What Is the Best Way to Heat a DIY Hot Tub? (2026 Running Cost Comparison)

Last updated: September 2026

What’s the best way to heat a DIY hot tub? It’s one of the first questions people ask me, and rightly so. Heating is one of the biggest parts of your setup, and it’s the biggest part of your running costs. Pick the wrong method and you’ll either be waiting all day for the water to warm up or wincing at every bill.

There isn’t one right answer for everyone. It depends on how you use the tub, what fuel you have, where you live and what you want to spend up front. In this guide I’ll go through every realistic option (electric, air source heat pump, gas, heat exchangers, wood and solar), what each one really costs to run at today’s prices, and which one I’d pick.

First: you still need a spa pack

Every method below works alongside a spa pack, not instead of one. The spa pack is the brains of the tub. It runs the jets, the blower and the filtration and purge cycles, which keep the water moving and the water clear. Almost every spa pack comes with an electric heater built in, and it’s no cheaper to buy one without.

Any extra heater sits in series in the plumbing, usually after the spa pack, so water goes pump, filter, spa pack heater, then your extra heater, then back to the jets. I leave the spa pack element in place on my own tub as a backup. If you want to understand how the element itself works and how fast it heats, see Hot tub heaters explained.

What does heat actually cost?

The fairest way to compare heating methods is the cost of putting one kilowatt hour (kWh) of heat into the water. An electric element turns every kWh of electricity into a kWh of heat. A gas heater loses some heat up the flue. A heat pump moves heat out of the air, so it gives you several kWh of heat for each kWh of electricity. That ratio is called the coefficient of performance (COP).

Here’s where the numbers come from:

• UK: the Ofgem price cap for 1 October to 31 December 2026 is 26.32p per kWh for electricity and 7.97p per kWh for gas (direct debit, average rates).
• USA: the EIA puts the average residential electricity price at 18.31 cents per kWh in July 2026. Residential natural gas averaged between $13.96 and $24.09 per thousand cubic feet from January to June 2026. Using the EIA’s conversion of 1,036 Btu per cubic foot and 3,412 Btu per kWh, that’s roughly 4.6–7.9 cents per kWh of gas.
• Assumptions: a gas heater around 85% efficient, and a heat pump COP of about 4 in mild weather and about 2.5 in cold weather at hot tub temperatures.

Heat source	UK cost per kWh of heat	USA cost per kWh of heat
Spa pack electric element	26.3p	18.3¢
Natural gas heater (85%)	9.4p	5.4–9.3¢
Heat pump, cold weather (COP 2.5)	10.5p	7.3¢
Heat pump, mild weather (COP 4)	6.6p	4.6¢

Two things jump out. First, heating with the spa pack element costs roughly three to four times as much per unit of heat as a heat pump. Second, in the USA, cheap natural gas can compete with a heat pump, especially in winter. In the UK the heat pump wins most of the year. Your own tariff will be different, so plug in the prices from your bill.

Propane is sold by the gallon or litre, so it’s harder to compare. A US gallon holds about 91,452 Btu (about 26.8kWh), so with an 85% efficient heater, divide your price per gallon by about 23 to get the cost per kWh of heat. It’s almost always dearer than mains gas.

Electric: the spa pack heater

Electric is the default. The heater is already in your spa pack, so there’s nothing extra to buy, no fuel to store and nothing extra to plumb. You’ll need an electrician to run the right supply, but you need that for the spa pack anyway. See A guide to hot tub electrics.

The downsides are speed and cost. A typical UK spa pack heater is 2–3kW, and in the USA it’s usually 4–5.5kW on 240V. That’s fine for a small shop-bought tub, but a DIY tub often holds a lot more water. My own tub holds about 2,800 litres (740 US gallons). On its 3kW element it heated at roughly 0.5°C (about 1°F) an hour. Impromptu dips were out of the question. You had to plan the week around it.

Electric makes sense if your tub is small and well insulated, if you keep it hot all the time, or as the backup to another heat source.

Air source heat pumps: what I use

If you’ve read any of my other posts or watched my videos, you’ll know I’m a big fan. An air source heat pump works like a fridge in reverse. It pulls heat out of the outside air and puts it into your water. For each kWh of electricity, you get several kWh of heat. The US Department of Energy says pool heat pump COPs usually range from 3 to 7. That’s measured at the federal test conditions of 80°F (27°C) air and 80°F water, though. A hot tub runs at 38–40°C (100–104°F), and the DOE notes that heat pumps get less efficient as the air gets colder, especially below about 45–50°F (7–10°C). So in a British or northern US winter, expect a lower COP than the brochure figure.

