Can you heat a pool with a water heater?
The output is not the problem, which is why this idea is so persistent. The gallons per minute are the problem.
Quick picks
The four appliances that do the job, in the order they make sense, plus the one that is technically closest to a water heater and is on the list as a warning.
| # | Product | Best for | Price |
|---|---|---|---|
| 1 |  Best overall Hayward HeatPro Pool Heat Pump Cheapest controllable heat there is, at about $10.75 per million BTU delivered. Slow, and that is the whole trade. | Lowest running cost per degree when you want real temperature control | |
| 2 |  Best when speed matters Raypak Digital Natural Gas Pool Heater 336,000 BTU/hr with a stated 82% efficiency. If the appeal of a tankless unit was instant heat, this is the appliance that actually delivers it. | Heating fast and occasionally, rather than holding a temperature | |
| 3 |  When the gas supply is the constraint Hayward Universal H-Series 150,000 BTU Natural Gas Heater 150,000 BTU/hr, under half the fire of a full-size unit — which is often the difference between using the gas line you have and upsizing it. | A modest pool where the gas supply, not the heater, is the real constraint | |
| 4 |  Do this first, whatever you buy Sun2Solar 1600 Series Ultimate Blue Solar Cover The heater you need gets smaller once the pool stops shedding its heat overnight. | An inground pool you cover most nights, every season | |
| — | No photo The trap this question leads to An electric resistance pool heater An element in a housing is the closest thing to a water heater you can plumb into a pool — and about $53.75 per million BTU, five times a heat pump on identical electricity. | Spas and very small above-ground pools only |
#ad We earn a commission if you buy through this link, at no extra cost to you. It never changes the ranking. How this site is funded. Prices last checked October 18, 2026.
Why the idea is reasonable, and still wrong
This question deserves a straight answer rather than a dismissal, because the instinct behind it is sound. A big residential tankless water heater is rated in the same order of magnitude as a small pool heater, it burns the same fuel, and it costs a fraction of what a pool heater costs. If the job is “put BTU into water”, it looks like the same appliance at a better price.
The energy side genuinely does work out. Here is the requirement that any heater has to meet, which is the same arithmetic we size every heater with:
| 20,000 gal x 8.33 lb x 10°F | 1,666,000 BTU |
| At 150,000 BTU/hr of input, at about 82% delivered | about 13.5 hours |
The output of a small pool heater is in the right range. So is a large tankless unit's.
Which is exactly why people try it. The BTU rating is the one specification where the two appliances look comparable, and it is the one specification that is not the obstacle.
The flow window is the whole story
A pool heater does not heat a tankful of water. It sits in the circulation loop and raises the entire pool by a fraction of a degree per pass, which means the whole filtration flow goes through it continuously. Manufacturers publish the window that flow has to stay inside, and it is not a suggestion: too little flow and the exchanger is cooked, too much and the pressure drop and erosion become the problem.
Hayward’s own page for the 150,000 BTU Universal H-Series — the smallest gas heater this site ranks — gives the window as minimum 20, maximum 125, with 2 in by 2‑1/2 in CPVC union connections, and advises keeping flow above the minimum with allowance for a dirty filter to protect the heat exchanger.
| Domestic water heater | Pool heater | |
|---|---|---|
| What it is asked to do | Raise a small flow by 60-70°F, on demand | Raise an enormous flow by a fraction of a degree, continuously |
| Flow it is plumbed for | Domestic pipe, sized for taps and showers | Published window — 20 to 125 on the Hayward H150 |
| Connections | Domestic hot and cold fittings | 2 in x 2-1/2 in CPVC unions on the H150 |
| Water it is built for | Potable water | Chlorinated or salted pool water — cupro nickel on the H150, titanium on heat pumps |
| Controls | A setpoint for drinkable hot water | A pool setpoint, flow proving, and freeze logic |
| Appliance class | Certified as a water heater for potable water | Certified as a pool and spa heater for non-potable water |
The last row is why this is not merely inadvisable. The two appliance classes are listed to different standards, so a domestic unit plumbed into a pool is outside its listing — which is the part that interests an insurer and a code inspector.
