Can I Heat My Hot Tub From My Home Boiler? Heat Exchangers Explained (2026)

Updated September 2026. I’ve rewritten this post with current UK gas and electricity prices, proper heat exchanger sizing from the manufacturer’s data, and what it means if your boiler is being replaced by a heat pump. I’ve also corrected a couple of things I got wrong first time round, including how the savings work.

Can you heat a hot tub from the boiler in your home? Yes, you can. With a heat exchanger and its own heating zone, your gas (or oil, LPG or biomass) boiler can heat your tub water without the two ever mixing. Regular readers know I usually point people towards an air source heat pump, and I use one on my own tub. But if you’ve already got a decent boiler close to where the tub is going, it’s a genuine option, and in 2026 it’s closer on running cost than you might think.

This post covers how it works, what you need, how to size the heat exchanger properly, what it costs to run compared with electric and heat pump heating, and the catches.

Why heat a hot tub from your home boiler?

Money, mostly. Gas is much cheaper per kWh than electricity in the UK, so heat from a gas boiler costs a lot less than heat from the 3kW electric element in a spa pack. And a boiler is powerful. A heat exchanger on a boiler can put far more heat into a tub than an element can, so heat-up times are shorter.

It makes most sense when:

• you have a gas (or oil, LPG or biomass) boiler with spare capacity,
• the tub is reasonably close to the boiler or the heating pipework, and
• you’re upgrading your heating or building new, so the tub can be designed into the system from the start.

How does a heat exchanger work?

It sounds like a complicated bit of kit, but it’s actually quite simple. Most pool and spa exchangers are “shell and tube”. Hot water from your boiler flows through the outer shell, around a bundle of tubes. Your hot tub water flows through the tubes. Heat passes through the tube walls from the boiler water to the tub water. The two circuits stay totally separate. Your chlorinated tub water never gets into your heating system, and your heating water (with its inhibitor) never gets into the tub.

If you want to see how the whole arrangement fits together, with the zone valve, circulator, thermostat and the spa heater in series, have a look at the diagram in my post on heating a hot tub with a ground source heat pump. It’s the same set-up, just with a heat pump in place of the boiler.

Does it really save money?

When I first wrote this, I said that because you’re already paying to heat water for the house, the heat exchanger lets you tap into heat you’re already paying for, so you don’t pay twice. I did say that was a bit simplistic. Truthfully, it’s wrong. The boiler has to burn extra gas for every kWh that goes into the tub. What saves you money is that gas is much cheaper per unit of heat than electricity.

Here’s how that looks at the UK Ofgem price cap for 1 October to 31 December 2026 (direct debit averages): gas 7.97p per kWh and electricity 26.32p per kWh.

Heat source	Efficiency (assumed)	Cost per kWh of heat in the tub
Spa pack electric element	100%	about 26p
Gas boiler via heat exchanger	85–90%	about 9p
Air source heat pump, cold weather	COP 3 (300%)	about 9p
Air source heat pump, mild weather	COP 5 (500%)	about 5p

These are my own sums, before pipe losses. The US Department of Energy puts pool heat pump COPs at 3–7, dropping below about 10°C (50°F) air temperature. So in the depths of winter a gas boiler and a heat pump cost about the same per kWh, and in spring, summer and autumn the heat pump wins. Both are roughly a third of the cost of the element in your spa pack.

To put a number on it: my tub holds about 2,800 litres. Heating it from 10°C to 38°C takes about 91kWh (1.16kWh per 1,000 litres per °C). That’s roughly £24 on the electric element, or around £8 through a gas boiler, before losses. In the USA, work it out the same way from your own gas and electricity bills. Prices vary a lot from state to state.

The bigger saving over a year comes from keeping the heat in. A good cover and an insulated shell reduce what any heater has to do. See DIY hot tub insulation and my guide to hot tub covers.

What you need

1. A boiler with spare capacity

You need a source of hot water circulating in a sealed heating circuit, just like the water that heats your radiators. That can be a gas, oil or LPG boiler, or a biomass boiler. Domestic boilers are often around 30kW, but what matters is how much of that is spare when you want to heat the tub.

If your boiler is working hard all winter to keep the house warm, there won’t be much left for the tub. If it’s barely ticking over, there’s plenty. A heating engineer can work this out from your home’s heat loss and the boiler’s output.

Combi boilers: a combi puts hot water for your taps first. When someone runs a shower, the heating (and the tub zone) pauses. That’s fine for a tub, but factor it in. A system or regular boiler with a cylinder is simpler to add a zone to.

2. A dedicated zone

The tub needs its own heating zone, taken directly off the boiler’s flow and return. Don’t run it through the underfloor heating first. Underfloor heating runs at low temperatures, and by the time the water gets to the tub there’s very little heat left in it. The zone should be able to run on its own, so the tub can heat in summer when the house heating is off.