My results

I fitted a 21kW Comfortline heat pump to my own tub. It changed how I use it:

• Running costs dropped by 66%. The full breakdown is in How I reduced the running costs of my hot tub by 66%.
• Heat-up went from about 0.5°C an hour to about 4°C an hour at 15°C (59°F) outside. That’s roughly 8 times faster, and taking the tub from 33°C to 39°C (91°F to 102°F) now takes about 1 hour 20 minutes instead of around 12 hours.
• No heat loss in use. The tub holds its temperature through a long session, where before it would drop a few degrees.

Is a heat pump really 8 times faster?

On my tub, yes. But I want to be clear about why, because the heat pump itself isn’t what makes it fast. Heat-up speed comes down to how many kW of heat go into the water. My old element was 3kW on a big, uninsulated tub. The heat pump is rated at 21kW, and even allowing for its output dropping in cooler air, it’s putting several times more heat into the water. For comparison, the current Comfortline 20kW model is rated at 20kW in warm conditions but 14kW at 15°C air.

So a heat pump heats faster only if it’s bigger than your element. My friend Paul’s 7kW heat pump heated his tub about 2.5 times faster than his 2kW element. A small 5kW heat pump in winter might put out 2–3kW, which is no faster than a spa pack heater. That’s why I tell people to buy the biggest unit their budget allows. There’s more on sizing in How to choose the right air source heat pump for your hot tub or pool.

The downsides

• Upfront cost. A decent unit plus a connection kit costs a lot more than a spa pack heater.
• Cold weather. Output and efficiency fall as the air gets colder, and many units have a minimum operating temperature. Check the spec sheet, and read how to defrost a Comfortline heat pump.
• Space, airflow and noise. It needs open air around it. See How noisy is an air source heat pump?

Installation is well within reach of a practical DIYer. You plumb it in after the spa pack with a bypass, and an electrician does the supply. My step-by-step is Can I add an air source heat pump to my hot tub? and the full background is in the ultimate guide to air source heat pumps for hot tubs. You can see the heat pumps and connection kits we sell on parts4tubs.co.uk.

Gas: natural gas and propane

Gas is the traditional way to heat a pool or hot tub, and it’s still the fastest. A gas heater can put a huge amount of heat into the water in a short time. The DOE says gas pool heaters are ideal for heating quickly and a good choice for pools that are only used periodically, and that some new models reach 89–95% efficiency. They also keep working whatever the weather, which a heat pump can’t promise.

• Natural gas needs a mains supply run to the heater. Where you have it, it’s usually the cheaper of the two.
• Propane (LPG) works anywhere, including off-grid, but you need a tank or cylinders, somewhere safe to put them, and regular refills. It usually costs more per unit of heat.

Both burn fuel, so both produce combustion gases. Every gas heater needs a proper flue and ventilation as the manufacturer specifies, and it shouldn’t sit in an enclosed space unless it’s designed for that. In the UK, the law says all gas work must be done by a Gas Safe registered engineer. In the USA, use a licensed gas fitter and get a permit. This isn’t a DIY job.

Gas makes sense if speed matters most, if you only fire the tub up now and then, or if you’re in the USA with cheap natural gas. My guide to hot tub gas heaters goes into more detail.

Heat exchangers: using heat you already make

A heat exchanger lets you borrow heat from your home’s boiler or heating system. Hot water from the boiler flows through one side of the exchanger, and your hot tub water flows through the other. The two never mix. The tub water simply picks up the heat.

They’re cheap for the amount of heat they can deliver, and if your boiler is on anyway, you’re not paying twice for the heat. The catch is that they only work when the boiler is heating, and in summer you may not want it running. So a heat exchanger is usually a second heat source rather than the only one. You’ll need a heating engineer to connect it, plus a pump and controls on the boiler side. See Can I heat my hot tub from the boiler in my home?

Wood-fired heaters and fire pit coils

People often ask me about wood. I’d only recommend it for off-grid setups where there’s no reliable power. The issues are:

• No power often means no pumps. No pumps means no jets and no filtration. Without filtration, you’ve basically got a bath that you empty after each use.
• No thermostat. You rely on the fire to get the water hot and keep it there without cooking you. Believe me, that’s a challenge.
• Time and effort. A big tub takes a long time to heat on a fire, and building one every time you want a soak gets old quickly (for me included).

A fire pit coil is a step up. A pump circulates the water through a coil in the fire and back to the tub, so you can add a filter and keep the water cleaner. It’s still hands-on, and tending a fire while you relax in the tub isn’t always relaxing, but it works well as an auxiliary heater. For a real example, see my off-grid wood-fired hot tub review.

