How to Add an Air Source Heat Pump to Your Hot Tub: Step-by-Step Guide

Last updated: September 2026

Electricity isn’t getting any cheaper, and a hot tub heated by a 3kW element is one of the thirstiest things in most homes. The good news is you don’t have to stop using it. Fit an air source heat pump and you can cut the heating bill dramatically. On my own tub it saves me around 66%, and heat-up is roughly eight times faster.

People assume this is a specialist job. It isn’t. If you’re a confident DIYer who can cut and glue PVC pipe, you can do most of it yourself, with an electrician for the power. In this guide I’ll go through it step by step: choosing the heat pump, the connection kit, where to put it, where to cut into the plumbing, how it’s powered and how it’s controlled.

If you want to see what the savings look like in real numbers, I’ve broken down my own bills in How I reduced the running costs of my hot tub by 66%. Here’s the video walkthrough of the install:

Six myths about heat pumps on hot tubs

Before we start, let’s clear up a few things I get asked all the time.

• “They don’t work on hot tubs.” Wrong. I’ve fitted one to my own tub and it works brilliantly. For the full background, see my ultimate guide to air source heat pumps for hot tubs.
• “They won’t get to 40°C (104°F).” Wrong. Spa heat pumps heat to 40°C without any trouble.
• “You need a heat pump that matches your spa pack brand.” Wrong. I have a Balboa system and a Comfortline heat pump, and I fitted a Comfortline to my friend’s Gecko system. Both work perfectly.
• “You’ll get an error code on the topside.” Not with the flow method in this guide. The topside has no idea a heat pump is there. Only the brand-matched units, such as Balboa’s Clim8zone, talk to the pack.
• “It has to be powered from the spa pack.” No. A standalone heat pump has its own supply. More on that below, because this is the step to get right.
• “You’ll need a bigger electricity supply.” Often not. If you disconnect the 3kW heater (about 13A), many heat pumps draw less than that. A Comfortline 12.8kW, for example, draws a maximum of 7.74A. The heat pump still needs its own correctly rated protection, though, so talk to your electrician.

Before you start: your warranty

I have to be upfront. Cutting into the pipework or drilling the cabinet will almost certainly void the manufacturer’s warranty on your tub. My honest view is that if you can fit a heat pump yourself, you can service the tub yourself too. But it’s your decision, and if the tub is only a few months old you might want to wait. I’ve covered this in more detail in 5 considerations before you install a heat pump on your hot tub.

Step 1: Choose the right air source heat pump

There are lots of heat pumps on the market. I have a Comfortline on my own tub. I chose it because it was well priced, it did what I needed and it’s been fantastic, and I have a full Comfortline heat pump review. Mine is a big 21kW unit on a big tub (around 2,700 litres), which is why my heat-up time is so quick.

The number that catches people out is the headline kW. Manufacturers quote output at warm conditions (around 26–27°C air). What matters in the UK for most of the year is output at 15°C air and 26°C water. Here’s how that looks on the Comfortline range, from the spec sheets on parts4tubs.co.uk:

Model	Headline output	Output at 15°C air	Max current	Rough fit
Comfortline 7kW	7.0kW	5.0kW	4.52A	Plastic shell tubs up to ~1,500 litres (400 US gal)
Comfortline 12.8kW	12.5kW	8.5kW	7.74A	Larger tubs, ~1,500–2,500 litres (400–660 US gal)
Comfortline 20kW	20.0kW	14.0kW	13.21A	Big DIY tubs and fast heat-up

As a rule, buy as big as your budget allows. A bigger unit doesn’t cost more per kWh of heat, it just gets there faster. To give you a feel for it: heating 1,000 litres by 1°C takes about 1.16kWh. So lifting a 1,500 litre tub from 30°C to 38°C needs roughly 14kWh. A 3kW element takes nearly five hours, plus losses. A 7kW heat pump giving 5kW at 15°C does it in under three. There’s a full walk-through in How to choose the right air source heat pump for your hot tub or pool.

You can see the current range below and in my air source heat pump store. In the USA, have a look at my Hydro Royal review.

Here’s one fitted by a customer, Barry, on the Isle of Skye. It’s a Comfortline, installed DIY:

Step 2: Choose a connection kit

To make life easy, I put together connection kits with the pipe and fittings you need to connect a heat pump to an existing tub. Measure your pipe run first (Step 4), then pick the size:

• Regular, Large or Extra Large. The difference is the amount of pipe and the number of elbows and couplers. The Regular kit, for example, has 5m of rigid pipe, 2m of flexible pipe, elbows, couplers, reducers, a bypass valve set-up and pipe cement.
• 1.5in or 2in. Match the pipe size inside your tub. Most spa packs use 2in unions, while many heat pumps have 1.5in (50mm) connections, so the kits include reducers.
• With a flapper valve. There’s a Regular 2in kit with a flapper valve for single-pump (two-speed pump) systems. If that’s your tub, read Can I add a heat pump to a hot tub with a dual speed pump?

