Comfortline Heat Pump Review (2026): The Air Source Heat Pump on My Own Hot Tub

Last updated: September 2026

The Comfortline is the air source heat pump on my own DIY hot tub. I’ve also supplied plenty of them to customers, so I’ve seen how they behave on all sorts of tubs, not just mine. In this review I’ll cover what the Comfortline actually is, the current range and specs, how it has performed for me (including in sub-zero weather), and the good and the bad.

Short answer: I have access to pretty much every heat pump on the UK market, and this is the one I chose to put on my own tub.

What is the Comfortline heat pump?

The Comfortline is an inverter air source heat pump for pools, hot tubs and spas, sold in the UK and Europe. It takes heat from the outside air and moves it into your water through a titanium heat exchanger. You plumb it into the tub’s circulation, and it does the heating while your spa pack’s electric heater takes a back seat.

I fitted mine after my electricity bill for the tub doubled in 2022. You can read the whole story in why I fitted an air source heat pump to my hot tub, and how I plumbed it in on how I added an air source heat pump to an existing hot tub.

The key features:

• Full DC inverter compressor (GMCC) that adjusts output from 20% to 100% to match demand. When you’re heating up from cold, it runs flat out. When it’s just holding temperature, it drops to low speed, which is where it’s most efficient and quietest.
• Spiral titanium heat exchanger in a PVC housing. It’s salt-water safe up to 3.5% salinity, so it’s fine with chlorine, bromine and salt systems.
• Auto defrost, which lets it keep running down to around 0°C. There’s also a manual defrost, which I’ve covered in how to defrost a Comfortline.
• Silence mode for night-time running. Parts4tubs lists it as about 7dB(A) quieter, and it caps output at 80% while it’s on.
• R32 refrigerant, with 50mm (1.5in) plain sockets for the water connections.
• Simple control panel. WiFi comes as standard on the 7kW, 9kW and 12.8kW. On the bigger models it’s an add-on adaptor.

The current Comfortline range (September 2026)

Here’s what’s listed on parts4tubs.co.uk at the time of writing. Prices change, and several sizes are on backorder, so check the product page for current lead times.

Model	WiFi	Price (inc VAT)	Typical use
7kW	Included	£897	Most hot tubs
9kW	Included	£1,167	Larger tubs, faster heat-up
12.8kW	Included	£1,819	Big tubs, swim spas, small plunge pools
16kW	Add-on adaptor	£1,987	Swim spas, plunge pools
20kW (mine)	Add-on adaptor	£2,329	Small pools, or a big tub where speed matters
24kW	Add-on adaptor	£2,645	Pools

The WiFi adaptor for the larger models is £199. If you’re comparing the Comfortline with the cheaper plug-in units, I’ve written up the difference between the Onda and the Comfortline, and there’s a full Onda Mini review too.

Key specs: the 7kW and the 20kW

These are the two I get asked about most: the 7kW because it suits most hot tubs, and the 20kW because it’s the one on my tub. Figures are from the parts4tubs.co.uk product pages.

Spec	7kW	20kW
Output at 26°C air / 26°C water	7.0kW	20.0kW
Output at 15°C air / 26°C water	5.0kW	14.0kW
COP at 26°C air (100% to 50% output)	10.3 / 9.3	up to 11.8
COP at 15°C air	5.8–6.0	4.6–6.5
Electrical input	0.29–1.04kW	0.75–3.04kW
Supply	230V single phase, hard-wired	230V single phase, hard-wired
Water flow needed	2–4m³/h (33–67 L/min, 9–18 US gpm)	8–10m³/h (133–167 L/min, 35–44 US gpm)
Air operating range	0–43°C (32–109°F)	0–43°C (32–109°F)
Noise at 1m	38.8–50.2dB(A)	45.3–57.1dB(A)
Size (L × W × H)	744 × 359 × 648mm	954 × 359 × 748mm
Weight	42kg	68kg

Warranty

Heat exchanger 5 years, compressor 2 years, and all other parts and the casing 2 years. It’s parts only after the first 12 months, so labour is on you (or your installer) after that.

How efficient is it, really?

You’ll see the Comfortline quoted with an average COP of around 9 on the 7kW. That’s measured at 26°C air, 26°C water and 80% humidity, which is a lovely summer’s day. It means 1kW of electricity gives you about 9kW of heat in those conditions.

The number I’d pay more attention to is the 15°C air figure, where the COP is about 4.6–6.5 depending on the model and how hard it’s working. That’s much closer to a typical UK spring or autumn. In winter it’ll be lower again. Even so, compare it with your spa pack’s electric heater, which has a COP of 1: every kW in gives you one kW of heat, whatever the weather.

The inverter is what makes the Comfortline good at the day-to-day part. Once the tub is up to temperature, it throttles back and ticks over, which is where the COP is highest and the noise is lowest. An on/off heat pump can only run at full power and then stop. There’s more on how to read COP figures in how to choose the right air source heat pump for your hot tub or pool.

My results: 66% cheaper and 8x faster

I have the 21kW model (it’s now listed as the 20kW). My tub has no insulation, which I’d never recommend now, and before the heat pump it was relying on a 3kW heater in the spa pack that managed about 0.5°C an hour.

• Running costs: about 66% cheaper than the 3kW electric heater. The full breakdown is in how I reduced the running costs of my hot tub by 66%.
• Heat-up speed: about 8x faster. In mild weather (around 15°C) I was seeing around 4°C an hour.
• In the cold: at -4°C it still managed about 2°C an hour and defrosted itself as it went. The full test is in I ran my air source at -4°C: the results.

