Which pool heater to buy

What we ranked these on

Almost every “best pool heater” list ranks machines against each other inside a single category, which quietly assumes you have already made the decision that matters. You have not. The expensive choice is which technology, and the gap between the best and worst answer to that question is a factor of five on your running cost — far more than the gap between any two heat pumps.

So these are the five criteria, in the order they decide it:

• What it costs to deliver a million BTU into the water. Not the fuel price, and not the efficiency rating on its own. The two divided through, which is the only figure that describes the actual job.
• How many hours it needs. A heater that costs half as much to run and takes five times as long is not cheaper if you swim on Saturdays and decide on Friday night.
• How much control it gives you. A thermostat you can set is a different product from a collector that delivers whatever the sky delivered.
• What the install really involves. A gas line sized for the burner, a 240 V circuit, unshaded roof area. This is where the surprise costs live, and it is routinely left out of comparisons.
• What it costs to own. Purchase price plus a decade of running cost, which frequently reverses the order of the purchase prices.

Nothing here is ranked on a temperature claim, because in this category those are unattributed almost without exception — and the ones that are attributed turn out to be measuring three different things.

The number the category will not publish

Every heater comparison you will read gives you fuel price and efficiency in separate paragraphs and leaves you to combine them. Combined, they look like this — dollars to put one million BTU into the water, at US average residential energy prices:

The reversal in that table is the most useful thing on this page. Electricity is the dearest energy you can buy per unit and natural gas is among the cheapest, and yet electricity produces both the cheapest row and the most expensive one. The machine on the end of it decides everything: a heat pump moves heat and delivers roughly five units for each one it draws, while a resistance element makes heat and delivers one.

One question picks your heater

Before any model comparison, answer this: do you want to hold a temperature all season, or heat the pool on demand?

If you hold a temperature, running cost dominates, because you will pay it every day for months. Buy the heat pump. If you heat on demand — a pool used two weekends a month, or a shoulder-season pool you warm for a visit — speed dominates, and the running cost applies to relatively few hours. Buy gas. That one question resolves most of this category, and almost nobody asks it before recommending a machine.

Speed is the second axis, and it is not close

Time to heat is one division: the energy needed, divided by what the machine actually delivers per hour. For that same 1,666,000 BTU job, using the ratings each manufacturer publishes:

Read the two Hayward rows next to each other. A 150,000 BTU gas heater and a 140,000 BTU heat pump look like the same size machine on a listing page, and the gas unit is slower. That is not an error in the table.

The rating trap that ruins the comparison

There is a second version of the same trap on the electric side, where one phrase covers two machines whose running costs differ by five times. Which electric heater is which is the page for that, and gas against electric runs the resulting three-way properly.

This is the most expensive misunderstanding in the category, because it leads people to buy a gas heater on a headline number and then wonder why it is not proportionally faster. The sizing guide works the conversion both ways, and the head-to-head comparison runs the two technologies against each other on equal terms.

Heat pumps: cheapest to run, slowest to arrive

A pool heat pump is an air conditioner working backwards. It extracts heat from outdoor air and pushes it into the water, which is why it can deliver several times the energy it draws and why its cost per delivered BTU is the lowest of anything that has a thermostat.

The trade is specific. It is slow, and it gets slower as the air cools. Coefficient of performance is not a constant — it falls with air temperature, so the machine is weakest in the cool shoulder season, which is precisely when you most wanted the heat. Below roughly 50°F of air temperature, most units deliver very little at all.

Buy one if you hold a temperature across a season. Do not buy one if the plan is to warm the pool on Friday for Saturday. What a heat pump costs to run, month by month works through the arithmetic, including what happens to the COP as autumn arrives. The heat pumps themselves, ranked, including the small ones for above-ground pools.

Gas: buy the speed, or do not buy gas

A gas heater’s case is entirely about time. It reaches temperature in hours rather than most of a day, it does not care how cold the air is, and it costs less to buy than a heat pump of comparable real output. Against that, it is roughly two and a half times the running cost per delivered BTU, and propane instead of natural gas adds about a quarter again on top of that.