Solar

Every so often I’m asked about solar. There are two ways to use it:

• Solar PV (panels that make electricity) can run your spa pack or, better, a heat pump. A heat pump turns every kWh from your panels into several kWh of heat. Running a hot tub from batteries overnight isn’t realistic for most people, so think of solar as cutting your daytime bill rather than taking you off-grid.
• Solar thermal (panels that heat water) can feed the tub through a heat exchanger. It’s cheap to run but depends entirely on the sun, so it’s a top-up, not a main heater, in most of the UK.

More in Can I heat a hot tub with solar power? and, for the free-heat ideas, How to heat a hot tub for free. If you have room for ground loops, a ground source heat pump is another option.

Side-by-side comparison

Method	Upfront cost	Running cost	Heat-up speed	Best for
Spa pack element	Included	Highest	Slow on big tubs	Small tubs, backup heat
Air source heat pump	Medium to high	Low	Fast if sized well; slower in cold air	Most DIY tubs, regular use
Natural gas	Medium to high (gas line, flue)	Low to medium	Fastest	Occasional use, speed, cheap gas areas
Propane	Medium to high (tank)	Medium to high	Fastest	Speed with no mains gas; off-grid
Heat exchanger	Low to medium	Depends on your boiler	Fast while the boiler runs	Second heat source in winter
Wood / fire pit coil	Low	Low (plus your time)	Slow, hands-on	Off-grid tubs
Solar	High	Very low	Weather dependent	Top-up, or powering a heat pump

So which would I choose?

For most DIY tubs that get used every week, I’d fit an air source heat pump, sized generously, and keep the spa pack element as a backup. That’s what I did, and I wouldn’t go back. If you only use the tub now and then and want it hot fast, or you’re in a part of the USA with cheap natural gas, a gas heater is a strong choice. If you’re off-grid, look at wood, a fire pit coil or propane.

Keep the heat in first

Whatever heater you choose, the cheapest heat is the heat you don’t lose. I didn’t insulate my own tub when I built it, and it’s the one thing I’d change. On a new build, insulate the floor and walls (DIY hot tub insulation), consider ICF walls, which come insulated (TubBlox kits), and get a good, well-fitting cover (the ultimate guide to hot tub covers). Wind shelter, a warm location and how you use the tub all matter too. There’s more in hot tub heating and energy efficiency.

Common mistakes

• Relying on a 2–3kW element for a big DIY tub. It works, but slowly, and it’s expensive.
• Buying a small heat pump. A 5kW unit in winter may be no faster than your element. Go bigger.
• Ignoring flow. Every heater needs water moving through it. Check the heat pump’s flow requirement against your pump.
• DIY gas work. Don’t. Gas needs a registered engineer, a flue and ventilation.
• Skipping insulation and paying for it every month afterwards.

Planning your own build?

Heating is much easier to get right on paper than to retrofit. My DIY hot tub plans include the plumbing layout, control room and parts list, so you can plan the heat pump or gas heater connections, the insulation and the electrics from day one.

If your tub is custom, I can draw up an architectural CAD and 3D plumbing design with the heating circuit included.

FAQs

What is the cheapest way to heat a hot tub?

For most people, an air source heat pump. At the UK price cap for late 2026, it delivers heat for roughly 7–11p per kWh against 26p for an electric element. In the USA, natural gas can be similarly cheap, depending on your local price and the season.

What is the fastest way to heat a hot tub?

Gas (natural gas or propane). A large, well-sized heat pump can also be quick in mild weather. Mine heats my 2,800-litre tub about 8 times faster than the old 3kW element did, but that’s because it’s a much bigger heater.

Do I still need a spa pack if I use a heat pump or gas heater?

Yes. The spa pack runs the pumps, filtration and controls. The extra heater sits in the plumbing after it, and most people keep the spa pack element as a backup.

Does a heat pump work in winter?

Yes, but it produces less heat and runs less efficiently as the air gets colder, and each model has a minimum operating temperature. Size it for your coldest months, and keep the element as a boost if you need it.

Can I install a gas hot tub heater myself?

You can do the plumbing, but the gas connection must be done by a Gas Safe registered engineer in the UK, or a licensed fitter with a permit in the USA. It also needs a proper flue.

Got a question about heating your own tub? Drop it in the comments below. I read them all.

Cheers,
Andi

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Thanks - Andi

 

Hi, Andi here. I own Buildahottub.com and also write all of the articles and info pages on the site. Some years back now, I built my own hot tub but struggled to find the information I needed. So, once my tub was complete, I started this website to help others in their own pursuit of hot tub and plunge pools DIY building information.

Fast forward to 2025, I've helped over 1400+ DIY customers just like you all over the world build hot tubs and pools. Have a good look around the site, there are lots of resources here. Please do get in touch if I can help you. - Cheers, Andi