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.

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Bob Bazley
Bob Bazley
2 years ago

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 !”

Bob Bazley
Bob Bazley
Reply to  Mars
2 years ago

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
Bob Bazley
Reply to  Bob Bazley
2 years ago

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

Bob Bazley
Bob Bazley
2 years ago

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 ?

2 years ago

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….?)

Reply to  Mars
2 years ago

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

Tom Kacandes
Reply to  Steffan
11 days ago

@Steffan I think you may be misstated what the 1% of coldest “days” actually are: the 24 hours that get averaged into a “day” nearly always vary in temp in a “diurnal” pattern being how energy engineers I hang about with precisely say “it gets cold at night” for about 6 hours in particular, which drags the average for the 24 hours down of course. Where I disagree with your statement is that VERY rarely (engineer speak for “never”) would this translate into “your house will be colder than usual” – no, it won’t because unless you live in a literal paper bag the house as a system has a fair bit of thermal mass that holds the interior temperature up through the extra cold night time. These bits of physics applied to buildings (in US, we use the term “building science”) are extremely consistent such that in my job deliberating heat pump policy we see these sorts of misunderstandings causing heat pumps to be quite oversized for no reason with negative consequences. Your concern that all homes with a heat pump would have this problem getting cold temps inside for days per year are likely not borne out in the general experience even in the relatively balmy UK. By the bye, it was -5C here in New York and our winter is just getting started – we will see overnight lows of -20C a dozen times in the months ahead, so thank you thermal mass for bouying the internal temps through those bits.

Derek Marsh
Derek Marsh
2 years ago

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.


Neill Anderson
Neill Anderson
2 years ago

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!

Lynn Beasant
Lynn Beasant
2 years ago

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
Jon C
2 years ago

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
Gary Hardman
2 years ago

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.


2 years ago

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.

Shajahan Miah
Shajahan Miah
2 years ago

Hi, Mars and Kirsten. Sorry for the late comment. This is my second winter and the two February cold weeks was a test for my system. I did feel the cold, but I must stress, we live in a cold country, and a little bit of cold does not hurt! During the coldest days, when it’s sub-zero temperature I can get the bedroom temperature to 19 degrees, which is sufficient. Just to reiterate I have two systems, 6 KW ASHP and a 3 KW Inverter Air Con. Without my air-con I would have really struggled during the cold weeks. I knew 6 KW is underpowered for a 3-bedroom post-war semi detached house and hence why I also have a 3KW AC. During the colder days I was hitting 35 KWh per day and not using any other heating system.