Three more things that go wrong
- The water is the wrong water. A pool heater’s exchanger is specified for chemically treated water: Hayward puts cupro nickel in the H-Series as standard and states salt and chemical corrosion resistance, and every heat pump worth buying uses titanium. The exchanger material is the single component that decides a heater’s life in treated water, and a potable-water appliance was never specified for it.
- The controls have the wrong job. A water heater holds a setpoint for water going to a tap. A pool heater proves flow before it fires, modulates against a pool setpoint it will take hours to reach, and protects itself against freezing. Those are not luxuries; they are what stops an appliance destroying itself in a loop it cannot satisfy.
- The economics are not even good. If the appeal was cheap heat, an element in a housing is the closest thing to a water heater you can legitimately plumb into a pool, and it costs about $53.75 per million BTU delivered — roughly five times a heat pump on the same electricity. The two machines sold as electric pool heaters are separated by exactly that factor.
The honest exceptions
Two cases where the instinct is not wrong, stated properly because flat-out “no” would be the wrong answer:
- An indirect heat exchanger off a boiler is a real product class. Pool water on one side, your boiler’s heating circuit on the other, with the pool water never entering the boiler. That is a genuine way to heat a pool from an existing heat source, and it is nothing like plumbing a water heater into the loop: it is a designed component, specified for the flow on both sides, and fitting one is a plumbing project rather than a purchase. We do not rank them, because they are not an affiliate product you choose from a listing.
- Hot water into a very small pool does do something. On a few hundred gallons, hot water from the house is a real intervention rather than a rounding error. On a full-size pool it is a rounding error, and the arithmetic there shows why: your water heater holds a trivial fraction of a pool’s volume and delivers it at electric-resistance or gas cost with no efficiency advantage.
So what should you buy?
Work from the thing that attracted you to the water-heater idea, which is the order of the ranked table above:
- It was the cheap running cost: a heat pump, at about $10.75 per million BTU delivered. It is the cheapest controllable heat available and it is slow, which is the entire trade.
- It was instant heat on demand: gas. That is the one thing gas genuinely buys you, at roughly $28.36 per million BTU delivered. The two costed against each other is the decision page.
- It was using something you already own: then the honest answer is the cover rather than any heater. It is the only purchase here under a few hundred dollars, and it attacks the loss rather than adding heat.
Every pick, in detail
What each one publishes, what is good and bad about it, and — the section this category usually omits — what we could not judge without owning one.
Hayward HeatPro Pool Heat Pump
If you are heating a pool with electricity, this is the shape of machine to do it with. A heat pump moves heat instead of making it, and at the assumptions we publish it delivers a million BTU into the water for about a fifth of what an electric resistance heater costs to deliver the same. It is slower than gas and it fades as the air gets colder, which is exactly when you want it — that trade is the whole decision.
Best for Lowest running cost per degree when you want real temperature control
| Type | Air-source heat pumpThe manufacturer publishes no retrievable specification sheet; figure is as listed at retail. | Retail listing |
|---|---|---|
| Rating convention | Rated by BTU/hr OUTPUTThis is the trap in the category. Gas heaters are rated by fuel INPUT, heat pumps by heat OUTPUT. A 140,000 BTU heat pump and a 140,000 BTU gas heater do not deliver the same heat. | Published standard |
| Cost per million BTU delivered | About $10.75 at our published assumptionsUS average residential electricity at 18.34 cents/kWh (EIA, June 2026) and an assumed COP of 5.0. Both are substitutable on our running-cost calculator. | Our arithmetic |
- Cheapest running cost per delivered BTU of any controllable heater
- No gas line, no combustion, no flue
- Holds a set temperature rather than depending on the weather
- Slow — expect most of a day to move a full-size pool ten degrees
- Efficiency falls with air temperature, so it is weakest in the shoulder season
- High purchase price against a gas heater of comparable output
- The COP this specific unit achieves at your conditions
- Compressor and control-board longevity
- Noise at the distance your deck actually sits from it
These need a unit in hand across a season. We do not have one, so we do not have an opinion worth printing on them.
