Why I Fitted an Air Source Heat Pump to My DIY Hot Tub (and How I Plumbed It In)

Last updated: September 2026

If you’ve read this blog or watched my YouTube channel, you’ll know I had no idea how to build a hot tub when I built mine. This website is what I’ve learned since. One thing I got wrong from the start was heating, and this is the story of how I fixed it: why I added an air source heat pump to my own DIY tub, and how I plumbed and wired it in without drilling a single new hole in the shell.

If you’re after the money side, that’s in How I reduced the running costs of my hot tub by 66%. If you have a shop-bought tub, Paul’s install is the better guide. This post is about the why, and the creative plumbing.

Why I needed a better way to heat my tub

It was slow

When I built my tub, John, my supplier, told me I was at the limit of what a 3kW spa pack could heat for a tub that size. I knew that from the outset. Over the years I ran it, the heater managed about 0.5°C an hour on an average day. That meant impromptu dips were out of the question. You had to plan every soak a day ahead.

It was expensive

Before April 2022, the tub cost me roughly £50 a week. Not cheap, I know, but it’s a luxury. After the April 2022 price rises, with more due that October, it was around £100 a week. Ouch.

It also has no insulation, because I didn’t think about it when I built it. I can’t change that now without a rebuild, but I could change how I heat it.

Paul’s numbers sealed it

I’d just helped my friend Paul fit a 7kW heat pump to his hot tub. The texts he sent me afterwards said it was heating about 2.5 times faster and costing roughly half as much on his smart meter. That was it. I had to get one for my tub. You can read about Paul’s install here.

The constraints: renovation, not a new build

Things happen for a reason. I was already in the middle of renovating my control room when I had my lightbulb moment. I woke up the morning after talking to Paul thinking of nothing but ways to add a heat pump to my tub. The numbers were so compelling that it simply had to happen.

But with any renovation, the challenge is what’s already there:

• My tub doesn’t leak. I wasn’t about to core new holes in it for new plumbing.
• I’d already renovated most of the plumbing outside the control room, so I wasn’t ripping that out.
• I have a western red cedar deck, which cost a fortune and would cost even more now. There was no chance I was wrecking that.

So I needed to be creative.

Where the heat pump went

I remodelled a small, messy area to the left of my office, where part of the office air conditioning already sat. Rather than pour a new concrete pad, I built a plinth from timber sleepers to match the rest of the garden.

The heat pump fits in that space, and the pipes run through the greenhouse. I said I’d need to be creative. I was hoping they’d give the greenhouse a bit of warmth in the cooler months too.

The plumbing: a separate heat pump loop

Here’s where it gets unconventional. Normally, each pump should have two lower drains on its suction side. I was going from a one-pump system to a two-pump system without drilling any new drains. So I split what I had:

• Loop 1 (the original dual-speed pump): the skimmer and one main drain feed the dual-speed pump. This loop keeps the electric heater, now on a switch and firmly off most of the time.
• Loop 2 (the new heat pump loop): the second main drain feeds a new pump, then the filter, then the heat pump, then back to the tub.

There was no room in the control room for the new pump, so it sits above ground. That meant it had to be self-priming. I went for a low-powered 0.5HP pump, because there’s about a 5m (16ft) run to and from the tub and it needs enough suction to prime. It’s easy to turn the flow down with a ball valve if there’s too much. It’s much harder to turn it up if there’s too little. For more on this, see What are the self-priming hot tub pump options? and Using a priming circuit above ground.

A quick safety point if you copy this. With one main drain feeding the new pump on its own, make sure that drain has a proper anti-entrapment cover rated for the flow. On a new build I’d give the heat pump loop two drains, as normal.

Getting the hot water back in

My other worry was returning the hot water into the jet lines. If the 3HP jet pump was running, it would push against the little 0.5HP pump, and could potentially burn it out. Not what anyone wants.

So here’s the creative bit. The skimmer I use in my builds, a Certikin HD100, has two knock-outs in the back for water filling and overflow. On my tub they’d never been used. I drilled them out and used them as the returns for the hot water, with flexible pipe angled down so the warm water goes into the base of the skimmer and straight into the dual-speed pump, which spreads it round the tub. They weren’t a perfect fit, so I epoxied the backs.

This is very much a DIY method, and I wouldn’t do it this way on a new build. A new design would have two main drains and two proper pool returns on the heat pump loop. But I’m a DIYer who happens to help people with their builds, and this kind of rule-bending is what I love.

The electrics: relays, not the spa pack

I love the planning stage of any project, especially the electrics. I have an electronics background, so this is the part I enjoy most.

I had a particular challenge. From my original build, I know that overloading a spa pack gets expensive when you melt a $300 circuit board. I wasn’t doing that again. So the new pump isn’t plugged into the spa pack. It runs on its own feed and is switched by relays.

Here’s the logic. I wanted the new pump to run whenever the dual-speed pump runs, on either speed, so the heat pump gets water whenever the tub is circulating. The catch is that a dual-speed pump has two lives (one for low speed, one for high) and a shared neutral. You can’t join them together through one relay, or you’d be feeding two supplies to the pump. I’m not sure how that would end, and I’m not going to find out.