The zone is made up of:

• A motorised zone valve. When I first wrote this I called it a fancy name for a diverter valve. It isn’t quite. A two-port zone valve is simply an on/off valve that opens when the tub calls for heat and closes when it’s up to temperature. A three-port diverter valve is a different thing. Your engineer will choose the right arrangement for your system.
• A circulating pump on the boiler side, if the existing one can’t push enough flow to the exchanger. Exchanger outputs depend heavily on how much boiler water flows through them (more on that below).
• A thermostat with a sensor in the tub water. It opens the zone valve and fires the boiler until the tub is at temperature. The sensor sits in a sensor pocket (a thermowell) in the tub pipework, so the probe reads the water without any leaks. You can also let the spa pack do the calling. See How to control a heat pump with any hot tub spa pack, as the same relay idea works for a boiler zone.
• Insulated pipework between the house and the tub. A long uninsulated run across the garden heats the garden. Use pre-insulated underground pipe on buried runs.

3. The right heat exchanger

For a hot tub I’d use a titanium shell and tube exchanger. Bowman, one of the best-known makers, says its titanium models come with a 10-year guarantee on the parts in contact with pool water, and that its stainless steel version is not for salt water pools. Titanium costs more, but it’s the “fit and forget” option with chlorine, bromine or salt. Bowman also makes a range specifically for spas and hot tubs, and says it works with gas, LPG and biomass boilers as well as heat pumps.

4. A bypass on the tub side

Pool heat exchangers have a maximum water flow. Bowman’s spa models list 200 litres per minute. A big jet pump can push more than that. Plumb the exchanger on the filtration or circulation loop (after the filter, before the spa pack heater), not on a jet pump, and fit valves so you can set a bypass and let any excess flow go round it. Isolation valves also let you take the exchanger out for servicing without draining the tub.

Sizing the heat exchanger

This is where most people (including me, first time round) go wrong.

Read the rating conditions

Exchanger outputs are quoted at specific temperatures. Bowman’s data sheets rate them with pool water at 28°C and boiler water at 82°C, 70°C, 60°C or 45°C, at different boiler water flow rates. Here’s the smaller spa model from its EC80 data sheet and the bigger one from its EC100 data sheet, at 60 litres per minute of boiler water:

Boiler water temperature	Bowman EC80 (small spa)	Bowman EC100 (large spa)
82°C	26kW	50kW
70°C	20kW	35kW
60°C	15kW	26kW
45°C (heat pump territory)	8kW	14kW

All of those are with the pool water at 28°C. A hot tub sits at 38–40°C, so the gap between the boiler water and the tub water is smaller, and you get less heat out. As a rough guide, the output falls in proportion to that gap. At 82°C boiler water, a 40°C tub gets roughly three-quarters of the rated figure. At 60°C, it’s nearer 60%. At 45°C, there’s only 5°C of difference, and you get very little. Check the maker’s chart or ask them to size it for your tub temperature.

Mind your boiler’s flow temperature

Modern condensing boilers are most efficient when they run cooler. The UK Money Saving Boiler Challenge recommends a flow temperature of 60°C or below so the boiler stays in condensing mode. That’s a real trade-off. Run the boiler hot and the exchanger gives more, but the boiler is less efficient. Run it at 60°C and you need a bigger exchanger for the same output. For a hot tub, I’d plan around 60–70°C and size the exchanger for it.

Work out how much heat you need

When I first wrote this, I used a rule of thumb of 1,000 BTU per square foot of water surface, plus 20%. For a big 4.5m x 2.5m (15ft x 8ft), 11,200 litre tub I was working on, that gave about 144,000 BTU/h (42kW), and I picked a 49kW exchanger. I’d now size it from the volume instead, because that tells you the heat-up time:

• It takes about 1.16kWh to raise 1,000 litres by 1°C (about 8.34 BTU per US gallon per °F).
• That 11,200 litre tub needs about 13kWh per degree, so about 365kWh to heat from 10°C to 38°C.
• At 60°C boiler water, the larger Bowman gives about 26kW at pool temperatures, and very roughly 16kW at hot tub temperatures. That’s about 1.2°C an hour, or a day to heat from cold.
• At 82°C, it’s nearer 39kW at tub temperature, or about 3°C an hour.

In the original post I also said that at 50°C boiler water, the exchanger would give 52kW. That was a misreading of the chart. The manufacturer’s figures show much less than that at low boiler temperatures, and less again with a hot tub rather than a pool. For a normal 1,500–3,000 litre DIY tub, the smaller spa model at 60–70°C is usually plenty. My heater size guide has the same maths for any heater.

What if your boiler is being replaced by a heat pump?

