It’s not been an efficient year for our ASHP so far

by Mars

The weather this year has been unstable, overcast, wet and cold. As the weather finally starts to improve and temperatures start to rise, it’s a good time to look at our air source heat pump’s performance so far in 2021.

In the table below, we can see that the average temperature has been steadily going up since in January, and with that the total air source heat pump consumption has also decreased. The consumption figures have all been extremely high, and even April and May are very high. Admittedly, we do want to keep the house warm (21C in most areas), and the consumption is definitely connected to the outside ambient temperature. The colder it is, the more kWh are used by the heat pump to drive the central heating.

ASHP consumption: January to May 2021

I pulled last year’s data for the same period, January to May 2020, and it made for some interesting reading and comparisons. For starters, last year it was a lot warmer. This naturally meant that the heat pump did not work as hard or as frequently. What I also found interesting was that the mean temperature in March 2020 was 6C and 6.68C in March 2021, and the air source heat pump’s consumption is nearly identical.

This clearly illustrates the correlation between ambient temperature and ASHP performance and this ultimately speaks to our heat pump’s performance for heating our property. Honestly, I don’t know if this is a good thing or a bad thing, so it’s best to think about this from a monetary and fuel comparison perspective. Had we still had our oil boiler, with colder outdoor temperatures, we would have consumed significantly more oil in 2021.

ASHP consumption: January to May 2020

At the end of the day we are talking about central heating. If temperatures outside are colder you will use more fuel to heat the house, irrespective of what that fuel is. The biggest sticking point for air source heat pumps though is the electricity tariff, and this is a point I have raised before. I can only hope that the UK government does something soon to reduce tariffs for ASHP users, because with the current trend, running costs will soon become much more expensive than oil and LPG.

It’s also worth mentioning that we haven’t factored in our solar production, which will definitely offset some of our consumption.

Our ASHP’s lifetime COP is now at 2.73 (with a flow temperature if 45C). We’d obviously like to get this above 3.

In April, we managed to improve the internal performance of our heating by putting in a larger circulation pump and upgrading radiators to K3s in rooms that were struggling to come to temperature.

This leads us to the next phase of the air source heat pump journey: decrease the total consumption of the ASHP, while keeping the house warm. The first way that we are going to try and address this is by potentially playing around with flow rate temperatures. We currently run at 45C, and I think we’ll drop to 40C this coming autumn and see how the numbers stack up against 2020 data. We will also address drafts and insulation, as we do every summer.

We’re also hoping that since the coldest part of the house will now come to temperature as a result of the new K3 radiators, this could lead to the ASHP turning on less frequently to satisfy heating demand. I’m not sure how much this will impact the consumption as our UFH will still be calling for heat for vast portions of the day and night in winter, but the radiator circuit will definitely have less demand. It’ll be interesting to see if this has a positive impact on our consumption.


CrashOverride 31 May 2021 - 09:17

Hi Mars, thanks for the update. The data is expected for the given circumstances. It appears to have been colder in The Netherlands as well thus increasing compressor consumption. The requested flow temperature of 45C is way high for efficient operation and even 40C is too high. By increasing the flow temperature the actual capacity of the heat pump is decreasing and also efficiency and inherently increase running cost. The best advice I can give is lowering the flow temp to the bare minimum where you can still heat the room to the desired temperature and perhaps increase the flow rate (Litre/Min) to extract as much heat from that heat exchanger as possible. At the moment of writing I’m installing my own brand new 9kW ASHP of which I will make a detailed article for everybody to enjoy and learn from.

Mars 2 June 2021 - 20:40

Thanks CrashOverride. We’ll continue to experiment and share our experiences.

As a complete aside, and unrelated to this topic, can you please give us an indication of how much it costs, labour wise, it costs to install an ASHP in The Netherlands?

John Hardie 31 May 2021 - 09:46

I have the same “beef” with the Electricity Companies. I also have an electric car for which I have a reduced tariff from EDF from 21.00 til 0700 and from Friday to Monday mornings. This is offset by a higher tariff at all other times. whilst I can choose the time to charge the car, I cannot regulate the times when the heat pump kicks in to maintain a comfortable temperature. This could result in the heat pump working for an hour either side of the cheaper tariff. If the Govt. really wants a shift to heat pumps, there should be a tariff introduced for that source. The reckonings for costing for mine were based on 11pper KWh. I am now paying 20p per KWh. with 9p at the cheaper rate. I could opt for a fixed rate of 14.5p which may be the route I will travel. I have compared most of the companies and there is very little difference when you factor in the standing charges, and different rates. Only legislation will make these things viable.

Mars 2 June 2021 - 20:42

Wow, calculations based on 11p/kWh were very optimistic. When was this costed?

