrigeration cycle. It doesn't create heat, it relocates it, which is why it's so much cheaper per degree.
Buy it if you want the pool warm all season and you're somewhere with reasonably warm air. It's the right answer for most of Florida, the Gulf Coast and southern California. The Hayward HeatPro 140k is the size most residential pools land on.
The catch that matters: output is quoted at 80°F air, 80% humidity and 80°F water. Hayward, Pentair and Jandy all print a second rating at 50°F air in their manuals, and on current models it is roughly 60 to 70 percent of the headline figure, at a COP of about 4. Around 50°F the coil also has to defrost, and heating stops while it does — about 15 minutes at a time on Pentair's UltraTemp, anything up to several hours by Hayward's manual.
Which is a problem, because the shoulder season is exactly when you want it. Size on the 50°F rating, not the headline number.
Watch for: they're physically large and they need airflow around them, so a heat pump crammed into a tight equipment enclosure won't perform. They're also not silent — roughly like an air conditioning condenser. Hayward's variable-speed HeatPro VS has a Silent Mode, and it gets its quiet by cutting output. Our pool heater picks cover the gas and heat pump models worth shortlisting, and the heater pages group every gas heater and heat pump with a page here.
Solar
Water pumped through panels on a roof. The sun does the work.
Buy it if you have suitable roof area, decent sun, and you want to extend the season rather than heat on demand.
Sizing: panel area of 50–100% of your pool surface. That's the constraint. A 512 sq ft pool wants 250–500 sq ft of panel with good exposure and the right orientation.
Watch for: it only works when the sun's out, so you can't plan around it. Roof penetrations need doing properly. And it's often best paired with something else — solar for the bulk of the season, a small heat pump for the edges.
Electric resistance
An immersion element. Simple, cheap to buy, brutally expensive to run because you're paying for every unit of heat directly with no multiplier.
Only sensible for small spas and very small pools. On a full-size pool the bills are genuinely shocking, and I've been called out to what people assumed was a faulty meter.
Running two, which more people should
The comparison above reads as a single choice, and on a lot of pools the better answer is two heaters doing different jobs.
Solar plus a heat pump is the common pairing and the strongest one. Solar carries the bulk of the season for nothing, and a modestly sized heat pump covers the cloudy week and the shoulder months when the panels are not delivering. Because the heat pump only fills gaps, it can be smaller than one sized to do the whole job alone.
Gas plus a heat pump suits a pool with an attached spa. The heat pump holds the pool through the season cheaply, and the gas heater does the spa on demand — which is the one job a heat pump genuinely cannot do. Nothing takes 400 gallons from cold to 102°F in half an hour except combustion.
Gas plus solar works where gas is the only practical option and the roof is available. The solar simply reduces how often the gas heater fires, and how often it fires is the entire running cost.
The reason this is not sold more often is that it is two line items on a quote instead of one, and the second reads as an upsell. It is worth pricing anyway, because the combination that costs a little more to install frequently costs dramatically less to run.
One installation note: two heaters on one pad want to be plumbed in series with isolation valves, so either can be bypassed for service without taking the pool out of use.
Sizing mistakes
Undersizing is worse than oversizing. An undersized heater runs constantly, never quite reaches temperature, and costs more overall because it's fighting losses without ever getting ahead. Oversizing costs more up front and gets there faster, then idles.
Forgetting minimum flow. It's the number one reason a heater won't fire. Most gas heaters need a minimum flow rate to fire. Run your variable-speed pump too slow and the heater won't light. Schedule a higher-RPM block during heating hours.
Ignoring the exchanger material. On a salt pool, or anywhere with aggressive water, specify cupronickel rather than standard copper. On most gas heaters it costs a few hundred more, though Hayward's Universal H-Series with OmniX has it on every size at no extra charge, and Pentair's condensing ETi 400 uses titanium instead. It's the difference between a heater lasting four years and fifteen.
Not planning for a cover. Whatever you buy, a cover cuts the running cost roughly in half. Budget for it in the same conversation.
Go deeperWhat each heater type actually costs to runMonthly numbers by climate, and why the cover matters more than the heater. Go deeperHeating a hot tub is a different problemStandby loss, not heat-up, is what a spa actually costs you every month. Go deeperConverting to salt with a heater?Check the heat exchanger first. Older copper units are less happy about salt.Frequently asked questions
What size pool heater do I need?
For gas, a working rule is 1.5 BTU per hour per square foot of pool surface per degree of temperature rise you want, then add margin for wind exposure and no cover. A 16×32 pool wanting a 20°F rise needs roughly 400,000 BTU with margin. Heat pumps size differently because output drops with air temperature.
Which pool heater is cheapest to run?
Solar, by a wide margin, since running cost is just the extra pump energy. Among active heaters, a heat pump usually lands between a third and two-thirds of gas per degree, and which end depends entirely on your utility rates — the advantage is largest against propane and smallest where natural gas is cheap and electricity is not because it moves heat rather than generating it. Gas is the most expensive to run and the fastest to heat.
How long does a pool heater last?
Gas heaters typically 8-12 years, heat pumps 10-15, solar panels 10-20. Water chemistry is the biggest factor — aggressive water eats heat exchangers, and I've seen copper exchangers fail in three or four years on pools with chronically low pH and alkalinity.
Can you use a heat pump in cold weather?
Up to a point. Output falls as the air cools: Hayward and Pentair rate their current heat pumps at roughly 60 to 70 percent of full output in 50°F air, and around that temperature the coil has to stop and defrost, sometimes for hours. That is exactly the shoulder-season weather people buy them for, so size on the 50°F rating in the manual rather than the headline figure quoted at 80°F.
Do I need a cover if I have a pool heater?
If you care about the bill, yes. Most heat leaves a pool through evaporation from the surface, and a cover cuts total heat loss 50-70%. On a heated pool a solar cover usually pays for itself within one season, which makes it better value than any heater upgrade.
