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

With so many air source heat pumps on the market right now, 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.

But here’s the good news — once you know what to look for, it’s really not that complicated. In this article, I’m going to walk you through the key things you need to consider before you part with your cash. By the end, you’ll know exactly what questions to ask and what to look out for.

Let’s get into it.

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1. Know Your Volume of Water {#volume}

Before you do anything else, you need to know how much water your pool or hot tub actually holds. This is the single most important number when it comes to sizing your heat pump correctly.

A 5 kW unit is perfectly fine for a hot tub. It is absolutely not fine for 50,000 gallons of pool water. Get the sizing wrong and you’ll either be waiting days for your pool to heat up, or you’ll be running an oversized unit that’s costing you far more than it should.

You also need to factor in how warm you actually want to get the water. Heating a hot tub to 40°C is a very different ask to warming a pool to a comfortable 28°C.

My tip: If you’re not sure where to start with sizing, open up a browser, head to ChatGPT, and ask: “What size air source heat pump do I need for X litres/gallons of water?” It’ll give you a solid ballpark figure to work from.

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2. When Are You Going to Use It? {#when}

This one catches a lot of people out. Are you planning to use your heat pump all year round, or just in the warmer months?

It matters because air source heat pumps are rated to operate down to a certain ambient temperature. Some units are only rated to work effectively down to around 10–12°C. Below that, the unit struggles to harvest heat from the air efficiently. It won’t just switch off — but in the worst case, you’ll basically get no gain at all. Every kilowatt you put in, you’ll get roughly one kilowatt of heat out. You lose all of the efficiency benefit that makes a heat pump worth having in the first place.

So if you’re in the UK and you want year-round heating — and realistically, that’s most of us — you need to make sure the unit you choose is rated for low ambient temperatures. Some heat pumps will operate efficiently down to -10°C or even lower, but you need to check the spec sheet rather than take the marketing copy at face value.

Match the temperature rating of the unit to the conditions you’ll actually be using it in.

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3. Inverter vs Non-Inverter {#inverter}

This is probably the most technical bit, but stick with me — it’s worth understanding.

A non-inverter heat pump is essentially on/off. It runs at 100% output or it doesn’t run at all.

An inverter heat pump is smarter than that. It can vary the speed of the compressor motor — running at say 60%, 70%, or 80% of its maximum output — rather than being flat out all the time. The clever bit is that the inverter technology is constantly calculating the most efficient output level based on the ambient temperature, the relative humidity, and the target water temperature. It’s adapting in real time to squeeze the most heat out of the air for the least amount of electricity.

The result? An inverter heat pump will generally be cheaper to run than a non-inverter equivalent, because it’s not burning energy it doesn’t need to.

If you’re looking at two heat pumps at a similar price point and one has inverter technology and one doesn’t, I know which one I’d be going for.

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4. Understanding COP — Coefficient of Performance {#cop}

COP stands for Coefficient of Performance, and you’ll see this number plastered all over heat pump spec sheets and marketing materials. It’s basically a measure of efficiency — a multiplier that tells you how much heat you get out for every unit of electricity you put in.

A COP of 5 means for every 1 kW of electricity you use, you get 5 kW of heat output. That’s a great return.

Here’s where you need to be careful though. Manufacturers are required to state COP at a specific set of conditions, and those conditions are almost always the most favourable ones — typically something like 26°C ambient air temperature and warm water. That’s a great COP figure. It’s also pretty much useless if you live in the UK.

When it’s 15°C outside — which is fairly average for a UK spring or autumn — the COP is going to be noticeably lower than the headline figure. When it’s 5°C outside, it’ll be lower still.

The good manufacturers will publish a full performance table in their spec sheet showing COP at different ambient temperatures. That’s the document you want to find. Don’t just look at the headline COP. Find out what the unit actually delivers at the temperatures you’re going to be running it in.

It’s a bit more legwork, but it makes a real difference when you’re comparing units side by side.

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5. Budget {#budget}

Finally, let’s talk money. Air source heat pumps vary enormously in price, and — just like cars — brand plays a big part in that.

There is a certain amount of brand snobbery in the heat pump market, exactly the same as you’d find with any other piece of kit. Some of the premium brands are genuinely excellent. Some of the budget options are surprisingly good too. The key is not to buy blind.

Here’s what I’d actually look at:

• The spec sheet — does it tell you what you need to know, or is it vague?
• The warranty — how long is it, and importantly, who honours it? Is there UK-based warranty support?
• The manufacturer’s track record — how long have they been making heat pumps?
• Reviews from real customers — not just the ones on the product listing

And don’t feel rushed into a decision. This is a considered purchase. Take your time, do your research, ask questions — and if you want a steer on specific brands or models, feel free to get in touch.

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Wrapping Up

So to summarise, here are the five things you need to nail down before you buy an air source heat pump:

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’ll save you 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

Can I Help You?

If I can help you in any way I would love to hear from you. You can get in touch using the form below.

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