Ours was calculated on 14.5p/kWh, which may providers offered for the first year – three years down the line, there’s only one provider that offers a tariff less than 14.5p.

Mark Crooks 31 May 2021 - 09:47

Watching with bated breath…

Our system was set up and MSC forecasts were calculated with a 50º flow temp. It seems counter intuitive but it would be interesting to see what affect this setting had on your running costs for a month.

CrashOverride 31 May 2021 - 10:19

Are you running an ASHP with 50 degree flow temperature? I think it might be cheaper to switch back to natural gas jf you need those high temperatures. With an ASHP you shouldn’t want to go higer than 35 degrees because of efficiency degradation. I don’t know about the UK but here in The Netherlands we pay 21-23 cents per kWh and 0,80 cents per m3 of gas. We need roughly a COP of 3.3 or higher to match gas in price. This is only feasible if you go low with the flow temp.

Mars 2 June 2021 - 20:43

Me too Mark. We’ll find out this winter. Fingers crossed.

NormanB 31 May 2021 - 10:08

Hi Mars reducing flow temperatures should help efficiency but the only way to reduce true demand in all conditions is to improve insulation. What are your future plans to do that?

Mars 2 June 2021 - 20:44

Norman, there isn’t a lot more we can do to the insulation. There are minor tweaks, but that’s about it. Any more ‘significant’ insulation would require breaking through the stud wall, which is not something we are up for doing any time soon.

NormanB 3 June 2021 - 09:20

Hi Mars – Not familiar with your house layout, construction or indeed planning constraints. However is it possible to convert your loft to a ‘warm loft’ by installing insulation under the roof rather than as is traditional installing insulation over the ceilings (or loft floor). More drastic is external insulation systems, especially on the more exposed elevations – clearly not cheap and planning issues may prevail.

Steffan Cook 8 June 2021 - 11:25

There is cause for concern in ‘warm loft’ insulation in the rafters. Better to keep the insulation layer at ceiling level rather than at ceiling and loft. Why? I don’t think spray foam will last (if you use spray foam rather than put in insulation boards), loft insulation might create condensation and wood rotting issues in the loft if airflow isn’t designed in , and insulation at the ceiling level is easier to control in general.

Derek Marsh 31 May 2021 - 17:27

Hi Everyone,

My wife and I met some close friends yesterday at a botanical garden after a long time not meeting face to face.
Whilst the ladies discussed very important lady things, my friend and I concentrated on the rather mundane task of putting the world to rights. He works for National Grid whose task is to convey electrical energy from the producers to the customers.
I am afraid that most, if not all of you, on the forum, will not be too happy about what he had to say.
His predictions are that electricity prices are more likely to increase, rather than reduce or even stay constant in the future, for the following reasons.
To transition from fossil fuels to green alternatives will require billions of pounds of investment, not only in wind farms or solar farms but also in the infrastructure required to transport the energy from A to B. At the end of the day the consumer will have to pay for this investment one way or the other.
When electricity transmission systems are being designed, the designers utilise what is called a diversity factor to calculate the size of cables and transformers etc. The diversity factor works on the theory that not everyone down a particular street will switch everything on at the same time. If everyone down that street now switches from gas to an ASHP and also buys an EV, it would not be too long before the lights go out.
So for the vast majority in this country to switch to ASHP’s and EV’s, it is not just a matter of installing a few extra wind turbines in the North Sea.
Every link in the chain from producer to customer will need to be assessed and quite possibly upgraded. Not a cheap, quick or easy task to achieve.

As far as electricity prices are concerned, the government do not have any direct influence. It is not like in the days of the Central Electricity Generating Board, when the government directive was to keep the lights on, not make more than 3% profit or 3% loss.
Nowadays there are probably 50 or more companies, all offering 20 or more different tariffs, all trying to make the maximum profit. It is my considered opinion that all the different tariffs are not designed to give consumers more choice, but are more like smoke and mirrors to confuse their customers and extract the maximum amount of money from them.

It is therefore down to each individual to do their best to reduce the heat loss, utilise solar gain as much as possible and optimise the operation of their systems to reduce their carbon footprint and reduce costs.

NormanB 3 June 2021 - 09:26

Good post Derek. I agree with the general thrust of it and it is exacerbated by the planning stasis in building new power stations which has the effects of reducing the threshold for brown outs. I can easily see a picture, not too far ahead, where national grid load shedding becomes a routine necessity.

Steffan Cook 8 June 2021 - 11:30

On the contrary, renewable power stations are coming online consistently. The load shedding comment is also mute, load shedding is now called ‘flexibility’ and is actively being planned as a cornerstone of grid operation.


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