To keep things simple I’m going to bullet point my opinions and recommendations:
* I think yours is a unique situation as your property is older and bigger so people cannot compare that to terraced or semi-detached.
* Your filter getting blocked regularly, I feel is a design issue and the manufacturer should deal with it. I just don’t understand why there has to be a filter, surely the pump can handle some sediment in the water. I feel that machine has been over engineered. You should not have to sort the filter out several times in a short period of time.
* I think a 9 kilowatt system would have been better for me. I could have just kept that running at a lower power through the whole day. I fitted the system myself so I had to do with what was available and 6 kilowatt was available. I got a very good price. I wish there are more choices.
*I feel my air-con is the lifesaver and the icing on the cake. During the cold days I can be in the living room with temperature as as high as I like. Just like people huddle around their wood fire during cold days! During the cold days I use my two pedestal fan to blow the warm air from the air-con around the house. Not a big deal for me. I also use TVR to do some balancing act! I got a timer TRV and that helps as I have the lounge off most of the time as the AC makes the room hot.
* “A 2015 study by Dutch scientists, for instance, found that women are comfortable at a temperature 2.5C warmer than men, typically between 24-25C. Men and women have roughly the same core body temperature, at over 37C; in fact, some studies have found the female core body temperature is slightly higher” – The Guardian. People like convenience. This is where the problem lies. We have to sacrifice some convenience in order to become more sustainable.
* People have called me a lot of things, I even call myself crazy for doing what I’ve done. Now on windy days I check the UK wind power generation and the variable unit rate at night falling. Although my night rated is cheap and fixed, still the fact that cheap wind energy is been used to heat up my house and water is so comforting. I work in evening from home so this works very well for me.
* Overall I am very happy with my setup and have no regrets. I am free of fossil fuel and gas engineers.
* The only one issue I have with my air source heat pump is maybe once a week or so the compressor makes louder noise for few minutes and then it is resets itself. The panel display a pressure issue. Not a problem for me.
* Had it not been for the reduced night-time rate I think my bill would have been a lot higher.
* I just hope my systems keep running like this and if it comes to the worst I will just simply replace either of the systems myself. I did not want to fork out thousands of pounds for a system with the the manufacturer providing the support. I’m not getting any RHI and don’t really care as I feel they give with one hand and take with other. I did not want to take this route as I do not think I would have the money.
* My solar is doing fine and I make the most of it, either its is heating the house or the hot water tank. Just a bit depressing during the coldest 3-4 months there’s hardly any sun.
* I would advise people to do the research and make sure you size your system appropriately, otherwise you’re going to get your family members complaining constantly. And you will lose a lot of sleep and money. Also to think about my 2 system as that might be a good solution. In hotter days my AC is the star! Everyone loves the cool temperature thanks to free solar electricity (Feed in would pay for my system).

I would just like to say, love you guys and your vlogs, keep up the effort. You are helping many people wonder about this subject.

I welcome any questions from anyone.


Shajahan Miah
Shajahan Miah
2 years ago

Quitting Gas
I live in a 3 bed semi and work from home. My old gas boiler died on me during an autumn. I knew it was a replacement job but as I was contemplating solar I was wondering if I could leave gas. Gas was only for hot water and heating. I decided to not replace the boiler and asked my gas man to fit a hot water tank. He kept delaying and I was left with using electric kettle for hot water for bathing for a month. I was utterly disappointed when this so called engineer said he does not have the certificate to install hot water tank but will get his mate and tried to charge me double! I have a gas safe mate who I did not want to bother as he is far and busy. He felt so sorry for me and called me crazy for wanting to quit gas. He said I had not thought of how I will heat the house. Initially I thought few covector heaters will do the job as it’s just me in the house. Was I wrong!

My mate installed the water tank and capped the gas meter. The 170 ltr solar direct tank was about £400, installation was £350 and electric work was another £300. He challenged me that I will call him again to reinstall a gas boiler and advised me not to remove the central heating radiators. Now my mind was buzzing about solar and how I will heat the house. I wanted to get in there before FiT subsidy expired.

I got a 4 kW solar system installed over east west roof with quite a bit of shading from neighbours roof. I do have optimisers to handle the shading and the solar is performing well considering everything. I get 2200 kWh in a full year. In the UK we cannot rely on solar for winter months. I have a Solar iBoost for dumping excess solar into the water tank before going to the grid. So in warmer months I have the comfort of knowing that sunlight provides me with free hot water!

I read about all the heating system there is out there. I moved from one electric rate to another and in one very cold day I used £10 worth of electricity just to keep 2 rooms warm with convector heaters. This was quite depressing and I felt the cold when I would go to the bathroom and then move back to the living room.

I managed to make it though December just to understand things. I appreciate how difficult it would have been for the ancient dwellers of this land! I originate from the tropic so feel extra cold! In the middle of January I installed a 3 kW inverter air con that can also provide heat. Ah, the feel of cosy rooms! This was an easy-fit DIY electriQ brand from aircondirect costing £500. I used to run this at full speed with doors open (in the rooms I used) and 2 pedestal fans to blow the air around the house. This almost did the job but I missed central heating!