The install is where gas surprises people. The heater has to be fed by a gas line sized for its input, and an existing residential line frequently is not. That is the real argument for the smaller 150,000 BTU class: it asks less than half as much of your supply, and on a modest pool the extra hours may cost far less than upsizing a gas run. The gas heaters, ranked, and Raypak against Hayward against Pentair for the shortlist stage.

Solar panels: nothing to run, nothing to control

Solar collector panels are the only entry on this page with a genuinely zero running cost, because your filtration pump is already circulating the water and no additional fuel is burned for the heat. Across a decade that is a very large number.

What you give up is control. There is no thermostat in any meaningful sense: you get the day the sky gave you, and on a cool overcast week the panels deliver almost nothing. They also need real unshaded collector area — the trade rule of thumb is 50 to 100% of the pool’s surface — and plumbing work to install. The full solar write-up is here, and solar against a heat pump is the comparison most people are really asking about.

Electric resistance: the one we tell you to skip

On a full-size pool an electric resistance heater is the most expensive way to heat water that exists, and the arithmetic is not close: about $53.75 per million BTU against about $10.75 for a heat pump running on the same electricity. Five times, for identical heat.

It has one legitimate home — a spa, or a very small above-ground pool where the volume is tiny and the install is a plug rather than a project. That is a real niche, and we say so on the above-ground page. On 20,000 gallons it is a mistake that keeps charging you every month.

Sizing: bigger is not automatically wrong

Oversizing a pool heater wastes purchase price, not fuel. The energy needed to raise a pool by ten degrees is fixed by the water, not by the machine — a bigger heater delivers the same BTU in fewer hours rather than burning more of them. Undersizing is the expensive error, because a machine that cannot outrun the pool’s losses never reaches the set point at all and runs continuously trying.

The one caveat is that a gas burner and a heat pump both have to be matched to their supply, so a bigger machine can carry a bigger install bill. Size from the energy rather than from a rule of thumb and the answer falls out.

What each of these has to survive

Cost of ownership is the fifth criterion and the hardest one to write about honestly, because service life is exactly the thing nobody can tell you from a specification sheet. What we can tell you is what each technology puts at risk, which is a genuinely different list.

• A gas heater lives or dies by its heat exchanger, and what attacks it is your water chemistry rather than your climate. This is why cupro nickel is worth asking about — it is the corrosion-tolerant option, standard on some models and a paid upgrade on others.
• A heat pump has a compressor and a control board, which is a different kind of exposure: fewer chemistry problems, more electronics sitting outdoors through winters. It is also the machine that runs the most hours per season, because it is the slow one.
• Solar panels have no moving parts at all, and their failure modes are the material giving up under a decade of UV and the plumbing joints leaking. In a climate that freezes they also have to be drained down every autumn, and a panel left full will split.

We have not owned any of these for a decade, so we do not publish a lifespan figure for any of them — every product write-up below lists the durability questions we could not answer rather than filling them in. What the list above does tell you is which question to put to an installer, and they are not the same question for each technology.

Is it worth waiting for the 2028 standard?

The Department of Energy has amended its energy conservation standards for consumer pool heaters, with compliance required from 30 May 2028. Manufacturers have to meet the new levels from that date, which will lift the efficiency floor of what is on sale.

Our answer is still no, do not wait, and the reason is arithmetic rather than opinion. A tightened floor mostly removes the worst products from the market; it does not make a heat pump cheaper to run than the physics allows. Two swimming seasons without heat costs more than the efficiency difference is likely to be worth — and if you are on the fence, the cover below is a much better use of the interval than waiting is.

The purchase that comes before any of this

Every figure on this page is about adding heat. The Department of Energy attributes about 70% of a pool’s heat loss to evaporation, and no heater here does anything about that — they replace what evaporation took, at the prices in the table above, every single night.

A solar cover is the only item on this page that attacks the loss instead of the symptom, and it costs about a hundredth of a heat pump. The cover-versus-heater comparison carries the measurement that tells you which half of the problem you actually have: read the water at dusk and again at dawn. A big overnight drop means you are losing heat, and buying a heater to replace heat you could simply have kept is the commonest expensive mistake in this category.

Buy the cover first, then size the heater to what is left. It is a smaller heater and a smaller bill. The cover picks are here.