How to Defrost a Comfortline Air Source Heat Pump (and Why It Ices Up)

Last updated: September 2026

The first proper cold snap of the winter always makes me wonder how my Comfortline air source heat pump is getting on. It’s only rated down to 0°C, so when I woke up to -2°C (28.4°F), with the cars thick with frost and ice, I was keen to find out. Here’s what I found, why heat pumps ice up, and exactly how to run a manual defrost on a Comfortline.

What happened at -2°C

We’d had a very mild start to the “winter”. Then an Arctic blast arrived, and it was the first frost of the year. When I woke up, I opened the WiFi app on my phone, half expecting to see the low ambient temperature warning.

I was pleasantly surprised. The temperature had probably dropped lower than -2°C overnight, but my hot tub was still holding at 30°C (86°F). I hadn’t had the electric heater connected since I installed the heat pump, so the heat pump was clearly still heating. That put paid to the idea that these units just stop working outside their rated range. As I’ve said all along, they’re just less efficient, and the manufacturer can’t guarantee what the COP will be.

After a cup of coffee I went out to have a look. The unit was thick with ice and frost. I’d assumed it would stop working if it frosted up that badly, but it was still going.

Why do air source heat pumps ice up?

Let me explain this as simply as I can. The heat pump pulls heat out of the outside air by running very cold refrigerant through the finned coil (the evaporator) while the fan pulls air across it. To take heat from air at, say, 2°C, the coil has to be colder than the air, so on a cold day its surface drops below freezing.

Air holds moisture, and when damp air hits a coil that’s below 0°C, the moisture freezes onto the fins as frost. That’s why icing is worst on those damp days around freezing, which describes most of a British winter.

Once ice starts to build up, you get a vicious circle:

1. The ice acts like insulation and blocks the air from reaching the fins.
2. Less air and less surface area means less heat gets into the refrigerant.
3. The coil runs even colder to compensate, so more ice forms.
4. The compressor works harder for less heat, and efficiency drops.

If nothing clears it, the heat output falls away and the unit will eventually stop heating. That’s what the defrost cycle is for.

How the defrost cycle works

The Comfortline has auto defrost. It watches the coil temperature and, when ice builds up, runs a defrost by itself. On my -4°C test it defrosted itself several times during a heat-up without me touching it.

During a reverse-cycle defrost, the heat pump briefly runs backwards, like an air conditioner, and the outdoor fan stays off. Hot, high-pressure refrigerant is sent through the outdoor coil instead, so the coil becomes the warm side and the ice melts from the inside out. InterNACHI says a typical defrost lasts from 30 seconds to a few minutes.

From the outside it looks as if the unit is spraying hot water down the coil. It isn’t. It’s the ice melting off in sheets, sometimes with a bit of steam. It’s one of the coolest things I’ve seen my heat pump do (apart from saving me a fortune and heating my tub 8x quicker). Yes, I know I need to get out more.

In a reverse-cycle defrost the heat has to come from somewhere, so the heat pump briefly takes a little heat back out of the water. That’s why the circulation pump must keep running during a defrost, and why heat-up times get longer in cold weather.

How to manually defrost a Comfortline

The manual describes both the ambient temperature sensor, which stops the unit when it’s too cold to run, and a manual defrost mode. Here’s how I did it on mine:

1. Make sure the heat pump has been running for at least 10 minutes. If you try before then, nothing happens. I found that out the hard way.
2. Press and hold the Mode key and the temperature down key together for 5–6 seconds.
3. The heating light starts to blink. That means you’re in manual defrost mode.
4. The fan stops, and after a minute or so the ice starts to come away.
5. Leave it to finish. On mine, the coil went from thick with ice to completely clear in under 2 minutes, and then it went back to heating.

You can only use manual defrost once every 30 minutes. At -2°C I only needed it once, and the coil was completely clear.

Check your own manual. Button combinations can vary between controller versions and model years. If your unit came with the newer WiFi controller, the procedure may be different.

What not to do

• Don’t chip or scrape the ice off. The aluminium fins bend easily, and one slip can puncture a refrigerant pipe, which is an expensive repair.
• Don’t pour boiling water over it. The water can refreeze in the base and drain, and pouring it near the electrics isn’t clever either.
• Don’t cover it while it’s running. A winter cover is for a unit that’s drained and switched off, not one that’s working.
• Don’t switch off the circulation. The heat pump needs water flowing through it, especially during a defrost.
• Don’t block the drainage. A defrost produces a lot of melt water. Stand the unit on feet or a plinth so the water drains away and doesn’t refreeze into a block of ice under it.

Troubleshooting: when icing isn’t normal

What you see	Likely cause	What to do
Light frost that clears on its own every so often	Normal cold-weather running	Nothing. Let auto defrost do its job.
Ice builds up even in mild weather	Restricted airflow: leaves, a dirty coil, or the unit boxed in	Clear the space around it, and gently clean the coil with a soft brush and a hose.
Ice on only part of the coil, or it never clears	Possible sensor fault or low refrigerant	Call a refrigeration engineer. Refrigerant work needs an F-Gas qualified engineer (EPA 608 certified in the USA).
The unit shows an error and stops	Low ambient cut-out, flow fault or pressure fault	Check the code in the manual. Check the pump, filter and valves.
No ice, but no heat either	Usually water flow or control settings	See my air source heat pump isn’t heating my hot tub.

Getting the best from a heat pump in winter

• Hold the temperature rather than heating from cold. I keep my tub at around 30°C (86°F) during the week and turn it up for the weekend. Heating up from cold in freezing weather takes a long time.
• Keep the cover on and in good shape. A waterlogged cover lets a lot of heat escape. See the ultimate guide to hot tub covers and DIY hot tub insulation.
• Insulate the pipe runs between the heat pump and the tub.
• Keep the electric heater as a backup for long freezing spells, if your setup allows it. How to control a heat pump with any hot tub spa pack explains how to set it up.
• Think about siting. Somewhere sheltered from the prevailing wind, but with open airflow, helps. Mind the noise too: see how noisy is an air source heat pump?

If you want to see how mine did when it got even colder, read I ran my air source at -4°C: the results. For the full review of the unit, see my Comfortline heat pump review. If you’re choosing a heat pump for a cold climate, how to choose the right air source heat pump and the ultimate guide to air source heat pumps for hot tubs cover the ratings to look for. Units like the Vian Power Plus are rated to -10°C.

Need parts?

For Comfortline heat pumps, WiFi adaptors, connection kits and pipe insulation in the UK, head to parts4tubs.co.uk. For the USA, it’s parts4tubs.com.

FAQs

Is it normal for my hot tub heat pump to ice up?

Yes. In cold, damp weather, frost on the coil is normal. As long as the unit clears it with its auto defrost every so often and keeps heating, it’s working as designed.

How do I force a defrost on a Comfortline heat pump?

Let it run for at least 10 minutes, then hold Mode and temperature down together for 5–6 seconds until the heating light blinks. You can do it once every 30 minutes. Check your own manual, as controllers vary.

Can I pour hot water on a frozen heat pump?

No. It can refreeze in the base, and it’s risky near the electrics. Use the defrost mode, or let the unit’s own defrost do it.

Will a heat pump still heat my hot tub below freezing?

Usually, yes, just more slowly and less efficiently. My Comfortline is rated to 0°C and still held my tub at 30°C at -2°C, and it kept heating at -4°C.

How long does a defrost take?

Usually 30 seconds to a few minutes. Mine cleared a thick layer of ice in under 2 minutes.

How does your heat pump cope in the cold? Let me know 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.

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