Can You Heat a Hot Tub for Free? Waste Heat, Solar and the Honest Truth

Last updated: September 2026

Can you really heat a hot tub for free? Sometimes, yes, if you’ve got heat going to waste. My customer Ben does exactly that, and I’ll show you how below. But let’s be honest from the start: for most people there’s no free lunch. Solar panels, solar thermal collectors and wood all cost money or time somewhere along the line, and your boiler certainly isn’t free.

So in this post I’ll show you Ben’s setup, then go through every “free heat” idea I get asked about, with what it really costs and whether it’s worth doing. Spoiler: the cheapest heat of all is the heat you don’t lose.

How Ben heats his hot tub for free

First things first: it’s not with the wood-fired burner you might have seen on the video thumbnail. That’s definitely not how Ben heats his tub for free.

Ben lives somewhere quite remote and is very much off-grid. He runs a diesel generator to make the electricity for his home, so it’s running all the time. Like any engine, the generator makes a lot of heat as a by-product, and it’s cooled by liquid coolant. Normally that heat is simply thrown away through the radiator.

What we did with his system was divert that coolant through a heat exchanger. The hot coolant goes round one side of the exchanger and heats it up. The hot tub water is pumped through the other side, picks up the heat and goes back to the tub. Because the generator runs all the time, the heat is there all the time, and it costs him nothing extra because it’s a by-product of electricity he was making anyway. Ben explains his setup himself in the video above.

How a heat exchanger works

A heat exchanger has two separate circuits. Hot water or coolant flows through one, and your hot tub water flows through the other. Heat passes through the metal walls between them, but the two liquids never mix. The hot tub water simply picks up the heat as it passes through and goes back to the tub warmer.

How much heat you get depends on the heat source. A heat exchanger doesn’t make heat. It moves it. On a boiler, it can put far more heat into the water than a spa pack element, which is why they’re popular. On a waste heat source, you get whatever that source has spare. I’ve covered sizing and the different types in detail in Can I heat my hot tub from my home boiler?

If you want to copy Ben

Waste heat from a generator (or any engine or process that runs anyway) is about as close to free heat as it gets. But there are some important catches, and this is not one to bodge:

• It’s only free if the engine is running anyway. Running a diesel generator to heat a hot tub is one of the most expensive ways to do it.
• Don’t compromise the engine’s cooling. The engine still has to be able to dump all its heat when the tub is up to temperature, so the radiator stays in the circuit. Get whoever services the generator to design the connection, and follow the manufacturer’s guidance.
• Engine coolant is antifreeze, which you don’t want anywhere near your tub water. Use a good-quality exchanger, check it regularly for leaks, and keep the two circuits completely separate. For chlorinated, bromine or salt water, a titanium exchanger is the fit-and-forget choice.
• Control it. A constant heat source will keep heating the tub. You need a thermostat and valve (or a pump that stops) so the water doesn’t go above 40°C (104°F), the maximum the US Consumer Product Safety Commission recommends.
• The pumps still use a little electricity. Small, but not zero.

For more on living without mains power, see Off-grid hot tub: luxury in the wild or impossible dream?

Other “free heat” ideas, honestly

Solar thermal: cheap to run, not free to buy

Solar thermal collectors heat water directly. Pool-style collectors pump the water through panels on a roof or frame and back again. The sun is free, but the kit isn’t. The US Department of Energy puts an installed solar pool heating system at $2,500–4,000, with a payback of one to seven years depending on your fuel costs and sunshine. In the UK, the Energy Saving Trust puts a typical domestic solar water heating system at around £5,300, providing around 90% of a home’s hot water in summer but only around 25% in winter.

A hot tub makes it harder still. The DOE notes that unglazed collectors suit warmer climates, while glazed systems are the ones that work year-round in colder areas. A hot tub at 38–40°C is much hotter than a pool, so the panels lose more heat. You’d want glazed or evacuated-tube collectors, a controller that only sends water through them when they’re hotter than the tub, and a heat exchanger or a dedicated loop. In most of the UK, think of solar thermal as a summer top-up alongside another heater, not your only heat source. The DIY coil-of-black-hose trick does warm water on a sunny day, but it’s a small, slow top-up and does nothing on a grey one.

Solar PV: free electricity, after you’ve paid for it

Solar panels make electricity, and that electricity can run your spa pack or, better, a heat pump. The Energy Saving Trust says an average home system costs around £7,600 and typically pays for itself in 9–12 years. So the heat isn’t free. You’ve paid for it up front.