Step 3: Pick a location

A heat pump pulls heat out of the air, so it needs plenty of air around it. That means it can’t be boxed in, covered, sunk into the ground or pushed up against a wall. The Comfortline installation clearances are a good guide:

• 2m (6.5ft) clear in front of the fan, so it isn’t blowing cold air straight at a wall or fence.
• 300mm (12in) at the sides and 500mm (20in) behind and above. Other makes vary, so check your manual.
• Within about 5m (16ft) of the tub if you can. Every metre of pipe and every bend reduces the flow from your circulation pump. Much further than that and you may need a bigger pump.
• On a solid, level base, raised slightly so it isn’t sitting in puddles or snow, with somewhere for the condensate water to drain.
• Away from the water’s edge where you can. The electrical rules for pools, which electricians apply to hot tubs, restrict equipment within 2m of the water. See Step 7.

They’re not especially noisy, but think about where the fan points in relation to your neighbours and bedroom windows. I’ve covered this in How noisy is an air source heat pump?

Step 4: Plan the pipe run

Next, decide the route the pipes will take to and from the tub.

• Keep bends and changes of direction to a minimum. Use sweeping bends where you can.
• If the pipes aren’t lying on the ground, support them properly.
• Insulate the pipes. Lagging stops you losing the heat you’ve just paid for, and it protects the pipe from frost and sunlight.
• Measure the full run, there and back, before you order your kit.

Every joint needs to be done properly or it will weep. This video shows how I make a leak-free joint, and there’s a written version in How to make the perfect PVC solvent weld joint. For pipe types and sizes, see my hot tub pipe guide.

Step 5: Decide where to cut into the plumbing

This is the bit most people are nervous about. Don’t be. It’s simple.

You’re looking for the pump that feeds the heater. On most tubs that’s the circulation pump, or pump 1 on low speed if you don’t have a circ pump. Follow the pipe from the top (the outlet) of that pump into the spa pack’s heater. You can cut in either just before or just after the spa pack. Then use elbows from the kit to take the pipe out of the cabinet to the heat pump and back.

I’d always fit a bypass: a valve on the line between the two tees, and a valve on the pipe to and from the heat pump. It lets you isolate the heat pump for servicing or winterising without draining the tub. In normal use, the bypass valve stays closed so all the flow goes through the heat pump. Only open it partly if your pump pushes more flow than the heat pump’s maximum (the spec sheet gives a flow range, for example 2–4m³/h on the Comfortline 7kW).

Step 6: Get the pipes out of the cabinet

Drilling a hole in an expensive hot tub cabinet feels scary, but it’s a clean job with a hole saw. Cut the holes just big enough for the pipe to pass through, low down and out of sight if you can.

Inside the cabinet, use flexible pipe to make the connections, as it’s far easier to route around the equipment. The kits include flexible pipe for this.

Top tip: flexible pipe is much easier to work with when it’s warm. If you’re struggling, pour some hot water over it to soften it up.

Step 7: Power the heat pump

A standalone heat pump isn’t powered from the spa pack’s outputs. It needs its own supply. The only exceptions are brand-matched units like the Balboa Clim8zone, which connect to specific packs.

I used to say you could simply take a loop off the spa pack’s main terminal block. I wouldn’t put it that way now. Technically the supply can come from the same circuit, but the heat pump’s cable must be protected by a breaker the right size for it. A 2.5mm² cable run straight off a 32A hot tub circuit isn’t protected properly. So:

• Check the heat pump’s spec sheet. The Comfortline 7kW calls for an 8A breaker with 1.5mm² cable, the 12.8kW a 15A breaker with 2.5mm², and the 20kW a 23.5A breaker with 6mm² cable.
• UK: 30mA RCD protection (Type A as a minimum), SWA cable for outdoor and buried runs, and a local isolator. The IET’s guidance treats outdoor hot tubs under the pool rules (BS 7671 Section 702), which restrict what can go within 2m of the water. New circuits outside are notifiable under Part P, according to Total Skills.
• USA: GFCI protection and a disconnect within sight, following NEC Article 680 and the heat pump maker’s instructions. Most areas want a permit.
• If you keep the electric heater connected as well, the total load goes up, so the supply may need upgrading.

Use an electrician for the connection. It’s usually a small job once the heat pump is sited. There’s more in Hot tub wiring explained and A guide to hot tub electrics.

