Air Source Heat Pumps for Hot Tubs: The Complete Guide (2026)

Last updated: September 2026

Air source heat pumps (ASHPs) are the biggest change to hot tub heating in years. They do the same job as an electric element, but for every 1kW of electricity they use they can deliver several kW of heat. I fitted one to my own tub and cut my running costs by 66%, so this is a subject I know well.

Fitting one is a straightforward job with minimal extra pipework. The unit can sit close to the tub, or further away if you add the right circulation pump. That makes them ideal for DIY hot tub builders. This guide explains how they work, what to look for when you buy one, and what I actually saw in my own tub.

What is an air source heat pump?

An air source heat pump uses the same vapour compression refrigeration cycle as a fridge or air conditioner, but in reverse. It uses a compressor and a heat exchanger to collect heat from the outside air and transfer it into your tub water. Reversible models can also run the other way, taking heat out of the water and pushing it into the air, so you can chill the tub and use it as a cold plunge in summer.

How does it work?

1. A fan draws outside air over an evaporator coil. Liquid refrigerant inside the coil boils at a very low temperature, so it absorbs heat from the air even when the air feels cold.
2. The compressor squeezes the refrigerant gas, which raises its temperature a lot.
3. The hot gas passes through a condenser (a titanium heat exchanger) where your tub water flows past. The heat passes into the water, and the refrigerant turns back to liquid.
4. An expansion valve drops the pressure again, and the cycle repeats.

Your circulation pump pushes the tub water through the unit. It’s the same water flow that feeds a normal heater, so the unit sits in the plumbing just after the filter. If you haven’t got one already, read my article on adding an air source heat pump to a hot tub and the step-by-step guide.

Advantages and disadvantages

Advantages	Disadvantages
Much cheaper to run than an element (see my numbers below)	Higher upfront cost, and they aren’t cheap kit
Heats up several times faster than a typical 3kW element	Output and efficiency fall as the air gets colder
Low carbon, with no fuel storage or flue	Needs electricity, and the right power supply
Some models can also chill the tub	Can be noisy (see how noisy is an air source heat pump)
Low maintenance and a long lifespan	Not as instant as a gas heater at very low temperatures

Payback depends on your tub, how often you use it and what you pay for electricity. My own worked out at about 10 months at the 2022 prices, but if you only use your tub occasionally, allow a couple of years. For reference, the Ofgem cap for October to December 2026 is 26.32p/kWh (I was paying about 28p in 2022) and the US EIA average is 18.31 cents/kWh, so my 2022 figures are still a fair guide.

Why aren’t all air source heat pumps the same?

Two heat pumps can have the same headline capacity and behave very differently. Think of cars: two 6-litre V8s can look alike and still differ hugely in features, performance and price. Here’s what to check.

Size (heat output)

For a typical DIY tub under 3,000 litres (650 gallons), around 12kW (40,000 BTU) is a good match. My tub holds nearly 3,000 litres, and I run a Comfortline, which is listed as 20kW (formerly 21kW). It’s big for my tub, and that headroom helps in the cold. A smaller tub of about 1,200 litres is happy with a 7kW unit. Any heat pump of 7kW or above will heat a hot tub, but a bigger one heats faster.

Cold weather performance

Some units stop working below 0°C (32°F) and some quote a minimum of +5°C or +10°C. If you’ll use your tub in winter, check the rating. Cheaper units often draw more power and give less heat in the cold, so ask for the performance table at several air temperatures. I ran my own at -4°C and it still worked. Read the results here, but treat that as my experience with one unit, not a promise for every model.

COP: read the small print

COP (coefficient of performance) is heat out divided by electricity in. The headline number is usually measured at 26°C air and 26°C water, which is very flattering. In a British spring or autumn it’s about 15°C, and a heat pump’s output at 15°C air is typically around 40% lower than the flattering figure. Compare units at the 15°C rating. My guide on how to choose the right air source heat pump explains this in detail.

Inverter compressor

A non-inverter unit is either on or off. An inverter unit varies the compressor speed to hold the temperature with the least energy, so it doesn’t waste power running flat out. Installers and manufacturers often quote savings of around a third on running costs, and an inverter is one of the first things I’d look for.

Soft start

A basic compressor draws a big surge of current for a split second when it starts. That spike can trip your breaker and shut down the whole tub. A soft start (which most inverter models have) avoids it. See 9 reasons your hot tub is tripping the breaker if you’re chasing tripping problems.

Defrost

Better units can sense when the coil is icing up and defrost automatically, sometimes by briefly reversing the cycle. Ice on the coil is normal in cold, damp weather. It’s only a problem if it builds and doesn’t clear. See how to defrost a Comfortline.

Warranty and noise

Cheap units often carry a one-year warranty. Ask who honours it in your country. Check the decibel rating too, especially if the unit will sit near a neighbour’s fence.

I’ve reviewed several units if you want real examples: the Comfortline, the Hydro Royal, the Vian Power, the Onda Mini and the Balboa Clim8Zone.

