Did our air source heat pump cope at -6C?

by Mars

Just when we thought we were rounding the corner of another mild winter, we were hit by freezing cold easterly winds in early February and our temperatures tumbled to -6C.

This is the coldest temperatures have dropped since we’ve had the heat pump, and the house certainly felt it, so we wanted to share our thoughts on ASHPs when things get really cold and icy.

As mentioned in the video, this some of our consumption from February 2021 during the cold snap. There were some heavy consumption days. As always, please ignore the room temperature column as this is the temperature inside our boiler cupboard.


Bob Bazley 19 February 2021 - 08:39

Great video, and the data is very interesting but I look at the average room temp and that seems high ? Isn’t around 21c meant to be a good temp ? I’m thinking is that due to you running the wood burners as well ? Having said that if the room feels cold then your obviously going to increase the temp of the room. The electric costs will be higher for sure and with the RHI and the production of solar to offset that it will certainly lessen the shock of the price of electric. I’d like to understand more about your Solar, the size of the array and do you use battery storage to benefit from the excess (is there excess if the air source is running ?) How much electric kWh does the air source use during a normal day / evening ? I think the idea of switching it off in the middle of the night might be a good idea as the underfloor heating will take time to lose the residual heat but where you have radiators its going to get cold fast. Ive been advised that I would need a 14kw air source which. Retrofitting underfloor on two floors is a major cost but sometimes its not about the ROI but that your comfortable in the house and thats part of the normal house cost. Thanks for the update much appreciated. PS you can see that Kirsten wants the house to be a lot higher temp than Mars, finding that middle ground balance well always be difficult for people living together and getting the heating right, what was that book called ? “ Women are from Venus and Men are from Mars !”

Mars 19 February 2021 - 09:44

Morning Bob.

Please ignore the room temperature column – that’s the reading from our panel which is in the water tank cupboard in the utility room, so it’s always super toasty in there.

I’ll try and share some data on the night versus day usage – to my mind, it’s not worlds apart.

And yes, the balance is an interesting one. When the house is at 21C, I get really warm (but Kirsten is comfortable) which is why I’m in shorts and T-shirts most of the time. When I have to go outside, I find it a drag to layer up so I collect logs and do veg garden inspections in my shorts too with a fleece top. Then when I get back inside the house, the 21C feels like a sauna.

Bob Bazley 19 February 2021 - 10:10

Do you feel warmer on the ground floor with underfloor heating than you do in the upstairs ? I ask this as its two very different ways of heating a room and the general opinion is that underfloor heating provides a better overall heat to the room. I think having the air source off for a short time at night would contribute a real saving in electrical use over the year potentially in the 10 – 15 % region as its at a time that would be completely grid generated and not from solar production. While this may seem small its all about incremental and ultimately that would make the difference. I found your earlier posts on solar and production so have read through them. My setup is similar with 12 Ja solar panels. (455w giving me an array of 5460kwh. The Inverter is a LuxPower 3.6 its that size as thats the limit for DNO to accept export energy but I also had the same thought as you, why cant i just produce the full array output. I have it tied to battery storage 4.8kv and agree the numbers might not work for a ROI but not everything needs to be based on that, when you buy a car or a new kitchen do you look at the ROI ? In the long run it provides benefit and on good days will help you be more green if you use battery charged from the sun. Overtime the tech will get better and more efficient. Over four days in from the 9 till the 12th of Feb I produced over 20kWh the array which bodes well for the longer days and higher sun.

Bob Bazley 19 February 2021 - 10:21

I meant to say that was over 20kWh per day over the 4 days so 80kWh in total 🙂

Mars 19 February 2021 - 22:18

That’s a good question Bob. I’d say our bedrooms, which are carpeted and above the living areas are probably a touch warmer. But the UFH is hugely effective too. Not much in it.

The solar question is answered by pitch, I’ve discovered. It’s the angle of the light (on our roof) that prevents full production.

