I Ran My Hot Tub Heat Pump at -4°C: The Real Results

Last updated: September 2026

A cold snap isn’t much fun in general, but it gave me a great chance to do some real-world testing on the air source heat pump on my hot tub in sub-zero conditions. So on a frozen Saturday morning at -4°C (24.8°F), I turned the thermostat up and timed it. Here’s exactly what happened, what it cost, and what it means if you’re thinking about a heat pump for your own tub.

The background

If you follow the blog or my YouTube channel, you’ll know I talk about air source heat pumps a lot, and in particular the Comfortline I have on my own hot tub. I have the 21kW model (it’s now listed as the 20kW).

The efficiency of an air source heat pump drops as the air gets colder. That’s just physics. This was my first proper cold spell with the heat pump, with temperatures well below zero (the mid-20s °F). I know many of you get far colder than that, but for us it was a proper freeze, and I wanted to document the results.

The grey area: what happens below the rating?

Sub-zero tends to be the grey area for heat pumps. What happens when the temperature drops below what the manufacturer recommends? Does the heat pump just stop? Does it freeze solid? I get asked this all the time, and for my unit the answer was unknown. The manufacturer doesn’t publish COPs at or below zero, and many others don’t either. (Vian is a welcome exception, and publishes figures down to -10°C.)

A few days earlier, at -2°C, I’d found my tub still holding at 30°C, with the heat pump thick with ice but still working. I’ve written that up in how to defrost a Comfortline air source heat pump. This time I wanted a proper heat-up test.

The test

It was just after 8am on a Saturday. I was outside at -4°C (24.8°F), the sun was just coming up, and everything was white and frozen. A beautiful morning.

During the week I hold my tub at around 30°C (86°F). At the weekend, when I want to use it, I turn it up to 39°C (102°F). When I first fitted the heat pump, I recorded a 66% saving on running costs and heat-ups 8x faster than on the electric heater, in moderate temperatures of around 15°C (59°F). The full breakdown is in how I reduced the running costs of my hot tub by 66%. Now it was time to see what it would do below zero.

I turned the thermostats on the hot tub and the heat pump up to 39°C. The unit kicked in, and we were off to the races.

The initial results

The early signs were promising. The unit was working, even though it’s only rated to 0°C. It didn’t fail or stop heating, which is what people always ask me about when a heat pump runs outside its temperature rating. It also defrosted itself every so often during the heat-up. It coped with the temperatures without a problem.

What was interesting, when I checked the WiFi app, was that the unit was clever enough to work out it couldn’t run at 100%. It was averaging about 75–80%. I assume it was too cold to get the benefit of full power. That’s one of the advantages of an inverter heat pump over an on/off model: it adjusts itself to the conditions instead of just hammering away.

An hour later, the app showed the water had risen by 2°C. That’s not bad at all. On full power in mild weather I’d been getting around 4°C an hour, and more in the summer. Two hours in, it was up 4°C. So the 2°C an hour was holding, and it was still -3°C (26.6°F) outside.

Compare that with the 3kW electric heater I disconnected, which managed around 0.5°C an hour in good conditions. It would have been slower still in this cold, although I don’t have a like-for-like reading at a similar temperature to compare against.

What about the cost at sub-zero?

The whole point of putting a heat pump on a hot tub is to save money. But what about when it’s below freezing?

I can’t measure the heat output (in kW) from the heat pump, so I concentrate on heat-up times, which are easy to measure and compare.

• The 3kW electric heater cost me around £1 ($1.20) an hour to run at the time, for less than 0.5°C an hour. Heating from 30°C to 39°C in these conditions would have taken 18–20 hours and cost roughly £20–25. (For the statisticians among you, I’m rounding and allowing some leeway.)
• The heat pump: my smart meter showed about 5kW being drawn that morning, which was costing about £1.66 an hour. At 2°C an hour, that’s 4.5 hours to reach 39°C, for about £7.50.

One caveat that makes the heat pump look even better: that 5kW was the smart meter reading for the whole house, so it included everything else running at the time. The Comfortline’s rated maximum input is about 3kW, so the true cost of the heat-up was probably lower than £7.50.