Step 8: Disconnect the element, fill and test

1. Disconnect the electric heater at the spa pack with the power isolated. On many packs it’s a couple of plug-in connections. Your pack doesn’t know it’s gone and will still “call for heat” as normal.
2. Check every joint has had time to cure. Then open the valves and fill or top up the tub.
3. Bleed the air. A new loop of pipe is a classic place for an air lock. Crack a union on the pump if you need to. See How to deal with a hot tub air lock.
4. Power up and check for leaks with the pump running.
5. Watch it start. The heat pump waits for steady flow before it starts the compressor. On the Comfortline that’s about 3 minutes.

How it’s controlled

Unless you have a brand-matched unit, you control the heat pump with water flow. When your spa pack wants heat, it runs the circulation pump. The heat pump’s flow switch senses the water moving, and if the water is below the heat pump’s own set point, it heats. When the pack is happy and stops the pump, the flow stops and so does the heat pump.

The heat pump has its own thermostat, independent of your topside. This is the one small downside. Say your heat pump is set to 40°C, but you’ve turned the tub down to 30°C while you’re away. Every time the pack runs the pump for a filter cycle, the heat pump sees flow, sees water below 40°C and starts heating. That costs you a little extra.

The fix is easy: keep the temperature on the heat pump the same as the temperature on your topside, and change both when you change one. I’ve gone into much more detail, including 24-hour circulation pumps and Balboa’s heat modes, in How to control a heat pump with any hot tub spa pack.

Common mistakes

• Buying on the headline kW. Check the output at 15°C, especially for winter use.
• Boxing it in. A pretty enclosure that blocks the airflow will kill its performance.
• Too far away, too many bends. Low flow means the flow switch may not trigger, or the unit throws flow errors.
• Uninsulated pipes in winter, which lose heat and can freeze.
• Different set points on the heat pump and the topside.
• Tapping power off the pack without checking the cable and breaker sizing.

Troubleshooting

Symptom	Likely cause	What to check
Heat pump never starts	No flow or low flow	Valves open, air lock, dirty filter, bypass not left wide open
Flow error code	Flow below the heat pump’s minimum	Pipe run length and bends, pump size, clogged filter
Heats when you don’t want it to	Heat pump set higher than the topside	Match the two set points
Slow heating in winter	Low air temperature or icing	Defrost cycles are normal; clearances; use a good cover
RCD or breaker trips	Wiring or a fault	Don’t keep resetting it. Get the circuit checked

For more, see Help! My air source heat pump is not heating my hot tub and How to defrost a Comfortline heat pump. I’ve also tested mine in a cold snap in I ran my air source at -4°C: the results.

What will it save?

It depends on your tub, your climate and your tariff, but upward of 50% is realistic. On my tub the saving is 66%. At 2022 prices, my heat pump paid for itself in about a year.

Here’s the simple maths. A 3kW element running for an hour uses 3kWh. At the Ofgem price cap average of 26.32p per kWh for October to December 2026, that’s about 79p. A heat pump with a COP of 5 delivers the same heat for 0.6kWh, which is about 16p. The warmer the air, the higher the COP and the bigger the saving. Pair it with a decent hot tub cover and you’ll save even more.

Building a new tub?

If you’re building a DIY hot tub from scratch, plan the heat pump from day one. Leave room in the control room, run the pipe ducts before the concrete goes in and get the electrician to allow for it. My DIY hot tub plans give you the CAD drawings, the plumbing layout and a full parts list. If your garden or backyard needs something custom, I can draw up architectural CAD and a 3D plumbing design for you.

For heat pumps, connection kits and spares, head to parts4tubs.co.uk in the UK or parts4tubs.com in the USA.

FAQs

Can I add an air source heat pump to any hot tub?

Almost any. As long as there’s a pump that runs when the tub calls for heat, you can control a heat pump by flow, whatever brand of spa pack you have. See Can I add an air source heat pump to my hot tub?

Do I need to remove the electric heater?

You disconnect it rather than remove it. Most people unplug the element at the spa pack so the heat pump does all the heating. If you leave it connected, both will run together when the pack calls for heat, which uses more electricity and needs a bigger supply.

Can I install a heat pump myself?

You can do the plumbing yourself if you’re a confident DIYer. The electrical connection should be done by an electrician, and in England and Wales a new outdoor circuit is notifiable under Part P.

How far can the heat pump be from the hot tub?

Try to keep it within about 5m (16ft). Beyond that, the extra pipe and bends can drop the flow below what the heat pump needs, and you may need a bigger pump.

Will it work in winter?

Yes, with less output. Check the rated minimum air temperature on the spec sheet. Below that, a heat pump may stop to protect itself, so a good cover and sensible sizing matter.

Will my topside show an error with the heater disconnected?

In my experience with Balboa and Gecko packs, no. The pack still runs the pump when it wants heat, and the heat pump does the rest.

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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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