My numbers are from 2022 tariffs, and your savings depend on your tub, your cover, your insulation and your electricity price. The percentage saving holds up better than the pounds, because a heat pump and an electric heater both get cheaper or dearer together when the unit price changes.

What about “all season” use?

In my opinion “all season” has become a bit of a marketing term. The Comfortline is rated from 0°C (32°F). That doesn’t mean it stops at 0°C. It means that’s the bottom of the range the manufacturer rates it for. Below that the output and efficiency drop, but that’s true of every heat pump, even ones rated to -15°C.

I’ve had mine running on mornings of -2°C and -4°C, and it kept heating. It iced up, defrosted itself, and carried on. If you live somewhere with long spells well below freezing, look at a unit rated lower (the Vian Power Plus models are rated to -10°C) and keep your spa heater connected as a backup.

How quiet is it?

Quiet enough that I don’t notice it from the tub. At the typical 50% output, the 7kW is listed at 42.8dB(A) at 1m, and silence mode knocks around another 7dB(A) off. The bigger models are louder at full power (the 20kW is up to 57.1dB(A) at 1m), but they spend most of their time at low speed once the water is hot. If it’s going near a bedroom window or a neighbour’s fence, read how noisy is an air source heat pump? first.

The good and the bad

What I like	What to watch
Full inverter, so it’s efficient and quiet when holding temperature	Only rated to 0°C. It works below that, but slower
Dead simple: set a temperature and leave it	WiFi is an extra on 16kW and above
Titanium heat exchanger, salt-water safe	Parts-only warranty after 12 months, and 2 years on the compressor
Strong value for the output	Needs hard-wiring, and the big models need a lot of flow

Controlling it with your spa pack

I love the simplicity of the unit. But the heat pump has its own thermostat, and your spa pack has another one, so you need to decide which one is in charge.

My own setup is a bit unusual: I control the heat pump through a relay rather than straight from the spa pack. Because my circulation pump runs during the filter cycles, if the heat pump is set higher than the spa pack, it starts heating whenever water flows through it. If I set both to the same temperature, that doesn’t happen. That’s the main reason WiFi became a “nice to have” for me, and I’ve since been using the WiFi app to check on it from my phone.

Most people don’t need anything that complicated. The usual approach is to set the heat pump a degree or so above the spa pack, so the heat pump does the work and the pack’s heater only kicks in as a backup. I’ve explained the options, including interlocking it with the pump, in how to control a heat pump with any hot tub spa pack.

Installation tips

• Size the flow, not just the kW. The 7kW wants 2–4m³/h through it, and the 20kW wants 8–10m³/h. Retailers generally don’t advise models over 7kW on a pool with only one inlet and one outlet. On a hot tub, a small circulation pump suits the 7kW, but the big models need a proper pump and plumbing. See how to add a circulation pump to a hot tub.
• Plumb it after the filter and before the spa pack heater, with a bypass so you can isolate it. Parts4tubs sells air source connection kits for this.
• Keep the pipe runs short and insulated. The 20kW listing gives a 10m maximum pipe run.
• Give it air. Don’t box it in or tuck it under a low roof. It needs clear space around the coil and in front of the fan.
• Get the electrics done properly. It’s hard-wired on a dedicated, RCD-protected circuit sized to the data plate. In the UK that’s notifiable work under Part P. See a guide to hot tub electrics.
• Insulate the tub and use a good cover. A heat pump can’t fix a tub that’s pouring heat away. That’s the lesson from my own uninsulated build. See DIY hot tub insulation.

If it isn’t heating as it should, work through my air source heat pump isn’t heating my hot tub. For the bigger picture on heat pumps, start with the ultimate guide to air source heat pumps for hot tubs.

My verdict

For me, the savings have been huge: 66% cheaper to run and about 8x faster to heat than my old 3kW heater. It’s simple, it’s quiet, and it has kept going in weather it isn’t even rated for.

If you want a year-round heat pump for a hot tub without paying premium-brand prices, the 7kW Comfortline is where I’d start for most tubs. Go bigger only if you have the flow to feed it, or a plunge pool or swim spa to heat. For the price, I think you’ll struggle to find better.

Planning a new build?

If you’re building a tub from scratch, it’s far easier to plan the heat pump in from day one: a proper plinth, a dedicated circulation line, insulation, and the cable route. My DIY hot tub plans include the plumbing layout and parts list, so there’s a place for all of it before you pour any concrete.

For parts, including the Comfortline range and connection kits, head to parts4tubs.co.uk in the UK.

More videos on the Comfortline and heat pumps

FAQs

Is the Comfortline heat pump still available?

Yes. As of September 2026, the Comfortline Inverter range (7, 9, 12.8, 16, 20 and 24kW) is listed on parts4tubs.co.uk for the UK and Europe. Some sizes are on backorder, so check the lead time.

What size Comfortline do I need for a hot tub?

For most hot tubs, the 7kW is plenty. Go bigger if you want faster heat-ups on a large or uninsulated tub, and only if your pump and plumbing can deliver the flow the bigger units need. For a swim spa or plunge pool, look at 12.8kW and up.

Does the Comfortline work below 0°C?

It’s rated from 0°C, but mine has kept heating at -4°C, just more slowly (about 2°C an hour against about 4°C an hour in mild weather). It defrosts itself as it goes.

Does the Comfortline have WiFi?

The 7kW, 9kW and 12.8kW come with WiFi. The 16kW, 20kW and 24kW need the optional WiFi adaptor.

Can I plug the Comfortline into a normal socket?

No. It needs hard-wiring on a dedicated, RCD-protected circuit installed by a qualified electrician. If you want plug-in, look at a mini heat pump like the Onda Mini, but it won’t run through the winter.

Got a question about the Comfortline or your own heat pump setup? 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