How to Defrost a Comfortline Air Source Heat Pump

Defrost a Comfortline


As the mercury has dropped this week, I did wonder how my Comfortline Air Source Heat Pump would cope in the cold weather. I’ve covered this product a lot on this blog and on my YouTube Channel.  In this article, I will explain how it is getting on.

We’re getting an Arctic blast this week which is dropping the temperatures somewhat. I think eventually, winter is upon us as to be honest, it has been very mild so far this “winter”.

Given that the Comfortline Air Source Heat Pump is only rated to 0C, I was really keen to see just how well it performed in the lower temperatures.

I woke up this morning to a -2C outside which is 28.4F – pretty chilly and the cars were thick with frost and ice. First time this year. I was keen to see how my Comfortline Air Source Heat Pump had faired to opened the WIFI App on my phone to check. To be honest, I was expecting to see the low ambient temperature warning.

I was pleasantly surprised – I am sure the temperatures would have dropped lower than that over night, but my hot tub temperature was still holding at the 30C or 86F – nice! Clearly the unit was still heating as I have not had the electric heater plugged in since I installed the heat pump.

So firstly, that put pay to the idea that these heat pumps just stop working outside of their recommended operating temperatures. As I have said all along, they are just not as efficient and cannot guarantee what the COP or Coefficient of Performance will actually be.

After a cup of coffee, it was time to go out and inspect the Comfortline Air Source Heat Pump to see how it was doing. I was surprised to find that it was thick with ice and frost. I would have thought that the unit would actually stop working if it frosted up and the ambient temperature was too low. Despite the ice, it was still going – great!

Why do Air Source Heat Pumps Freeze Up?

OK, let me try and explain this as simply as I can. The air source heat pump extracts heat from cold moist air (e.g. 0oC / 32F) as per usual. However, as it is extracting this heat, the moisture in the air freezes onto the heat exchanger coils.

The difference between the temperature of the refrigerant and the air temperature is smaller, so less heat is absorbed and less refrigerant evaporates.

If the temperatures don’t rise, then the heat pump is working constantly to meet the increased heat demand of the Hot tub. As the temperature remains low there isn’t enough time for the warm ambient air to thaw the ice. What you will see is that more and more ice forms over the heat exchanger coils. Now the downside to this is that a bit like insulation, the ice actually blocks the air from hitting the coils, reducing the total surface area for the heat transfer.

Any warm air that does reach the heat exchanger coils is already cooled by the ice block, which further reducing the heat differential. As you can see this is a bit of a viscous cycle.

As a result, less refrigerant evaporates into gas, and the compressor must do more work to meet the heat demand. The heat pump’s efficiency drops. If you don’t defrost the system, then it will eventually stop working.

Time to get defrosting!

Defrosting The Comfortline Air Source Heat Pump

I had read in the manual that there was both ambient temperature sensors for when the temperate us too low to operate and I also read that there was a defrosting mode too. It was time to give that defrosting mode a go. Time to defrost the comfortline!


The first thing you need to know is that the air source must be running previously for 10 minutes before you can set the manual defrosting mode. If you try and set the mode as I did before this, it just does nothing.

Once the 10 minutes has passed, press the “mode key” and the “temperature down” buttons on the control pad simultaneously and hold for 5-6 seconds and the heating light will start to blink.

This means you have entered manual defrosting mode. The fan will then stop and what happens next is just cool – no pun intended!

The unit after a minute or so begins to spray hot water down the heating element which totally defrosts the unit. That is what it looks like – I know this is not the case at all.

What is in fact happening is that The defrost cycle basically reverses the ASHP’s valve to air-conditioning mode, which then turns off the outdoor fan and switches the evaporator into a condenser. What this does is that this then warms the high-pressure refrigerant, melting the ice build-up on your heating system. Giving the effect that it is “sprayed” with hot water

In less than 2 minutes the unit went from being thick with ice to being totally clear.

A little sad I know, but that is one of the coolest things I have seen my Air Source Heat Pump do (apart from save me a fortune and heat my hot tub up 8x quicker) – yes, I know I need to get out more! Guess it is the little things in life and all that.

This mode can only be used once every 30 minutes but with a -2C or 28.4F ambient temperature, I only needed to use it once and it was totally thawed.

As I am sure you can tell, I am pretty pleased with the results

Happy “defrost the Comfortline” Hot Tubbin’

Can I Help You?

If I can help you in any way I would love to hear from you. You can get in touch using the form below.

Thanks - Andi


Hi, Andi here. I own 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.

Today, I've helped over 1000 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

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