I understand your ROI point. For “nice to have” items, personally, I’d like for them to be able to pay themselves off. But batteries will get cheaper, and maybe instead of investing $1.5 billion in Bitcoin, Mr. Musk could have provided a subsidy to potential Powerwall customers to incentivize uptake.

Bob Bazley 19 February 2021 - 08:44

Just found your solar production post so thank you for that but do you have a post with the specifications and layout of your entire solar array setup ?

Mars 19 February 2021 - 12:12

I think we may have covered it here: https://myhomefarm.co.uk/do-solar-pv-panels-work-in-the-uk

I think we’re overdue in doing a solar PV video. ASHPs have dominated our lives for the past few months.

Steffan 19 February 2021 - 09:17

Its nice to see an honest video. Sadly, we have all been spoilt by the comfort of gas boilers and the sheer available heating power they can give for such a low cost. Heat pumps , at the moment, cant replicate that flexibility and instead should be sized as a compromise. According to MCS, they should be sized to cover a home for 99% of cold weather. That means 1% of the time your house will be colder than usual and thats about 3/4 days a year! Its not only those in the country that will have to put up with this but all homes with a heat pump.

If it was me personally, I would be complaining to the manufacturer that if the heat exchanger in their product is so sensitive to rust then why a magsafe filter is not mandatory in the installation!!? Get a plumber to fit a magsafe on your return indoors and learn to clean that instead. I have one, and clean it every year or so.

p.s. are you sure its rust debris? could it be an air blockage instead? flushing like that is no good way to remove rust but is a good way to remove air blocks. If its an air bubble problem than an auto valve indoors at a high point could be an relatively easy patch (I dont know if they recommend auto bleed air valves for heat pumps….?)

Mars 19 February 2021 - 12:17

Great insights and comments as aways Steffan.

The installers are coming back out in early March to reevaluate things. There’s clearly a low wrong, and the crazy thing is that as first time ASHP owners, it’s taken us two years to establish that things probably aren’t running the way they should.

We have a Fernox TF1 installed on the return, and to be honest it’s a bit useless. It hardly ‘catches’ anything. I agree with your argument completely though – why is there no better filter added, as systems are bound to develop corrosion and rust.

We have an auto bleed valve in our utility room – it has a little red release valve, which we open regularly to release air. There’s been no trapped air for months now.

anna+mycoe 19 February 2021 - 13:42

Hi Mars
Magna clean is a good system filter and dead easy to clean

Mars 19 February 2021 - 17:33

Thanks Anna. I’ll google it this weekend.

Derek Marsh 19 February 2021 - 10:48

Hi Mars,

I have just watched your latest video and I was quite surprised at the amount of maintenance you have had to perform just to keep your ASHP operational. I was also surprised that you did not get an alarm or warning that your system was about to fail or had actually failed. It would appear that the control system for ASHP could do with a good dose of improvement.
Blocked filters that have to be cleared to get your ASHP back up and running are obviously a design weakness. If this is going to be a regular issue, I would have expected the designer to have incorporated a duplex filter system, whereby you can switch over from the dirty filter to a clean one with a simple lever, which then allows for the dirty filter to be cleaned at leisure. The changeover could be performed without the need to stop the ASHP. More sophisticated filter system would have an alarm to indicate that the filter is starting to block and there are even systems available that can automatically perform the changeover and then backflush the dirty filter to clean it.
Do you have any rust inhibitor in your system?
I have been working on the article that you asked me to write, and in the process I have created a spreadsheet to model the effects that changes in operating conditions have on an ASHP system compared to a gas boiler system. It has highlighted that very careful thought needs to be put in at the design stage to ensure that an ASHP system will function under all anticipated operating conditions. This is particularly true in a retrofit scenario.


Mars 19 February 2021 - 13:45

Truthfully, the last few weeks have been frustrating, especially given how much it has cost is to heat the house. But at least the installers are coming out and hopefully we’ll get to some kind of resolution.