In summer after much research I found a 6 kW inverter air source heat pump. It usually retails for about £1900 but I managed to get it for £1100. The model is AHP-60 from airconditioningcentre. I managed to install both units myself as a competent DIY person! I do not have a buffer tank. Some will make you buy one but it’s not always necessary, like in my case. The plumbing materials etc cost me about another £500. I also upgraded 2 radiators to bigger size and fitted all with TRVs. When I first turned the system on and the radiators were getting hot, I felt I did it! No gas engineer in sight!

I now have cosy warm house heated efficiently with electricity and no gas in sight! Little things like having towel warm and dry in the bathroom is actually a big thing. I used to wash the towel every few days as the dampness caused smell to build up. Now, I must emphasise nothing beats the convenience of gas, whether for cooking or heating but if we want to move towards a more sustainable future we must compromise. In the coldest days when temp is around zero I do have to do some balancing acts such as use both my units to heat only the rooms I use. I’m conscious of my electric consumption so do not run the systems all the time. I can get the bedroom temp to 18/19C during sub zero temp. The living room can be made hot with the air con! It heats the room in about 7 mins! This is positive compared to central heating. I’m a bit paranoid about temperature and have small LCD thermometers all over the house! I have 2 energy meter units displaying my solar generation and grid consumption. I can see at a glance what’s going on.

In warmer months my electricity bill drops to practically nothing. I use excess electricity to cool the house with my air con. In the hottest few days I drag my mattress into the living room and sleep like a baby under the air con and get people jealous. It only consumes about 250 W during the night to keep the room at about 23C.

An argument has been that gas is burned to generate electricity which I use so in effect I have not left gas. My argument is I am on a renewable electricity plan so at least I can say I use green electricity. I do want the whole country to be on renewable but I cannot wait for everyone. I cannot agree with that argument because electricity becomes so cheap at night as demand drops and wind power is available meaning gas power station are turned down. I uses this night time “wind” electricity to heat my water tank and run both my heating units to heat the house as well as run other appliances. I am on 5p/kWh from 12.30 am – 4.30 am and 13.5p rest of the day. In fact electricity can be so cheap, it can be free during night when you look at the actual half hourly rates. Also I have simply plumbed the solar coil in the water tank to the central heating so that it preheats the water up to 55C there by reducing the more expensive direct electric immersion heating.

I will retain the gas meter and gas line as the cost to remove them is £800 and can devalue the house. Gas gets rewarded on EPC and by people. I have to pay the standing charge for the meter. I was on a no standing charge package but these have disappeared now. Sad situation really. You would think I would be rewarded for leaving gas but no.

My gas mate recently visited and said “you did it”! I do agree that I was crazy to make the move! I must be the only one in my areas with such system. I wish there were more people so support for maintenance would be easy to access. I am prepared for any major issues, if I cannot get it repaired reasonably I will simply replace the unit. This is my 2nd year and my mind is at peace knowing that I have managed to leave gas with a reasonable alternative system. I do like the fact that my excess solar goes to my neighbours! My neighbour loves me for what I have managed to do. I also do a lot of gardening and in fact part of the front garden is now a veg bed! I recycle as much as I can. I rear few quails and chickens for egg and meat which makes me appreciate the effort meat production takes. I try to be conscious of my impact on the environment. We live in an old county with old infrastructure and housing stock but it is possible to make a move to renewable if we are willing to sacrifice convenience. I do get a little annoyed when out gardening and a little sales fellow says “do you want a free gas boiler?” I do politely give them a lecture! In fact one wanted a tour of my system and said he will consider such system for himself!

After going through all this I now celebrate Winter Solstice (Yule) and Summer Solstice! During one Yule, I couldn’t get the fire going. Once I did my niece shouted, yeah he helped us! I said who? She replied the sun god!

2 years ago

[…] winter was telling for our air source heat pump, especially after a bitterly cold spell in February, and the main area of concern was that the rooms to the north of the house were not getting to […]

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