In the UK, the government is pushing homes away from gas boilers towards heat pumps. The Boiler Upgrade Scheme gives £7,500 towards an air source or ground source heat pump that replaces your boiler, with an extra £1,500 until March 2027 for homes on oil or LPG with no mains gas.

If you’re doing that, a house heat pump can heat your tub through the same kind of exchanger. But look at the 45°C row in the table above. Heat pumps run at low flow temperatures, so you need a much bigger exchanger, and pushing the house heat pump hotter to suit the tub costs it efficiency. I’ve gone through all of this, including why the grant won’t pay for a heat pump that’s just for your tub, in Can you heat a hot tub with a ground source heat pump?

If you’re getting rid of the boiler anyway, a small dedicated air source heat pump on the tub is usually simpler. It plumbs straight into the tub loop and doesn’t compete with the house. That’s what I did. See Why I fitted an air source heat pump to my hot tub and How I saved 66% on my hot tub electricity cost.

Boiler vs the alternatives

	Home boiler + exchanger	Dedicated air source heat pump	Spa pack element
Running cost	Low (gas)	Low, lowest in mild weather	High
Heat-up speed	Fast, if the boiler has spare capacity	Fast in mild weather, slower in the cold	Slow (3kW is about 1°C an hour on 2,800 litres)
Installation	Heating engineer, pipe run from the house	Plumbs into the tub loop, electrics by a professional	Built into the pack
Best for	Tubs near a boiler with spare capacity	Most DIY tubs	Backup and small tubs

If you’re in the USA, gas heating usually means a dedicated gas spa heater rather than a home boiler, as many American homes have forced-air furnaces rather than wet heating systems. I’ve covered that in Everything you need to know about hot tub gas heaters. For every option side by side, read What is the best method to heat my DIY hot tub? and Hot tub heaters explained.

Common mistakes

• Sizing from the pool rating. The headline kW is at 28°C pool water and 82°C boiler water. A 40°C tub with a 60°C boiler gets far less.
• Taking the feed after the underfloor heating instead of directly from the boiler.
• No independent zone, so the tub only heats when the house does, or never stops heating.
• A stainless or cupro-nickel exchanger with salt water. Use titanium.
• Putting the exchanger on a jet pump line with no bypass, pushing more flow than it’s rated for.
• Uninsulated pipe runs across the garden.
• Removing the spa pack heater. Keep it for summer, servicing and backup.
• DIY work on the boiler. In the UK, anything that touches a gas appliance must be done by a Gas Safe registered engineer.

Troubleshooting

• Tub heats very slowly: check the boiler flow temperature and the boiler-side flow rate (a weak circulator starves the exchanger), and that the zone valve is actually opening. Check the house isn’t taking all the heat at the same time.
• Tub doesn’t heat at all: is the tub pump running? The exchanger only heats water that’s moving through it. Check the thermostat, the sensor and the zone valve.
• Tub overheats: the zone valve isn’t closing, or the thermostat sensor is in the wrong place (it should read the tub water, not the boiler water).
• Spa pack flow error: the bypass is set so too little water reaches the spa heater. See Hot tub FLO error and Help! My hot tub is not heating.

Planning a new build?

If you’re building a tub from scratch, decide how you’re heating it at the design stage. It affects where the control room goes, the pipe sizes and how the pumps are plumbed. My DIY hot tub plans include the plumbing layout and a full parts list. If you want a heat exchanger or heat pump designed into a custom build, my architectural CAD and 3D plumbing design service can draw it in for you.

For valves, heaters and spa packs, head to parts4tubs.co.uk in the UK or parts4tubs.com in the USA.

FAQs

Can I heat my hot tub with my central heating?

Yes. A heat exchanger on its own zone off the boiler heats the tub water, with the two circuits kept separate. A heating engineer should check the boiler has spare capacity and fit the zone.

Is it cheaper to heat a hot tub with gas or electricity?

At the October 2026 UK price cap, heat from a gas boiler costs about 9p per kWh, against about 26p from a spa pack’s electric element. An air source heat pump is about level with gas in winter and cheaper in milder weather.

What size heat exchanger do I need for a hot tub?

Size it from your tub’s volume, the heat-up time you want and your boiler’s flow temperature, remembering ratings are quoted for 28°C pool water. For a typical DIY tub, a small spa-rated titanium model is usually enough at 60–70°C boiler water.

Can I use a combi boiler?

Yes, from the heating circuit, as its own zone. The tub zone will pause whenever someone runs hot water.

Why titanium?

It resists chlorine, bromine and salt. Stainless steel isn’t suitable for salt water, and titanium models typically carry longer warranties.

Have you heated your tub from your boiler? Let me know how it went in the comments below. I read them all.

Cheers,
Andi

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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