How I Added an Air Source Heat Pump to an Existing Hot Tub (Paul’s Acrylic Tub)

Last updated: September 2026

My general guide, Can I add an air source heat pump to my hot tub?, explains the theory. This post is the real thing: the day I fitted a 7kW heat pump to my friend Paul’s shop-bought acrylic hot tub, step by step, including the bits that didn’t go to plan.

For full disclosure, Paul isn’t a customer. He’s a good friend of mine, and a good enough friend to let me loose on his “pride and joy”, a hot tub that was just six weeks old. I don’t do on-site work. This was a favour for a mate, and a chance to show you how it’s done.

A word on warranties first

If you modify the plumbing or electrics of a shop-bought hot tub, you’ll more than likely void the manufacturer’s warranty. Paul was fine with that, because I’ll look after him if anything goes wrong. If you’re doing this on your own tub, go in knowing you’ll be on your own if something fails.

Why Paul wanted a heat pump

With energy prices rising, Paul didn’t like what his smart meter was showing him each day, and most of it was the hot tub. He already had an air source heat pump heating his house, so one for the tub was a logical step.

We looked at lots of options. Paul went for the Comfortline 7kW because the specification and the price came out on top. After plenty of photos and video calls, we put a parts list together and a date in the diary.

The location

We put the tub’s heat pump next to the existing house heat pump, which looked the neatest. Normally I’d want more clearance around the unit and more open air in front of it, typically at least 30cm (12in) from walls. But that side of Paul’s house is a real wind tunnel, so there’s no shortage of air moving around the units. The heating engineer who fitted his home heat pump was happy with the spot too.

If your site isn’t like Paul’s, stick to the manufacturer’s clearances. A heat pump starved of air won’t perform.

Wall-mounting the heat pump

Most hot tub heat pumps sit on the ground, but we wall-mounted this one to match the unit already there. The bracket is heavy duty and so were the fixings: we drilled 14mm holes. The last thing you want is a heat pump coming off the wall.

The electrics: upgrading the breaker

Like many acrylic hot tubs, Paul’s was on a 32A circuit. With the extra load of the heat pump, that wasn’t going to be enough to run everything at once. So we swapped the RCBO for a 40A one, to power both the tub and the heat pump from the same circuit.

Paul’s hot tub supply was already a 10mm² cable run. Never fit a bigger breaker unless a qualified electrician has confirmed the cable and the rest of the circuit can take it. A breaker bigger than the cable is rated for won’t protect it. We had all the electrics checked by a qualified electrician before anything was switched back on. For the UK and US rules, see A guide to hot tub electrics.

We wired the heat pump into the spa pack’s main terminal block, using it purely as a connector. The spa pack doesn’t switch the heat pump. It just shares the supply. That was the simplest and cleanest way to do it.

Planning the plumbing

Paul’s tub has two jet pumps and a separate circulation pump. The circulation pump was the one we were interested in, because it’s the one that pushes water through the heater. The plan was to intercept its hot water return. Instead of going from the electric heater straight back to the tub, the water would go out to the heat pump first.

A separate circulation pump makes life much easier. You worry far less about the heat pump affecting the jets. If your tub doesn’t have one, you need to be careful with flow and use the bypass valve to keep the jets strong. See How to add a circulation pump to a hot tub.

The challenge with an acrylic tub is space. There isn’t much under there. Our plan was to move the heater closer to the pump and break into the circuit after it. Paul also wanted the pipes to leave the cabinet at the side rather than the front, where they’d look ugly and get in the way of the main access panel.

I used rigid pipe inside the cabinet. If I did it again, I’d definitely use flexible pipe. Rigid pipe has no give at all, which made getting it into the right place at the right angle much harder. We got there in the end.

A switch for the electric heater

We wanted Paul to be able to choose between the heat pump alone or the heat pump plus the 2kW Gecko heater as a boost on very cold days. The rest of the time it would just run on the heat pump.

Loving a bit of DIY, I went for the simple option: a single-pole switch in the heater’s live feed. Nothing complicated, but it does the job. It’s a clear warranty violation, and it’s mains-voltage work inside the spa pack, so if you want the same thing, get an electrician to fit a properly rated switch. We planned to make it a smart switch later, so Paul wouldn’t need to open the cabinet.

The bypass and isolation valves

Next, we fitted a bypass with isolation valves on the heat pump. That lets us adjust the flow if we need to, and it means we can take the heat pump out of the circuit completely for servicing. The layout is shown in the diagram in my main guide.

My tip: take your time and build everything in place. That way you know it will all fit.

Running the pipes

With the valves in, it was time to lift some of Paul’s newly laid landscaping to bury the pipes as far as we could. The pipes were a bit too big to hide completely, and we didn’t want to cut his new slabs, so he’s living with a few raised stones rather than a perfectly flat path.

• We used rigid 1.5in pipe in full lengths, so there were no joins underground.
• Keep everything at 90°. It makes life so much easier. I’ll admit a couple of my angles weren’t 100% perfect, but they were close.
• A well-made joint is a joint that won’t leak. If in doubt, cut it out and redo it. Otherwise it’ll only leak later.

