I ran my Air Source at -4 Celcius Sub Zero – The Results

Whilst a little unwelcome in general this cold snap, it has provided a certain opportunity. The uncharacteristically cold weather has provided a prime opportunity to do some real-world testing with my Air Source Heat Pump in sub zero conditions.


The Background

For those of you that follow the blog or my YouTube Channel, you will know that I talk about air source heat pumps a lot. Specifically, the Comfortline which I have on my own Hot Tub.

As I am sure you are all aware, the efficiency of Air Source heat pumps does drop as the ambient temperature decreases. This week, we have had a really cold spell (for us anyway I know many readers have seriously cold weather!) and temperatures have plummeted sub-zero – or late 20s in degrees F.

For me this was going to be the first cold spell where I was going to be using the heat pump and I wanted to document the results.

The Grey Area

Sub-Zero – this tends to be the grey area for heat pumps. What happens when the temperatures go outside of what the manufacturer recommends? Will the heat pump just stop. Will it not work? Will it just freeze up? All of these are somewhat unknowns. Likewise, the performance of the unit at sub-zero or even zero for me was an unknown too. The manufacturer kindly doesn’t state the COPs at this level. Neither do many others just so we are clear – not sure why, but that is a whole other question.

So, it’s just after 8am on a Saturday morning and I am outside in the -4C or 24.8F. The sun is just coming up – everything is white and frozen. Beautiful morning. Time to fire up the air source.

I tend to hold my hot tub during the week at around 30C or 86F. Then, at the weekend when I want to use it I fire it up to 39C or 102F. I documented how the air source when I fitted it gave me a 66% saving on running cost and an 8X heat up speed. This was in moderate temperatures of around 15C or 59F. Now it was time to see what it was going to do in sub zero temperatures.

I cranked the thermostats up on the hot tub and the air source to 39C. The unit kicked in and we were off to the races.

The Initial results

The initial results were promising. Firstly, the unit was working despite it only being rated to zero. It has not failed, stopped heating or anything else like that which I regularly get asked about heat pumps outside of their temperature ratings. It also defrosted itself periodically during the heat up which was good too. Coped with the temperatures no problem.

What was interesting when I was checking the WIFI App, was that the unit was intelligent enough to work out that it could not run at 100% so was running around 75-80% on average. I guess it was too cold for it to get the benefit of the full power which I thought was clever. This is also one of the advantages of having an inverted heat pump versus an on/off model.

An hour later I came back to the App and saw the that temperature had risen by 2C – not bad at all when on full power when it was mild outside I was getting around 4C an hour and more when it was hot in the summer. 2C – that was good.  Two hours later and yes, it was up 4C so that 2C an hour was holding and it was still -3C or 26.6F outside.

If I contrast this to the 3KW electric heater I disconnected, this was doing around 0.5C an hour in good conditions. It would have been less than that in these very cold conditions but I don’t recall having a similar ambient temperature to compare with unfortunately.

What about cost in these sub-zero temperatures?

The thing with the air source heat pumps is that we want them on our Hot Tubs to save us some money. Mine, as previously mentioned saved my 66% in my general running costs when I last did the calculations in moderate temperatures. But what about at sub zero?

OK, so my 3KW electric heater would cost at present around 1 GBP / 1.2 USD an hour for me to run and I would get less than 0.5C temperature increase an hour. So, that would cost me around £20-25 to heat up from my holding temperature in these conditions and take 18-20 hours to do so. (For the statisticians amongst you, I am rounding and generalising allowing for some leeway too)

I’m not able to measure what the KW output is from the Air Source, which is why I am concentrating on heat up times as that can easily be measured and compared. My Air Source was pulling around 5KW when I checked the Smart Meter this morning and that was costing around £1.66 an hour to run. At 2C an hour, that is 4.5 hours to heat up to 39C and will cost £7.50.

The Sub-Zero Results

3KW Electric Heater – 18-20 hours heat up time and £20-25 in cost.

Air Source Heat Pump – 4.5 hours to heat up and £7.50 in cost.

That is roughly 4x quicker and 62% cheaper

I take that as a win win – even more so when the model is not rated for sub zero use!

What have others found?

I have this almost “mini community” of hot tub air source heat pump users that always get in touch to discuss results and performance in different conditions. It’s awesome! It was the same when it was roasting hot this summer.

James P – check out my YouTube channel he comments regularly on the air source videos. He is massively data driven and has everything connected to a logging system so he can see exactly what is costing him what.

He has found that it is taking roughly twice as long to heat up but it is not using twice the energy – you can see his very cool usage graph below.

James W –  Just a short email from him “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

What is evident here is that the speed of heating certainly decreases as the units are not able to extract as much heat from the colder air. What is also clear is that even if the units are only rated to zero, they still work and they still produce results.

For me – sub zero, 4X quicker and 63% cheaper than the equivalent 3KW electric heater that was in my spa pack.

Happy 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 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.

Today, I've helped over 700 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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