Heat Pump vs Gas Pool Heater: What Each Costs to Run, With a Calculator

Most comparisons of these two heaters give you a monthly cost with no energy price behind it, and a rule that a heat pump “stops working at 50 degrees.” The first is not checkable and the second is not true. This guide shows the arithmetic, names the prices it used, and then gets to the part that decides most real purchases, which is not the heater at all.

The short version, by situation

How each one makes heat

A gas heater burns natural gas or propane and passes pool water through a heat exchanger above the flame. Federal law sets the floor: a gas pool heater made since 2013 must be at least 82 percent efficient, and most residential models are rated 82 to 84 percent, which means 82 to 84 cents of every fuel dollar reaches the water. Condensing models reach the mid-90s and cost more.

A heat pump burns nothing. A fan pulls outside air across a coil, a compressor concentrates the heat in that air, and the heat is handed to the water. Electricity runs the fan and the compressor; it does not make the heat. That is why a heat pump is rated by COP, the units of heat delivered per unit of electricity used, and why a COP of 5 is normal in warm air.

The mix-up to watch for: you will see a heat pump described as “100% efficient.” That is the figure for an electric resistance heater, which turns a unit of electricity into a unit of heat. A heat pump at a COP of 5 delivers five. The two are different machines with very different bills.

What a unit of heat costs, with the prices stated

Every figure below is the cost of putting 100,000 BTU into the water, using the U.S. average residential prices the Energy Information Administration published for 2026. Your bill will differ, which is the point of showing the inputs.

Two things in that table are worth a second look. The residential price of natural gas nearly doubles from winter to summer, from $13.96 per thousand cubic feet in January 2026 to $24.09 in June, because a household’s fixed monthly charges are spread over very little gas in summer. If your pool is the only thing burning gas in July, look at your own summer bill before trusting any annual average. And propane is its own category: more than three times the cost of a heat pump’s heat, before any tank rental or delivery charge your supplier adds.

Run it for your pool

Use the rates printed on your own bills. For gas, divide the total bill by the therms used, in a month when the pool heater would actually be running, so the fixed charges are counted the way you will pay them. Switch the fuel to propane if you are on a tank. Tick the cover box and watch every number fall by about half: a cover is the cheapest heat you will ever buy, whichever heater you own.

A heat pump does not switch off at 50°F. It shrinks

The most repeated line about pool heat pumps is that they stop working at 50°F. What actually happens is in the makers’ own rating tables. Heat pump pool heaters are rated at standard conditions written as air temperature, relative humidity and water temperature. Pentair publishes all three for its UltraTemp line:

That is about a third less heat and a COP that falls from 5.6 to 4.0. Every model in the table lands at about 4.0 in 50°F air. So a heat pump on a cool morning is still delivering four units of heat for each unit of electricity, which the cost table above prices at about $1.34 per 100,000 BTU, still under natural gas at the average price. What it cannot do is keep up. The pool is losing heat faster in cool air at the same moment the heater is delivering less of it.

Compare models on the 80/63/80 and 50/63/80 columns. The larger number some brochures lead with is measured in 80°F air at 80 percent humidity, which flatters the unit. And a COP printed with no conditions beside it tells you nothing.

Below 50°F, models differ. Frost forms on the coil, and a unit either defrosts itself and carries on or shuts down until the air warms. Makers sell both kinds, sometimes in the same product line, so read the spec sheet for the exact model. If you want a warm pool when the air is near freezing, buy gas. For a pool that will not heat at all, start with our troubleshooting checklist, because a heat pump idling in cold air is working as designed.

Speed: where gas earns its keep

Heating water is arithmetic. A gallon weighs 8.34 pounds, and one BTU raises one pound by one degree.

• A 20,000-gallon pool, raised 10°F, needs about 1.67 million BTU. A 400,000 BTU gas heater at 84 percent efficiency delivers about 336,000 BTU an hour: about 5 hours. A heat pump rated 134,000 BTU an hour needs about 12.5 hours in warm air and about 19 in 50°F air.
• A 500-gallon spa, raised 20°F, needs about 83,000 BTU: about 15 minutes on that gas heater, and 40 minutes to an hour on the heat pump.

Those are floors. They ignore the heat the water loses while it warms, so real times are longer, and much longer uncovered on a breezy night. The pattern is what matters. A gas heater can take a cold pool to swimming temperature in an afternoon, so you can leave it off between uses. A heat pump cannot, so it is run to hold a temperature. That is why usage decides this more than climate does: heat held for a season favours the heat pump, and heat on demand favours gas. It is also why an attached spa almost always gets a gas heater.

The quote that decides it

Every comparison ranks these heaters on running cost. Most purchases are decided earlier, by what it costs to get energy to the equipment pad.