Which pump and controls will I need?

Most heat pumps want a steady flow of water, so a circulation pump running continuously often makes more sense than the main jet pump. If you’ve a single-speed pump, you might need a bypass. Read about that in using an air source with a dual speed pump. To make the heat pump and your spa pack work together, see how to control a heat pump with any spa pack. If you want to compare heat pumps with ground source, look at ground source heat pumps.

My own case study: what did I actually save?

Here are my measured figures. They were recorded in 2022, so I’ve worked out the prices at today’s rates alongside, but the percentages are what matter. Full detail is in reduced hot tub costs and why I fitted an air source heat pump to my hot tub.

My tub holds nearly 3,000 litres (660 gallons). When I built it, I had no insulation in it, which is something I wouldn’t recommend to anyone. It ran on a 3kW electric heater, and after the April 2022 price rises, my weekly running cost was about £100 ($124).

Heat-up costs

Before, I’d turn the tub up from 33°C to 39°C (91.4°F to 102.2°F) on a Thursday night and leave it heating for about 12 hours, using around 36kWh. With the heat pump, the same jump takes about 1 hour 20 minutes at 15°C air, and uses around 10kWh. That’s a 72% saving on the heat-up. At the Ofgem cap for October to December 2026 (26.32p/kWh), that’s about £9.50 before and £2.60 after. At the US EIA average of 18.31 cents/kWh, it’s roughly $6.60 before and $1.85 after.

Holding the temperature

The heat pump also keeps up. With my old element, the tub dropped from 39°C to 36°C in a 90 to 120 minute session. With the heat pump, at about 15°C outside, I saw zero heat loss. I also hold it at 31°C (87.8°F) during the week now, rather than 33°C, because I can heat up so quickly.

The weekly totals (2022 prices)

Weekly cost	3kW electric heater	Air source heat pump
Initial heat-up	£11 ($14)	£2.50 ($3.10)
Weekend usage	£56 ($69)	£16.50 ($20.48)
Keeping it warm in the week	£33 ($41)	£14.48 ($17.97)
Total per week	£100 ($124)	£33.48 ($41.54)

That’s a saving of 66% on my running costs. I worked the maintenance figures out from smart meter readings (average home usage of 22.94kWh a day before, and 35.86kWh a day with the heat pump holding the tub at 31°C), so the numbers rest on real readings, not on the spec sheet.

My friend Paul’s tub

Before fitting mine, I helped my friend Paul fit a 7kW unit to his plastic shell hot tub (1,200 litres). It heated 2.5 times faster than his old 2kW element. At the time, his smart meter showed 93p ($1.16) an hour on the element, and between 22p ($0.27) and 43p ($0.54) an hour on the heat pump, depending on the air temperature. That’s more than half cheaper. You can read how we added it.

My results, at my prices and with my tub, aren’t a guarantee for yours. A well-insulated tub uses less energy whatever heats it. See DIY hot tub insulation.

Common mistakes

• Connecting the flow the wrong way round. The water must go in the “cold” inlet and come out of the “hot” outlet.
• Leaving the bypass open, so the water takes the easy route around the heat pump.
• Trapped air after installation, which shows up as a flow error.
• Buying on headline COP and ignoring the low-temperature rating.
• Undersizing the pipework or the circulation pump.

Every one of those is covered in air source heat pump not heating my hot tub. Also read my heat pump considerations.

Want a heat pump for your tub?

I sell heat pumps and the fittings to go with them at parts4tubs.co.uk (UK), including the Comfortline 20kW that I use and the Comfortline 7kW with WiFi for smaller tubs. In the USA, take a look at parts4tubs.com. If you’d like help with the parts or fitting one to your own tub, get in touch through the site.

Building a new tub? My hot tub plans give you the dimensions, plumbing and a parts list to build around.

FAQs

How much does an air source heat pump save on a hot tub?

It depends on your tub, use and prices. I measured a 66% saving on my weekly running costs compared with a 3kW element, and a friend saw more than half off with a 7kW unit.

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

As a rough guide, 7kW suits small tubs of around 1,200 litres, and 12kW or more suits tubs up to about 3,000 litres (650 gallons). Bigger units heat faster. Any unit of 7kW or above will heat a hot tub.

Do air source heat pumps work in winter?

Yes, but output and efficiency fall as it gets colder, and units have a minimum temperature rating. Check the rating, and look for a model with automatic defrost.

Are heat pumps noisy?

They make about the same noise as a large fridge or a quiet air conditioner. Check the decibel rating, and position the unit away from bedrooms and neighbours. See how noisy is an air source heat pump.

Can I use a heat pump with my existing spa pack?

Yes. You’ll need to connect a suitable control so the spa pack and the heat pump work together. See how to control a heat pump with any spa pack.

Can a heat pump cool my tub?

Only reversible (heating and cooling) models can. If you want a cold plunge in summer, check the unit is described as reversible before you buy.

Got a question about 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