#ad We earn a commission if you buy through this link, at no extra cost to you. It never changes the ranking. How this site is funded. Prices last checked October 18, 2026.
Raypak Digital Natural Gas Pool Heater
The right answer when speed matters more than running cost — a weekend pool you want warm on Friday afternoon rather than one you hold at temperature all season. At the assumptions we publish, gas costs roughly two and a half times what a heat pump costs per delivered BTU, and it will get there in a fifth of the time. Buy the speed if you need the speed.
Best for Heating fast and occasionally, rather than holding a temperature
| Type | Natural gas or propane fired, digital controlThe manufacturer publishes no retrievable specification sheet; figure is as listed at retail. | Retail listing |
|---|---|---|
| Rating convention | Rated by BTU/hr INPUTA 400,000 BTU gas heater burns 400,000 BTU an hour and delivers roughly 82% of it. Do not compare that number directly against a heat pump's output rating. | Published standard |
| Cost per million BTU delivered | About $28.36 at our published assumptionsUS average residential natural gas at $24.09/Mcf (EIA, June 2026) and an assumed 82% thermal efficiency. | Our arithmetic |
- Fast — hours rather than most of a day on a full-size pool
- Works the same in cold air, unlike a heat pump
- Lower purchase price than a heat pump of equivalent real output
- Roughly two and a half times a heat pump's cost per delivered BTU
- Needs a gas line sized for it, which is often the real cost
- Propane instead of natural gas raises the running cost by about a quarter again
- The efficiency this specific unit achieves in service
- Heat exchanger life against your water chemistry
- Whether your existing gas supply can feed it without an upgrade
These need a unit in hand across a season. We do not have one, so we do not have an opinion worth printing on them.
#ad We earn a commission if you buy through this link, at no extra cost to you. It never changes the ranking. How this site is funded. Prices last checked October 18, 2026.
Hayward Universal H-Series 150,000 BTU Natural Gas Heater
The gas heater for a pool that does not need 336,000 BTU an hour thrown at it. At 150,000 BTU/hr of input it is under half the fire of a full-size unit, which costs you speed and buys you something people discover too late: a much smaller gas supply to feed. On a modest inground pool or a big above-ground one, that is often the difference between using the gas line you already have and paying to upsize it.
Best for A modest pool where the gas supply, not the heater, is the real constraint
| Rated input | 150,000 BTU/hrHayward publishes this as a BTU/hr rating, in the gas convention: fuel burned per hour, not heat delivered into the water. | Manufacturer |
|---|---|---|
| Current part number | W3HS150FDN, which Hayward's listing states replaces the H150FDNSame heater class, current SKU, with the OmniX control. The older H150FDN part number still appears in older reviews and stock. | Retail listing |
| Heat exchanger | Standard cupro nickelCupro nickel is the corrosion-resistant option on most gas heaters and a paid upgrade on some. Hayward lists it as standard on this one. | Manufacturer |
| Thermal efficiency | Not stated for the current W3HS150FDNConfirmed on Hayward's own product page, read 17 October 2026: there is no thermal efficiency percentage anywhere on it. The only efficiency-shaped figure is a claim of savings of 'up to 18%', which refers to reduced PUMP run time rather than to combustion. An 84% figure circulated in retail copy for the older H150FDN; it does not appear for the current part, so we no longer attach it and run this heater at the 82% we assume for gas generally. | Manufacturer |
| Water flow window | Min 20, max 125Quoted exactly as Hayward publishes it — the specification table does not state the unit, though gallons per minute is the convention and the figures only make sense that way. Hayward advises keeping flow above the minimum with allowance for a dirty filter, to protect the exchanger. | Manufacturer |
| Connections | 2 in x 2-1/2 in CPVC unions | Manufacturer |
| Electrical | 120/240V, 60 Hz single phase, 5.5 circuit amps, 15 A max breakerHayward's table also carries a 'Power: 44 kW' row, which cannot describe this appliance: 5.5 A at 240 V is about 1.3 kW, and 44 kW would not sit behind a 15-amp breaker. We read it as a data-entry artifact and flag it rather than repeat it. | Manufacturer |