So I used two relays with a common goal. One is driven by the low-speed live, and the other by the high-speed live. Either one switches on the new circulation pump. If the water isn’t up to temperature, the heat pump’s own thermostat kicks in after about three minutes and starts heating. If it is, the heat pump just lets the water pass through.

The new pump takes its power from the heat pump’s main terminal block, which is fed from my main hot tub circuit on 10mm² cable, with plenty of current to spare. There’s also a switch so I can turn the pump off on its own, and I looped everything through a rotary isolator, so one turn of the dial isolates the heat pump and all the hot tub equipment. Perfect.

This is all mains-voltage work. I’m comfortable with it, but if you’re not, get an electrician to design and install it, and have any change to a hot tub circuit checked and certified. See A guide to hot tub electrics and, for the dual-speed pump side, Can I add an air source heat pump to a hot tub with a dual speed pump?

The heat pump I chose

I went for a Comfortline, one of the more affordable models. It’s a full inverter unit, and its spec sheet quotes an average COP of around 6 with highs of 10. In plain English, that’s around 6kW of heat for each 1kW of electricity. Those figures are measured with warm air and pool-temperature water, though. At hot tub temperatures and in cooler weather, the real COP is lower. My own figures are in the running costs post.

There are models that advertise more, but weighing up the cost, I took the economical route that most of my customers would take. I couldn’t see how fancy features like Wi-Fi or running down to -15°C were going to help me. What I did do was go big. If I was going to the trouble of fitting one, I wanted faster heat-ups as well as lower bills. So I chose a 21kW unit: seven times the power of my old heater.

The range has changed since. On parts4tubs.co.uk, the nearest models today are the Comfortline 20kW and 24kW. There’s more in my Comfortline heat pump review.

Testing in stages

I’m always a bit apprehensive about testing new plumbing, so I did it in stages:

1. Fill and check. I found a small leak at the bottom of the skimmer, on the old plumbing. It had been hidden under the old control room floor, so it had probably been there for ages. Epoxy paste, then an encasement of waterproof concrete, dealt with it.
2. The old loop on low speed. A small turn of one union nut and it was dry. I left it running while I checked everything.
3. The old loop on high speed. All good.
4. The relays. With nothing connected yet, I checked they had power and that each relay switched with its pump speed. All good.
5. The heat pump loop. I finished the plumbing, then wired in the new pump, the relay box and the heat pump.

That last step should have taken ten minutes. The manufacturer had done up two terminal screws so tight that I stripped both heads trying to undo them. An hour and a half and one cut-out screw later, I had the three cables wired in, and some choice words for the factory.

The results

First, it works, and it works perfectly. I’m super happy with it.

Second, heat-up time. I’m getting about 4°C an hour at 15°C (59°F) outside, and more on a warm day. Compared with 0.5°C an hour on the old 3kW heater, that’s roughly 8 times faster. Yes, 8 times.

To be clear about why: it’s mainly because the heat pump is so much bigger than the old heater, not because heat pumps are fast by nature. My big, uninsulated tub was a poor match for 3kW. A 21kW heat pump, even with its output reduced in cooler air, puts several times more heat into the water. A heat pump the same size as your element won’t heat any faster. That’s why I always tell people to size generously. See How to choose the right air source heat pump for your hot tub or pool.

Third, the savings. They came out at 66%, and I’ve broken them down in How I reduced the running costs of my hot tub by 66%.

If I were building from scratch

I’m really pleased with how this turned out, but it’s a retrofit, with compromises. If I were building a new tub today, I’d:

• Insulate the floor and walls from day one, or build with ICF. See DIY hot tub insulation.
• Plan a dedicated heat pump loop with two main drains, a proper pump and two returns.
• Build a bigger control room with space for the heat pump pipework and a second pump.

For the general how-to, see Can I add an air source heat pump to my hot tub? and the ultimate guide to air source heat pumps for hot tubs.

Planning your own build?

My DIY hot tub plans include the plumbing layout, control room and full parts list, so you can build the heat pump in properly rather than bending the rules like I did. If you’ve already got a tub and want help planning the plumbing, my plumbing consultancy is there for that, and the ultimate hot tub control room guide covers the equipment side.

FAQs

Can you add a heat pump to a DIY hot tub without drilling new holes?

I did, by splitting my existing drains between two pumps and using the skimmer’s unused knock-outs as returns. It’s a compromise. On a new build, plan a dedicated loop with its own drains and returns.

Why use a separate pump for the heat pump?

It keeps the heat pump’s flow independent of the jets, and it avoids overloading the spa pack. Mine is switched by relays from the main pump, so it runs whenever the tub circulates.

What size heat pump did you use?

A 21kW Comfortline for about 2,800 litres (740 US gallons). The nearest models today are the 20kW and 24kW.

How much faster does it heat?

About 4°C an hour at 15°C outside, against about 0.5°C an hour on my old 3kW heater. That’s roughly 8 times faster, mostly because the heat pump is much more powerful.

Did you keep the electric heater?

Yes. It stays on the original pump loop, on a switch, and it’s firmly off most of the time. It’s there as a backup.

Got a tricky retrofit of your own? Tell me about it in the comments below.

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