The sub-zero results

30°C to 39°C at -4°C to -3°C	3kW electric heater	Comfortline heat pump
Heating rate	under 0.5°C/hour	about 2°C/hour
Heat-up time	18–20 hours	4.5 hours
Cost (Dec 2022, about 33p/kWh)	£20–25	about £7.50
Same heat-up at about 26p/kWh (2026 price cap)	about £14–16	about £5.90

That’s roughly 4x quicker and about two-thirds cheaper (62–70%, depending on which end of the electric heater estimate you take). I take that as a win-win, especially from a model that isn’t rated for sub-zero use.

The 2026 figures use Ofgem’s average price cap unit rate of 26.11p per kWh (July to September 2026) and the same energy use as my test. Electricity is cheaper than it was in the 2022 crunch, so the pounds saved are smaller, but the percentage saving is the same, because the heat pump and the heater both get cheaper together.

What have others found?

I’ve got an almost “mini community” of hot tub heat pump users who get in touch to compare results in different conditions. It’s awesome, and it was the same during the summer heat.

James P comments regularly on the air source videos on my YouTube channel. He’s massively data driven, with everything connected to a logging system so he can see exactly what’s costing him what. He found it was taking roughly twice as long to heat up in the cold, but it wasn’t using twice the energy. Here’s his usage graph:

James W sent a short email: “the heat pump has been working great, honestly 40C no problem and efficient as hell! Obviously things have got a bit colder now but it’s still nothing cost wise and is heating up.”

My conclusion

Heat-up speed definitely drops in the cold, because the unit can’t pull as much heat out of freezing air. But even units only rated to 0°C still work below zero, and they still produce results. For me, at sub-zero, the heat pump was about 4x quicker and around two-thirds cheaper than the 3kW heater in my spa pack.

Getting the best from a heat pump in winter

• Hold the temperature during the week rather than heating from cold. Topping up from 30°C is far quicker than heating from 15°C in January. Balboa heat modes explained covers how to set the spa pack side of this up.
• Choose an inverter model, so it can throttle itself to suit the conditions like mine did.
• If you live somewhere properly cold, pick a unit rated below zero and look at its published low-temperature COPs. See how to choose the right air source heat pump for your hot tub or pool.
• Size for winter, not summer. A heat pump that’s plenty in July can struggle in January. My 21kW is big for my tub, and that headroom helps in the cold.
• Insulate and cover. My tub has no insulation, which I wouldn’t do again. A good cover and insulation make a big difference in winter. See DIY hot tub insulation and the ultimate guide to hot tub covers.
• Let it defrost. Ice on the coil is normal. Don’t chip at it.
• Keep the electric heater as a backup for long freezes if you can. How to control a heat pump with any hot tub spa pack explains how.

If you’re still deciding whether a heat pump is right for you, start with the ultimate guide to air source heat pumps for hot tubs and why I fitted an air source heat pump to my hot tub. If yours isn’t heating as it should in the cold, see my air source heat pump isn’t heating my hot tub.

Want a heat pump on your tub?

The Comfortline range I use, from 7kW up, plus connection kits, is on parts4tubs.co.uk for the UK and Europe. If you’re building a new tub, my DIY hot tub plans include the plumbing layout, so there’s a place for the heat pump from day one, along with the insulation I skipped.

FAQs

Do air source heat pumps work below 0°C?

Yes. Mine is only rated to 0°C and still heated my hot tub at about 2°C an hour at -4°C. Output and efficiency fall as it gets colder, and the unit defrosts itself periodically.

Is a heat pump still cheaper than an electric heater in freezing weather?

In my test, yes, by about two-thirds. A resistance heater always gives 1kW of heat per kW of electricity, and a heat pump still gives more than that below zero.

How long does a heat pump take to heat a hot tub in winter?

It depends on the size of the heat pump, the tub and the weather. Mine took about 4.5 hours to go from 30°C to 39°C at -4°C, about half its mild-weather speed.

Will my heat pump freeze up?

The coil will frost over in cold, damp weather. That’s normal, and the auto defrost clears it. Keep the water circulating, and see how to defrost a Comfortline.

Should I switch to the electric heater when it’s freezing?

Not usually. Even below zero, a heat pump is normally cheaper to run. Keep the electric heater as a backup for very long or very severe freezes, or if the heat pump can’t keep up.

How does your heat pump cope in the cold? Share your numbers 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