The installer added a lot of glycol to the system, which they said had inhibitor in it. I would contest now, that perhaps it was not enough.

I can see that there have been issues during our design stage, which we will get into with the installers on March 2. Watch this space for updates.

Neill Anderson 19 February 2021 - 11:11

I’m no expert in this field but am gathering information for a potential self build. It’s refreshing to see real life situations with honest feedback, positive and negative, and real data. In addition to the research, you did you have also paid with your own money to then follow your beliefs: fair credit for that!
You also clearly spend time and effort in making and editing the videos and keeping on top of your website. I don’t follow many social media entries but I think you deserve more followers.
Anyway, based on what I have understood I suspect that your building and installation are only just suitable for the size of ASHP you have. From memory you have an 18kW unit and your house is pretty large with quite a proportion of glazing. It probably isn’t that well insulated overall and I would guess is not that airtight.
I am sure that a full heat loss calculation was undertaken prior to installation but wonder whether there was a representative air tightness value used: was this physically measured? In addition it seems odd that the new refit kept the old oil boiler but there is no simple facility to isolate the ASHP loop to enable easy sue as a backup?
The fact that when the heat supply is off the temperature drops quickly indicates either loss through the fabric of the building (i.e. less insulation than desired) or loss through air movement (i.e. carrying heat away via leaks). People talk about “thermal mass” which isn’t a real parameter but usually means a combination of heat storage capacity and the speed at which heat energy can be gained/lost by the building shell. Some construction/insulation types retain heat for longer than others (search “decrement delay” for lots of discussions!).
Radiators need a much higher temperature differential than underfloor heating (UFH) (due to their much smaller heating surface) and from the ASHP efficiency (CoP) graphs I have seen all lose efficiency as the system flow temperature increases. A system running rads and UFH is always therefore going to be compromised as to efficiency.
I am sure your old oil boiler would probably be at least double the power (i.e. around 35kW) of your ASHP and therefore able to raise the temperature twice as quickly. Hence why such systems can be on in blocks rather than continuously.
I suspect an air tightness test would provide valuable information as would a look around the main structure with a thermal imaging camera whilst it is cold!
My conclusions, as you hinted at yourself, are that the ASHP starts with an advantage (i.e. the CoP potential to extract “free” heat energy from external air) but this advantage works best with a building that stays at a constant temperature by virtue of real air tightness (approaching passive standards, around 0.6 air changes per hour), good insulation (way above Building Regs and likely above retrofit potential) and slow decrement delay (PIR insulation boards are poor in this regard although highly insulative for their thickness).
You also note that you feel warmer on blue sky days: possibly good solar gain from the large glass rea of your conservatory. But just as equally this works against you on cold cloudy days; especially if the glazing isn’t really high spec!
I live in a reasonably modern large detached house (about 2200 sq ft) that is of the modest spec and not that well built. The oil boiler is around 35kW. It’s not greatly insulated and self evidently not very airtight! It’s really hard to keep it above about 19 degrees once the temperature drops below about 4 degrees and it’s really noticeable that whenever it is even moderately breezy the heat is simply sucked out of the house. Once the heating goes off it’s noticeably colder within 30 minutes. Without the brilliant wood burner in the open plan downstairs lounge/diner/kitchen we’d be pretty frozen.
I am sure 20 years ago someone did the standard heat calculations and sized the rad accordingly but I have never understood why not use bigger/biggest ones with thermostatic valves: the added cost in a complete house build would be a few hundred pounds at best. In my previous house I redid all the central heating, massive radiators (compared to calculated sizes), 22mm pipe everywhere and a condensing gas combi boiler and the result was a house that was much warmer and smaller gas bills!
All of which perhaps explains why I am now looking to downsize to a smaller home but one that will hopefully be purpose designed/built to a much higher standard: not quite fully passive levels but close. But much more research etc. left to do; hope this may be useful and good luck.
I also believe that introducing new inhibitor in any old system always tends to scour loose any debris etc. and also often finds new leaks. I am sure there must be better (bigger) filters that can be added into a more convenient location leaving the strainer at the ASHP as the last line of defence!

