5 Things to Consider Before Fitting an Air Source Heat Pump to Your Hot Tub

Last updated: September 2026

Fitting an air source heat pump to your hot tub is one of the best ways to cut its running costs. Heating a tub on a 3kW electric element alone is expensive for most of us. On my own tub the heat pump saves around 66%.

If you’re weighing one up, read this first. These are the five things I’d want you to think about before you spend any money. I won’t cover the installation itself here, because that’s all in my step-by-step guide to adding an air source heat pump to your hot tub.

1. Is your hot tub under warranty?

Why does this matter? The moment you cut into a pipe or put a hole in the cabinet, you’re probably going to void your warranty. Most manufacturers have a clause saying you can’t modify the tub in any way.

My take is a little different, because I design these for a living. If you’re comfortable adding a heat pump to your hot tub, you’re comfortable servicing it too, so you don’t really need the warranty.

I’ve also seen that a lot of the parts that fail aren’t covered anyway, because the manufacturer finds water somewhere it shouldn’t be:

• A pump fails because the coil has rusted. That’s water where it shouldn’t be.
• A blower fails and trips the breaker. That’s water where it shouldn’t be too. See Water in a hot tub air blower.

Manufacturers have plenty of ways to wriggle out of a claim, so personally I’d go for it and start saving on your running costs. That’s just my opinion, though. Make up your own mind whether you’re comfortable going outside your tub’s warranty. If the tub is only a few months old, waiting until the warranty runs out is a sensible middle ground.

2. What size air source heat pump do I need?

This is the question I get asked most. As a general rule, bigger is better, because bigger means faster heat-up. But bigger doesn’t mean more economical. Across most ranges the efficiency is similar from model to model, so you’re paying for speed, not a cheaper kWh.

My rule is simple: install as big a heat pump as your budget allows. As a minimum:

Water volume	Smallest heat pump I’d fit
1,000–1,500 litres (265–400 US gal)	7kW
1,500–2,000 litres (400–530 US gal)	9kW
2,000–2,500 litres (530–660 US gal)	12–13kW
DIY and in-ground tubs, 2,500 litres and up	12kW or more (mine is 21kW on around 2,700 litres)

That covers most plastic shell hot tubs. These are headline sizes. On a cool day you’ll get less, which is why the next section matters. For a proper walk-through of sizing, see How to choose the right air source heat pump for your hot tub or pool and Determining the perfect heater size for your pool or hot tub.

3. What specifications should I look for?

COP, and the conditions it’s measured at

The COP (coefficient of performance) is the key number. It tells you how much heat you get out for the electricity you put in. A COP of 6 means that for every 1kW of electricity in, you get 6kW of heat out.

The catch is that manufacturers quote COP at the most flattering conditions, typically 26–27°C air and 26°C water. At 15°C air, which is much closer to a typical UK day, both the COP and the output drop. Here’s the Comfortline 7kW as an example, from its spec sheet:

Air temperature	Heat output	COP at full output
26°C (79°F)	7.0kW	10.3
15°C (59°F)	5.0kW	6.0

So when you compare heat pumps, compare them at 15°C. A COP of 5–10 at warm conditions is normal for a good inverter unit. I’m sceptical of headline figures far above that. At 15°C, look for something around 4–6.

Inverter or on/off?

An inverter heat pump varies its speed. It runs flat out to heat up, then throttles back to hold temperature, which is quieter and more efficient. A cheaper on/off unit runs at full power until the water is up to temperature, then stops. Both work, but I’d choose an inverter if the budget stretches.

Operating temperature range

Every heat pump has a minimum air temperature on its spec sheet. Take these with a pinch of salt, but be mindful of your winters. If you live somewhere that sits below freezing for days at a time, you want a unit rated well below 0°C (32°F). In somewhere like the UK, which doesn’t spend long below zero, a unit rated to -15°C (5°F) is overkill.

Don’t assume a heat pump stops dead the moment it’s colder than its rating, and don’t assume it keeps going either. Output and COP fall as the air gets colder, and it’ll spend time defrosting. Below its rated minimum, many units stop to protect themselves until it warms up. Manufacturers won’t guarantee performance there. I’ve tested mine in a cold snap. See I ran my air source at -4°C: the results. If you really need heat in the depths of winter, leaving the electric element available as a backup is an option.

Which brand?

I have access to pretty much every model on the market, and the Comfortline is the one I chose for my own tub. For the US market, I recommend the Hydro Royal range of heat pumps.

