How to Choose the Right Air Source Heat Pump for Your Hot Tub or Pool

Last updated: September 2026

With so many air source heat pumps on the market, choosing the right one can feel overwhelming. There are dozens of brands, hundreds of models, and enough jargon in the spec sheets to make your head spin.

The good news is that once you know what to look for, it’s really not that complicated. In this article I’m going to walk you through the five things to nail down before you part with your cash, so you know what questions to ask.

1. Know your volume of water

Before anything else, work out how much water your pool or hot tub holds. It’s the single most important number when sizing a heat pump. A 7kW unit is perfectly fine for a small hot tub, and absolutely not fine for 50,000 gallons of pool water. Get it wrong and you’ll either wait days for your pool to warm up, or run an oversized unit that costs more than it should.

You also need to consider the temperature you’re aiming for. Heating a hot tub to 40°C (104°F) is a very different job to keeping a pool at a comfortable 28°C (82°F). Here are rules of thumb for hot tubs:

Hot tub volume	Typical heat pump size	Notes
Up to about 1,500 litres (400 gal)	7kW	My friend Paul’s 1,200 litre tub runs on 7kW
1,500 to 3,000 litres (400 to 800 gal)	12kW or more	A bigger unit heats up faster. Mine is a Comfortline listed as 20kW (formerly 21kW) on a tub of nearly 3,000 litres
Over 3,000 litres or a swim spa	16 to 24kW, or ask for advice	Wind, cover and insulation matter more

Any heat pump of 7kW or above will heat a hot tub. A bigger unit doesn’t use more electricity to hold the temperature, it just gets there quicker. If you don’t know your volume, work it out from the tub’s dimensions, or see the heater size guide. Pools depend far more on cover, wind and the target temperature, so ask the supplier for a sizing check before you buy.

2. When are you going to use it?

Are you planning to use it all year, or just in the warmer months? It matters because heat pumps are rated to work down to a certain air temperature. Some units are only rated down to around 10 to 12°C. Below that they struggle to collect heat from the air. They won’t just switch off, but in the worst case you get almost no gain: every kilowatt you put in gives you roughly one kilowatt of heat out, and you lose the efficiency that makes a heat pump worth having.

If you’re in the UK and want year-round heating (and realistically that’s most of us) choose a unit rated for low air temperatures. Some heat pumps work efficiently down to -10°C or lower, but read the spec sheet rather than trusting the marketing. For a spring-to-autumn tub, a unit rated to +10°C is fine. I ran my own, which is rated to 0°C, in sub-zero temperatures and it still worked (see the -4°C results), but don’t count on that from a different make.

3. Inverter or non-inverter?

A non-inverter heat pump is essentially on or off: 100% output, or nothing. An inverter unit varies the compressor speed, so it might run at 60, 70 or 80% of its maximum instead of flat out. It’s constantly working out the most efficient output for the air temperature, the humidity and the water temperature you’ve asked for.

An inverter heat pump will generally be cheaper to run because it isn’t burning energy it doesn’t need to. If two heat pumps are at a similar price and only one is an inverter, I know which one I’d choose. Inverter units also tend to have a soft start, which avoids the current spike that can trip your breaker. See my ultimate guide to air source heat pumps for more on this.

4. Understanding COP (coefficient of performance)

COP shows up all over spec sheets. It’s a measure of efficiency: how much heat you get out for each unit of electricity you put in. A COP of 5 means 1kW of electricity gives 5kW of heat.

The catch is that manufacturers quote it under the most favourable conditions, typically 26°C air and warm water. That’s a great number, and pretty much useless in the UK. At 15°C, which is about average for a British spring or autumn, the output and COP are noticeably lower, and at 5°C they’re lower again. I’ve seen output at 15°C run around 40% below the 26°C headline.

Good manufacturers publish a full performance table showing COP and heat output at different air temperatures. That’s the document you want. Find out what the unit delivers at the temperatures you’ll actually use it in. It’s a bit more legwork, but it makes comparing units side by side much fairer. As a rough guide, a well-chosen unit should give about 3 to 6kW of heat per 1kW of electricity at typical UK temperatures.

5. Budget, warranty and support

Heat pump prices vary enormously, and just like cars, the brand has a lot to do with it. There’s some brand snobbery in this market. Some premium brands are genuinely excellent, and some budget options are surprisingly good, so don’t buy blind. Here’s what I’d check:

• The spec sheet. Does it tell you what you need to know, or is it vague?
• The warranty. How long is it, and who honours it? Is there UK-based (or US-based) support?
• The manufacturer’s track record. How long have they been making heat pumps?
• Real customer reviews, not only the ones on the product listing.
• Noise. Look for the decibel rating, and see how noisy is an air source heat pump.
• Power supply. Most home units are single phase, but check the electrical requirements before you buy.

Don’t feel rushed. This is a considered purchase. If you want a steer on specific brands or models, feel free to get in touch. I’ve also reviewed several units: the Comfortline, the Hydro Royal, the Vian Power and the Onda Mini.

Wrapping up

1. Volume: size the unit correctly for your pool or hot tub.
2. Season: make sure it’s rated for the temperatures you’ll be using it in.
3. Inverter: go inverter if you can. It saves money in the long run.
4. COP: look past the headline figure and check performance at real-world temperatures.
5. Budget: shop around, check the warranty and don’t buy on impulse.

Once you’ve chosen, the next step is fitting it. Read how to add an air source heat pump to your hot tub, and if it isn’t heating properly afterwards, see heat pump not heating. My own running costs fell by 66% (the full story is in reduced hot tub costs).

Looking for a heat pump?

I sell air source heat pumps at parts4tubs.co.uk (UK), including the Comfortline 20kW and the Comfortline 7kW with WiFi. In the USA, see parts4tubs.com. Building a new tub? My hot tub plans give you the dimensions, plumbing and a parts list to build around.

FAQs

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

As a rule of thumb, 7kW suits tubs up to about 1,500 litres (400 gallons) and 12kW or more suits tubs up to about 3,000 litres (800 gallons). Bigger units heat up faster.

What is a good COP for a hot tub heat pump?

Ignore the headline figure at 26°C. Look for a COP of around 4 or more at 15°C air, and check the performance table for colder temperatures.

Is an inverter heat pump worth it?

Usually yes. It adapts its output to the conditions, so it typically costs less to run than an on/off unit, and it starts more gently.

Can I use a heat pump all year round?

Yes, if it’s rated for the winter air temperatures where you live. Check the minimum operating temperature and the performance at low temperatures.

Is a cheap heat pump a bad idea?

Not necessarily, but check the low-temperature performance, the warranty length and who supports it. Cheaper units often draw more power and give less heat in the cold.

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