There’s another catch. If you’d otherwise export your spare solar power, you’d be paid for it. Every kWh you put into the tub instead is export income you’re not getting. It’s still usually a good use of daytime surplus, especially through a heat pump, which turns each kWh from your panels into several kWh of heat. But running a hot tub from batteries overnight isn’t realistic for most people. I’ve gone into this in Can I heat a hot tub with solar power?

Your home boiler: cheaper, not free

People often tell me they’ll heat the tub “for free” from the boiler because it’s on anyway. I’m afraid it doesn’t work like that. The boiler burns extra gas for every kWh that goes into the tub. What saves money is that gas is much cheaper than electricity. At the Ofgem price cap for October to December 2026, gas is 7.97p per kWh against 26.32p for electricity. The details are in Can I heat my hot tub from my home boiler?

Wood: free fuel, if you have it, and your time

If you’ve got your own supply of seasoned wood, a wood-fired stove or a fire pit coil really can heat a tub without an energy bill. But it’s hands-on, there’s no thermostat, and building a fire every time you want a soak gets old quickly. You also need somewhere safe for it and to follow any local smoke rules. See my wood-fired hot tub review for what it’s really like.

Ground source heat pumps

If you’ve got room for ground loops, a ground source heat pump is very efficient, but the installation is expensive. It’s only really worth it if you’re putting one in for the house anyway. See Can you heat a hot tub with a ground source heat pump?

Free heat options compared

Option	Upfront cost	Running cost	Is it really free?
Waste heat (generator, process)	Low to medium (exchanger, pump, controls)	Almost nothing	Yes, if the source runs anyway
Solar thermal	Medium to high	Very low (a pump)	After payback, and only when it’s sunny
Solar PV	High	Lost export income	No, but daytime surplus is cheap heat
Home boiler	Medium (zone, exchanger, engineer)	Gas price	No
Wood	Low to medium	Your wood and your time	Only with your own wood
Air source heat pump	Medium	Low	No, but it’s the cheapest heat for most people

The closest most of us get: cheap heat

For most people, the realistic goal isn’t free heat. It’s cheap heat, and not wasting it.

• Keep the heat in. A good, well-fitting cover is the biggest single saving. For pools, the US DOE says evaporation is by far the largest source of energy loss and that covers can save 50–70% of heating costs. Add insulation and shelter from the wind. See hot tub heating and energy efficiency and DIY hot tub insulation.
• Fit an air source heat pump. It moves heat from the air into the water, giving several kWh of heat for each kWh of electricity. On my own tub it cut my running costs by 66%. The numbers are in How I cut my hot tub running costs by 66%, and the full guide is the ultimate guide to air source heat pumps for hot tubs. For choosing one, see How to choose the right air source heat pump.
• Compare all the options with today’s prices in What is the best way to heat a DIY hot tub?

You’ll find heat pumps and connection kits at parts4tubs.co.uk, and pumps, valves and fittings at parts4tubs.com in the USA.

Planning your own build?

If you’re designing a tub around an unusual heat source, whether that’s waste heat, solar or a boiler, it’s much easier to plan the heat exchanger loop, pumps and controls on paper than to retrofit them. My DIY hot tub plans give you the plumbing layout and parts list, and for something truly custom I can draw up an architectural CAD and 3D plumbing design with the heating circuit included.

FAQs

Can you heat a hot tub for free?

Only if you have heat going to waste, such as an off-grid generator that runs anyway. Solar, wood and boilers can make heat cheap, but they all cost money or time somewhere.

Can I heat my hot tub with solar panels?

Yes. Solar PV can power the spa pack or, better, a heat pump, and solar thermal collectors can heat the water directly. Both cost thousands to install, and in the UK they work best as a summer top-up.

Is heating a hot tub from my boiler free?

No. The boiler burns extra gas for every kWh that goes into the tub. It’s cheaper than the electric element because gas costs less per kWh, but it isn’t free.

What is the cheapest way to heat a hot tub?

For most people, a good cover plus an air source heat pump. If you have genuine waste heat, like Ben, a heat exchanger can be cheaper still.

Does a black hose coil really heat a hot tub?

It warms water on a sunny day, but slowly and only a little. It’s a fun top-up in summer, not a heater you can rely on.

Got a crazy idea for heating your tub, like Ben? Tell me about 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