You don’t need to match the heat pump to your spa pack brand. Balboa and Gecko would love you to, but you don’t have to. I have a Balboa system and a Comfortline heat pump. Brand-matched units like the Balboa Clim8zone II only work with specific packs and topsides, which rules out a lot of older tubs.

4. Where should I put the heat pump?

Every manufacturer has its own rules for siting, so check your manual. Here are the general guidelines:

• Don’t push it against a wall. It needs air flow all the way round. The Comfortline wants 300mm (12in) at the sides and 500mm (20in) behind and above.
• Give the fan space. Whichever way it blows needs clear space, at least 1m (3ft) and 2m (6.5ft) for the Comfortline. Don’t have it blowing straight onto a wall or fence.
• Don’t box it in. It has to be outside, in the open, so it can pull heat from the air. Inside a wooden box it can’t do its job.
• Keep it close. Ideally within about 5m (16ft) of the tub, so the extra pipe doesn’t kill your flow.
• Think about noise and neighbours. They’re not loud, but see How noisy is an air source heat pump?
• Mind the electrics. Under the UK wiring rules for pools, which the IET applies to outdoor hot tubs, equipment close to the water is restricted. Siting the heat pump more than 2m from the water’s edge keeps things simple.

5. Where will the pipes connect, and how is it powered?

I cover the plumbing in detail in the installation guide. In short, you cut into the pipe from the circulation pump (or pump 1 on low speed) to the heater, ideally just before or after the existing heater. My connection kits have the pipe and fittings you need.

The heat pump is then controlled by water flow. When your spa pack runs the pump to heat, the heat pump senses the flow and heats. That’s why it works with almost any spa pack. I explain it fully in How to control a heat pump with any hot tub spa pack. If your tub only has one two-speed pump, read Can I add a heat pump to a hot tub with a dual speed pump? first.

For power, the heat pump needs a supply with cable and a breaker sized to its own rating, 30mA RCD protection (GFCI in the USA) and an electrician to connect it. If you disconnect the electric heater, the total load often doesn’t go up. A 3kW element draws about 13A, while the Comfortline 12.8kW draws a maximum of 7.74A. See Hot tub wiring explained.

If you’re not sure where yours would connect, get in touch through parts4tubs.co.uk with a photo of the inside of your tub’s cabinet showing the pump and spa pack, and we can advise.

Is it worth it? Costs and savings

On my tub, the heat pump cut my running costs by 66% and heats the water about eight times faster. I’ve shown all the numbers in How I reduced the running costs of my hot tub by 66%. Your savings depend on your tub, climate, cover and tariff, but more than 50% is realistic.

A quick sum: at the Ofgem price cap average of 26.32p per kWh for October to December 2026, an hour of a 3kW element costs about 79p. The same heat from a heat pump running at a COP of 5 costs about 16p.

Common mistakes I see

• Buying on the headline kW and being disappointed in November.
• Hiding the heat pump in an enclosure or a corner with no airflow.
• Siting it at the far end of the garden, then wondering why the flow switch won’t trigger.
• Setting the heat pump and the topside to different temperatures.
• Skimping on the cover. A heat pump can’t make up for heat pouring out of a tired cover. See my guide to hot tub covers.

For the bigger picture, including how heat pumps compare with gas, solar and electric, read my ultimate guide to air source heat pumps for hot tubs.

Can I help you with your air source heat pump?

If you’d like a heat pump for your tub, I have a range of models and connection kits in my online store here. In the USA, see parts4tubs.com. If you’d like some advice on where to connect one, do let us know.

Building a tub from scratch? Plan the heat pump in from the start. My DIY hot tub plans give you the full plumbing layout and a parts list.

FAQs

Will fitting a heat pump void my hot tub warranty?

Almost certainly, because you have to cut into the pipework and usually the cabinet. Check your warranty terms. If the tub is new, you might choose to wait.

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

As a minimum, around 7kW for 1,000–1,500 litres, 9kW for 1,500–2,000 litres and 12–13kW for 2,000–2,500 litres. Bigger heats faster, so go as big as your budget allows.

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

At warm test conditions (26–27°C air), 5–10 is normal for a good inverter unit. At 15°C air, around 4–6 is realistic. Always compare at the same conditions.

Does the heat pump need to be the same brand as my spa pack?

No. A standalone heat pump is controlled by flow, so it works with Balboa, Gecko and most other packs.

Can I put my heat pump in a shed or enclosure?

No. It needs free air around it to work. Keep it outside with the clearances in the manual.

Got a question about adding a heat pump to 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