| Warranty | 1 year parts, no laborAt the shorter end of this category. For contrast, FibroPool states a Limited Lifetime warranty and TURBRO gives 2 years on the machine with 5 on the compressor, both on much cheaper units. | Manufacturer |
| Cost per million BTU delivered | About $28.36 at our 82% assumptionUS average residential natural gas at $24.09/Mcf (EIA, June 2026) divided by 82%, the same assumption we apply to every gas heater without a published efficiency figure. | Our arithmetic |
- Roughly half the gas demand of a full-size heater, which is often what decides the install
- Cupro nickel heat exchanger listed as standard rather than as an upgrade
- Low NOx certified, so it is permitted in the air districts that require it
- Slow for a gas heater: about 14 hours to lift 20,000 gallons ten degrees
- Still gas, so still roughly two and a half times a heat pump's cost per delivered BTU
- Hayward publishes no thermal efficiency figure for it on its own product page
- The thermal efficiency it actually delivers — no figure is published for the current part
- How the cupro nickel exchanger holds up against your water chemistry
- Whether your gas meter and line can feed it without an upgrade
These need a unit in hand across a season. We do not have one, so we do not have an opinion worth printing on them.
#ad We earn a commission if you buy through this link, at no extra cost to you. It never changes the ranking. How this site is funded. Prices last checked October 18, 2026.
Sun2Solar 1600 Series Ultimate Blue Solar Cover
The cover to buy if you intend to keep using it in five years. 16 mil is the thickest tier the category sells, and thickness on a solar cover buys durability rather than heat — Bob Vila measured every cover in its hands-on group at the same 4 to 5 degrees, from 12-mil clear to 16-mil blue. Pay the extra for the one that survives being dragged over the coping twice a day, not for a temperature difference that did not show up in the only published one-hour test in this category.
Best for An inground pool you cover most nights, every season
| Price shown for | 24 ft roundThis product is sold in several sizes on one listing. The live price and photograph on this page are the ones Amazon returns for this variant; other sizes are priced separately. | Retail listing |
|---|---|---|
| Thickness | 16 milThe manufacturer publishes no retrievable specification sheet; figure is as listed at retail. | Retail listing |
| Color | Blue | Retail listing |
| Stated lifespan | 5 to 10 years for the 16-mil tierSwim University's published figure for 16-mil covers generally, not a Sun2Solar warranty statement. | Retail listing |
| Bubble side | Bubbles face the waterCategory-wide instruction, not brand-specific. The bubbles are the insulating layer and they have to touch the water. | Published standard |
4 to 5 degrees F in one hour
Claimed by Bob Vila, measured across all covers in their group. Conditions: One hour under 2pm sun on 89 to 92 degree F days. Source
Their figure was the same for every cover they put on the water. Read our reconciliation of what a one-hour reading actually measures before you plan around this number.
- Thickest tier sold, which is where the durability difference actually is
- Blue absorbs more of the incoming energy at the surface than clear does
- Cuts oversize to your pool shape with household scissors
- Heavy and awkward at 16 mil — a large inground cover is genuinely a two-person job without a reel
- Costs meaningfully more than the 12-mil version for no measured heating advantage
- Whether the seams hold up over five seasons of daily handling
- How the bubble layer survives repeated dragging over rough coping
- Whether the stated lifespan tier holds for this specific product line
These need a unit in hand across a season. We do not have one, so we do not have an opinion worth printing on them.
#ad We earn a commission if you buy through this link, at no extra cost to you. It never changes the ranking. How this site is funded. Prices last checked October 18, 2026.
An electric resistance pool heater
Skip it for a full-size pool, and the arithmetic is not close. At the same electricity price, a resistance heater costs about five times what a heat pump costs to deliver the same heat, because it makes heat instead of moving it. It has a legitimate niche on a small above-ground pool or a spa where the volume is tiny and the install is a plug. On 20,000 gallons it is the most expensive way to heat water that exists.