Making good solvent-weld joints

1. Clean and lightly roughen the pipe and fitting with glass paper, then use primer if your cement calls for it.
2. Mark the depth the pipe needs to go into the fitting, so you know when it’s fully home.
3. Mark where the fitting should line up with the pipe, so you finish at the right angle.
4. Apply cement to both the pipe and the fitting, and work quickly.
5. Push the pipe in with a slight twist to avoid “channelling”, which causes leaks.
6. Hold it for about 10 seconds and wipe off the excess. Follow the cement’s cure times before you pressurise it.

There’s a full guide in How to make the perfect PVC solvent weld joint.

Insulating the pipes

There are a few metres of pipe between the tub and the heat pump. To cut heat loss, we wrapped them in foil pipe wrap. We did that before cementing them in place, because it would have been very awkward afterwards.

Testing

Then came the moment of truth. I suspected a gasket was missing from one of the joints inside the tub that used to connect to the heater. Sure enough, as soon as I opened the ball valve, water poured out of it. Nothing a 50p gasket couldn’t fix.

One corner joint that wasn’t quite at 90° also had the tiniest leak. Being the plumbing perfectionist I now am, we cut it out and remade it. Now it’s perfect.

How it works

The heat pump is controlled by the flow from the circulation pump. We set the heat pump’s own thermostat to 40°C (104°F) and set the spa pack to whatever temperature Paul wants.

When the spa pack decides the water is warm enough, it stops the circulation pump. With no flow, the heat pump’s flow sensor tells it to stop heating. Paul’s model actually shows an error code when that happens, which is nothing to worry about. When the spa pack calls for heat again, the pump starts, the flow comes back and the heat pump kicks in about three minutes later. So the spa pack is controlling the heat pump indirectly. More in How to control a heat pump with any hot tub spa pack.

The results

On the first run, the heat pump raised the water temperature by 7°C in about 2.5 hours, far quicker than the 2kW electric heater could. Paul reckoned it was heating 2–3 times faster, and later settled on about 2.5 times.

That makes sense. The Comfortline 7kW is rated at 7kW in warm conditions and 5kW at 15°C air, against 2kW from the element. Speed is about how many kW go into the water, and the heat pump simply puts in more.

Paul also ran tests on his smart meter:

• On day one, at 10°C (50°F) outside and heating from cold, the heat pump cost about 33p an hour. Switching on the electric heater at the same time added about 57p an hour, so the heater was costing almost twice as much as the heat pump, for much less heat.
• Once he’d lived with it, the heat pump cost between 22p and 43p an hour depending on the weather, against about 93p an hour on the electric heater. On average, he estimates it at about half the cost of the electric heater.
• He worked out it would pay for itself in about 17 months.

Those figures are what convinced me to fit one to my own tub. That story is in Why I fitted an air source heat pump to my hot tub, and my own before-and-after numbers are in How I reduced the running costs of my hot tub by 66%.

What I’d change next time

Only one thing: flexible pipe inside the cabinet of an acrylic tub. There’s so little room, and rigid pipe is so unforgiving, that flexible pipe would have saved a couple of hours and a lot of stress getting the joints and angles to meet. Apart from that and a couple of small leaks, which are fairly normal, the job went to plan.

Doing your own?

Follow the step-by-step in Can I add an air source heat pump to my hot tub? and check the 5 considerations before you install an air source heat pump. For sizing, see How to choose the right air source heat pump for your hot tub or pool, and for the bigger picture, the ultimate guide to air source heat pumps for hot tubs. There’s another real-world example in Chris’s journey.

In the UK, we sell the Comfortline range and ready-made connection kits at parts4tubs.co.uk. In the USA, parts4tubs.com has the pumps, valves and fittings. And if you’re building a tub from scratch, my DIY hot tub plans let you plan the heat pump in from day one.

FAQs

Can you add a heat pump to a shop-bought acrylic hot tub?

Yes. Paul’s was six weeks old. You cut into the return after the heater, add a bypass and run pipes out to the heat pump. Space in the cabinet is tight, so use flexible pipe.

Will it void my hot tub warranty?

Very likely. Changing the plumbing or electrics of a shop-bought tub usually voids the manufacturer’s warranty, so check before you start.

Can the heat pump run off the hot tub’s supply?

Sometimes, if the supply cable and breaker can carry both. Paul’s needed a 40A RCBO, which is only OK if the cable and the rest of the circuit are rated for it. Always get an electrician to confirm this.

Do I still need the hot tub’s electric heater?

No, but it’s handy as a boost in cold weather. We put Paul’s on a switch so he can choose.

Why does the heat pump show an error when the tub is up to temperature?

Because the spa pack has stopped the circulation pump, so there’s no flow. It’s normal on this setup. The heat pump restarts a few minutes after the flow comes back.

Thinking of doing the same to your tub? Ask away in the comments below.

Cheers,
Andi

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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