Mars 19 February 2021 - 22:38

Thank you Neill for a detailed, thoughtful and helpful comment. We agree fully about the insulation, but we’re also fairly certain there has been a lack of flow implementation consideration for the oil boiler to ASHP switch, which we hope will be resolved on the installers’s visit in March.

Interesting comments about the inhibitor, and I wonder if that didn’t happen in our case.

Good luck with your new house hunt. If we move in the future, insulation will be a top priority in that new home.

Lynn Beasant 19 February 2021 - 14:23

Really interesting. We have never touched our filter or anything! Installed in July this year. Retrofit but our radiators were 31 years old so they were all replaced. Maybe that’s why we have no issues? We have used less than you but yes our bills are more than oil this winter. Solar is on our list now. Off to find your solar video.

Jon C 19 February 2021 - 20:53

Thanks for the update, I agree with what you’re saying about the suitability of ASHP for everyone and my experience is similar to yours although not had any filter issues over 3 years but we’ve just got new plastic UFH pipes. Interested in your consumption table, what is the Booster kWh table showing ? 2kWh for Feb 12 ?.
That was the highest usage day for me too, but I reckon our electric assist added 50 to 80 kWh that day.

Here are my figures for that period, so I think your doing quite well. I don’t have a temperature recorder but using figures from RAF Watisham (9 miles away) I get:

1 2 °C 4 °C 0 °C 132
2 7 °C 11 °C 1 °C 84
3 7 °C 9 °C 4 °C 90
4 6 °C 8 °C 3 °C 100
5 7 °C 10 °C 5 °C 88
6 5 °C 7 °C 3 °C 86
7 -1 °C 2 °C -3 °C 168
8 -3 °C -2 °C -3 °C 164
9 -2 °C 0 °C -4 °C 132
10 -2 °C 0 °C -5 °C 160
11 -2 °C 0 °C -5 °C 164
12 -2 °C -1 °C -3 °C 174
13 -1 °C 1 °C -3 °C 168
14 1 °C 2 °C -1 °C 136
15 6 °C 10 °C 2 °C 92
16 8 °C 9 °C 6 °C 84
17 9 °C 11 °C 6 °C 74
18 7 °C 9 °C 4 °C 72

I reckon the Storage AGA uses 40 to 50 kWh per day, irrespective of the external temperature. So it would be cost effective to replace with a second ASHP but not something I’d be allowed to do.

Looking forward to your next update, I still think the occasional very high day usage does not justify the extra cost of a Ground Source Heat Pump.

Gary Hardman 19 February 2021 - 23:49

Mars, good update about your filtration problems caused by the older plumbing and radiators in your house. As your probably aware, you might have to get your system flushed out once per year.

But let’s think about this a minute. The rusty parts are in the house and the new non-rusty and probably antifreeze treated parts are outside as the heat-pump and underground pipe.

Why not isolate those two parts by placing a heat exchanger in your utility closet. This way you can use distilled water in the heat-pump loop along with the antifreeze. Then in the house you can have the heat exchanger transfer the heat-pump heat to the dirty radiator water. This will also allow you to have a filter in the nice warm utility closet for the dirty radiator water.

You can read about the flat-plate heat exchangers starting on page 39 of the following Caleffi PDF article.


Lisa-Marie 24 February 2021 - 21:38

Hi there,

Really useful video folks and good to have ‘inside’ information.
Finally decided I am up for the challenge of an ASHP and awaiting my SMART SMETS 2 meter so I can be up and running with my solar soon.
My roof I have been told is 41 degree tilt and with a 1% shading factor not sure if I need a motorised solar arrangement, can you tell me what the website was again Miles you mentioned about the data into that?
Applied for my green homes grant too and very excited to be helping the planet in general.
Thank you for being an inspiring couple. It’s awesome.


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