Best for Spas and very small above-ground pools only
| Cost per million BTU delivered | About $53.75 at our published assumptionsUS average residential electricity at 18.34 cents/kWh (EIA, June 2026) at a coefficient of performance of 1.0. Compare with about $10.75 for a heat pump on the same electricity. | Our arithmetic |
|---|---|---|
| Efficiency | About 1:1 — no efficiency to argue about | Published standard |
- Cheap to buy and often just plugs in
- Genuinely sensible on a spa or a very small pool
- About five times a heat pump's running cost for identical heat
- Draws serious current, which frequently means an electrical upgrade
- On a full-size pool the running cost is punishing
- Element life in your water chemistry
- Whether your existing circuit can carry it
These need a unit in hand across a season. We do not have one, so we do not have an opinion worth printing on them.
#ad We earn a commission if you buy through this link, at no extra cost to you. It never changes the ranking. How this site is funded. Prices last checked October 18, 2026.
Questions people actually ask
Can I use a tankless water heater for my pool?
Is a pool heater just a big water heater?
What if I plumb a water heater into the pool return line?
Can I heat the pool from my house boiler?
Would filling the pool with hot water work?
- Hayward's own product page for the Universal H-Series W3H150FDN: heating capacity 150,000 BTU/hr, "Standard cupro nickel heat exchanger", "Excellent salt water and chemical corrosion resistance", Water Flow Rate Min 20 and Max 125 (the table does not state the unit), 2 in x 2-1/2 in CPVC union plumbing connections, 24 x 19 x 29.5 in, 135 lb, 120/240V 60 Hz single phase, circuit amps 5.5, maximum breaker 15 amps, a "Power: 44 kW" row that cannot be reconciled with that breaker size, warranty of 1 year parts and no labor, SCAQMD Rule 1146.2 low-NOx compliance, and no thermal efficiency figure anywhere on the pageHayward Industries · read 2026-10-17
- Average retail price of electricity to US residential customers: 18.34 cents/kWh, June 2026U.S. Energy Information Administration · read 2026-09-02
- US residential natural gas price: $24.09 per thousand cubic feet, June 2026U.S. Energy Information Administration · read 2026-09-02
- US residential propane price: $2.674 per gallon, week ending 30 March 2026U.S. Energy Information Administration · read 2026-09-02
- Energy content conversion factors: 3,412 BTU per kWh, 1,036 BTU per cubic foot of natural gas, 91,452 BTU per gallon of propaneU.S. Energy Information Administration · read 2026-09-02
- U.S. Department of Energy pool-energy figures (evaporation share, BTU per pound evaporated, cover savings, recommended temperatures, wind effect)Recreonics, reproducing and attributing the U.S. Department of Energy · read 2026-09-02
- Pool heater roundup: the 8.33 BTU per gallon per °F figure, attributed on the page to Alicia Toedter of Leslie'sBob Vila · read 2026-09-02
Every URL above was fetched and returned content on the date shown. Where a figure is ours rather than somebody else’s, the arithmetic is on the page and on the methodology page. If you find an error in it, we would genuinely like to know.
Read next
Which pool heater to buy
Every type of pool heater ranked on what it costs to deliver a million BTU into the water, how many hours it needs to do it, and what the install actually involves.
Electric pool heaters
Heat pump or resistance element? Both are sold as electric pool heaters and one costs five times what the other does to deliver the same heat. How to tell which is which before you buy.
Heat pump vs gas pool heater
The two options priced per delivered BTU and timed on the same pool, with the one question that decides which trade suits you.
What size pool heater do I need?
How to size a heater from your pool's actual energy requirement, the input-versus-output rating trap, and why the answer gets smaller once you own a cover.
How to heat a pool fast
What actually governs heating speed, the input-versus-output rating trap that makes gas heaters look faster than they are, and the tricks that do and do not help.
The cheapest way to heat a pool
Cheapest to buy is a cover. Cheapest to run is solar, then a heat pump. The arithmetic for both, and a spending order that works at any budget.