• A gas heater needs a gas line sized for it. A pool heater is often the largest gas appliance at the house, and an undersized line starves it. Pentair’s MasterTemp manual requires supply pressure of “not less than 4” wc and not more than 14” wc” at the heater while it runs. If the meter is on the far side of the house, or the run crosses a driveway or a deck, the line can cost more than the heater. The cost aggregators put a new gas line at $500 to $2,000 (Angi, August 2026) or $15 to $50 and up per linear foot (HomeGuide, January 2026), and those are estimates, not quotes. On propane, add the tank.
• A heat pump needs a dedicated 240V circuit. Pentair lists a 40 or 50 amp breaker for every model in its UltraTemp line. If your panel has room and sits near the pad, that is a small job: $250 to $900 by HomeGuide’s figures, up to $2,100 by Angi’s. If the panel is full or on the other side of the house, it is not.

Get both quotes before you read another running-cost table. At many houses one of them is a few hundred dollars and the other is several thousand, and no fuel saving closes that gap quickly. The same two aggregators put the installed heaters themselves in overlapping ranges, $1,500 to $6,000 for gas and $2,000 to $7,500 for a heat pump, so the heater is rarely the deciding line.

Above-ground pools

The same physics applies, and four things change.

1. Gas is usually not at the pad. Most above-ground pools sit away from the house with no gas line near them, which is why the heater market for them is almost entirely heat pumps.
2. Check the voltage, not the label. “For above-ground pools” does not mean it plugs in. Some small heat pumps run on a standard 120V outlet. Many sold for above-ground pools are 208 to 230V and must be hardwired by an electrician. The 120V units are small, so match the maker’s stated pool size to yours, and read the plug-in catch in our budget heat pump showdown before you count on an existing outlet.
3. Check minimum flow against your pump. A heater will not fire without enough water moving through it, and the pumps that ship with above-ground pools are small. The minimum is printed in the heater’s manual in gallons per minute. If you are also choosing a pump, size it by flow.
4. Mind the distance from the wall. Gas heater manuals set a minimum distance from the pool. Pentair’s says the heater “may not be installed within 5 ft.” of the inside surface of a pool or spa unless a permanent barrier separates them. Plan the pad before you plan the plumbing.

A small above-ground pool also loses heat fast for its size, so the cover matters even more here than on an in-ground pool.

Three things no comparison mentions

Where you buy it can void most of the warranty. Several of the big pool-equipment makers cut coverage sharply on heaters bought online, down to a year of parts with no labor in some cases, and some honor a warranty only through sellers they authorize. This applies to gas heaters and heat pumps alike, and it can matter more than the brand. The makers’ own wording, brand by brand, is in our pool equipment warranty guide, and it is the reason our in-ground heat pump pick is the one whose warranty survives an online purchase.

Both die of the same two things: bad water and low flow. The heat exchanger is the expensive part of either heater, and low pH eats it. Pentair’s MasterTemp manual is blunt: “Failure to adhere to the listed water chemistry parameters may result in premature failure of the heat exchanger and will void the heater warranty.” Keep your five numbers in range. Low flow is the other killer, and the most common reason a working heater will not fire: the same manual sets a minimum of 20 gallons per minute on its smallest models and 40 on its largest, which a variable-speed pump set too low will not deliver.

A screen enclosure changes the math. A pool under a cage gets roughly 30 to 40 percent less solar energy (Florida Solar Energy Center), so it runs cooler through spring and fall and leans on its heater more. A heat pump also needs open air around it, because it cools the air it uses, and one that re-breathes its own cold exhaust loses output. Give it the clearances in its manual and do not box it in.

One more thing a heat pump can do

Some heat pumps run in reverse. Pentair’s UltraTemp 140 H/C is rated to remove 71,000 BTU an hour from the water at a COP of 4.1. In Florida, where a pool can pass 90°F in August, a heat-and-cool model is a heater for the shoulder months and a chiller for the summer. No gas heater can do that. Our pool cooler showdown covers the options.

What we’d do

• Florida, the Gulf Coast, the Sun Belt, year-round swimming: a heat pump, sized up a step, with a cover. Add gas only for a spa.
• A northern pool heated all summer: a heat pump if the 240V circuit is easy, and accept that it will struggle in May and late September. Gas if you mostly want those shoulder weeks.
• A northern pool heated for occasional weekends: gas on a natural gas main. On propane, do the math in the calculator first.
• Pool and spa together: a heat pump holding the pool and a gas heater for the spa, if the budget allows. Otherwise gas.
• Above-ground: a heat pump matched to your gallons and your outlet, and a cover.

Then buy the specific heater from the showdown that matches: in-ground heat pumps, gas heaters, budget, mid-size and large above-ground heat pumps, or the pound-for-pound roundup if you are still choosing a type.

Neither heater earns a federal tax credit. The home energy credit never listed pool heaters, and it does not apply to anything placed in service after December 31, 2025. Your utility or state may offer a rebate on a pool heat pump, and that is worth a phone call before you buy.