Friends, a few weeks ago I wrote four articles about using the idea of Heating Degree Days to make simple calculations about heat losses from one’s home.
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Article 1 was an introduction to the idea of Heating Degree Days as a general measure of the heating demand from a dwelling.
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Article 2 explained how and why the idea of Heating Degree Days works.
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Article 3 looked at the variability of Heating Degree Days across the UK, at locations around London, and from year to year.
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Article 4 introduced some rules of thumb for estimating the Heat Transfer Coefficient for a dwelling and the size of heat pump it requires.
The Rule of Thumb for Heat Pump Sizing is dramatically simple:
The video above is about using these ‘Rules of Thumb’.
I feel these rules could be helpful to both heat pump installers and their clients.
The Powerpoint slides (.pptx) I use in the presentation can be found here.
Protons for Breakfast 
June 3, 2023 at 8:00 am |
[…] Protons for Breakfast blog proposed that if you have a gas boiler, you should take your annual gas usage and divide it by […]
September 1, 2023 at 10:54 am |
Hello Michael,
Apologies I can’t find my past posts on your blog site. I have been in touch with a few companies to get quotes for heat pumps and on reflection found I lack confidence in the affordable ones (£10k inc £5k off) and don’t want to pay £17k to a company I feel more confident in. Even then, I can’t understand why the MCS installers don’t ask to see previous KWh heating for us rather than producing what I consider to be oversized estimates for the size of heat pumps. I did want to ask how you arrived at the number 57.3 (i.e. the maths behind it, not the reasoning) to divide my KWh by?
For an update I was going to get British Gas to do our heat pump but they were unable to tell us when it might be done. The manpower seems to be lacking. I cancelled that but see that Octopus are developing the RED heat pump which looks promising as a product and may ultimately be better for us.
I really enjoy reading your articles and greatly appreciate the effort you have put in to explain heat pumps. I might know more now than a lot of MCS installers which is a worry.
Thanks
Jonathan Walker
September 1, 2023 at 2:21 pm |
Jonathan,
Good Afternoon.
It is a very fraught affair trying to find an installer. Have you tried looking at the Heat Geek Map to find one of their trained installers?
If you are feeling unsure then I would probably encourage you to wait for a winter, and use the winter to make measurements that will give you confidence that you will eventually be installing teh right kit.
1. Lower the flow temperature of your boiler and see if your house stays warm – if you can reduce teh temperature to around 50 °C a heat pump will work fine.
2. If you don’t have one, buy a thermocouple thermometer. You can use it to measure air temperature or pipe temperatures.
3. Measure gas use weekly – and when it’s really cold, measure how much you use day by day. This is teh best measure of the heat pump power you require.
In any gas, good luck with your endeavours – I wish it was less hard than it is.
Best wishes
Michael
September 1, 2023 at 5:16 pm
Hello Michael,
Good tips. I’ll do that.
Thanks
May 16, 2024 at 6:06 pm
Hello Michael, you have advised me in the past to find a heat geek which I have done. I have much more confidence in them and I have a proposal. As it stands I’m looking at a 12kw unit. I am toying with the idea of EWI and had 1 rough quote working out at about £200/sq metre. You have helpfully quoted how much you paid. I was wondering how many square metres your EWI was and did the total include scaffolding and VAT?
Thanks for inspiring me
Jonathan
May 17, 2024 at 9:22 am
Jonathan,
Good Morning. I’ll send you some EWI details by e-mail. If you haven’t received anything by midday today (17th May) let me know and I’ll find another way to send it.
Three things.
My first thought is that 12 kW is a BIG heat pump. I can’t remember details of your house but to require a 12 kW heat pump it should be a larger house or a very poorly insulated one, or both.
Regarding EWI, We had 100 mm of Kingspan K5 over 134 m^2 of the house. The initial quotation was for just under £20k. During the work we decided to have a chimney removed, some render on one wall completely removed and replaced before EWI could be done, a new roof over the existing flat roof, and the soffit boards replaced. The final cost was I think £26k if I remember correctly.
So £200 per square metre sounds on the expensive side, but not crazy.
Remember that if you have EWI it will change the heating requirement – and hence teh required heat pump size!
Do send details if you would like to discuss them further: I’ll e-mail you now,
Best wishes
Michael
January 1, 2024 at 11:28 am |
Michael
Thank you for your informative videos and this website on heat pumps. It has been very helpful to me.
I am at the beginning of this journey and desperately trying to understand the science and technology behind it.
I have a largish 4 bedroom detached house built in the 1970s. I have made various improvements including two extensions where cavity wall insulation was installed and I upgraded the loft insulation to more modern standards. A 7kW solar roof with batteries was installed in 2022.
This has been a huge success. Currently my house is comfortably heated by a Worcester Bosch gas boiler of some 15 years of age. So, I am trying to prepare for the time when it will need to be replaced.
I like the simplicity of your “rule of thumb” method of calculating heat pump size. My gas usage last year (2023) was around 16000 kWh when calculated from my gas bills. We were comfortable and that includes all our hot water and cooking. So, I feel using a COP of 3 should require a heat pump of around 6kW. However, when I use the traditional longhand method of calculating the requirement for my house (I used an online calculator) inputting all the required information, it comes out at a way different answer ie. a 15 kWh pump. Perhaps this is why people are being quoted for much larger pumps than they actually need.
So, I am left wondering if the on-line calculator is making assumptions. For example, would one assumption be a four bedroom house has two adults and two teenage children living in it. In my case, my wife and I are both retired. Our children are grown up and have families of their own. We are in pretty much all of the time as neither of us work but we are not profligate with the heating. It is pretty much turned off for seven/eight months of the year. This has left me wondering if I should install a heat pump of much larger capacity just in case the house is eventually sold to a family in the future.
In the meantime I am going to tweak my current system to run at a lower temperature as you suggest .
Thanks
Les
January 1, 2024 at 5:53 pm |
Les, Good Evening, and thank you for your kind words.
Excellent.
Your experience is far from unique. And it is unsettling to have two methods give such discrepant answers. Having been around this block a few times, I am confident that the lower answer is nearer the correct answer than the upper answer. Things that can make the rule of thumb inaccurate are if you (a) aren’t in the house very much and just warm it up for a few hours a day, or (b) use large amounts of hot water – in which case the oversizing effect gets worse!
To check, I recommend you try the cold weather test – I believe we are due some cold weather in the coming weeks. Try reading the gas meter at midday just before a cold night and at midday just after a cold night. The colder the better.
A 6 kW heat pump will deliver 6 kW x 24 hours = 144 kWh of heat to your home.
A 15 kW heat pump will deliver 15 kW x 24 hours = 360 kWh of heat to your home.
Let’s say your gas reading is 180 kWh of gas consumed on the cold day. Then a 90% efficient boiler will have delivered 90% x 180 kWh = 162 kWh of heat to your home (in space heating and hot water).
So by making measurements on one or two cold days, you can get a real sense for what your true heating requirement is.
If you get back to me when you have made the measurements, we can see what seems to be happening.
Best wishes for an exciting 2024.
M
January 17, 2024 at 8:00 pm |
Michael
You asked to let you how I got on with using the coldest day of the year approach to size a heat pump.
Last night was the coldest so far this winter with a low of -6 degrees C. We have been very comfortable during this freezing period.
The amount of gas used over the 24 hour period was 13.148 cubic metres.
I used this in the formula Octopus provides to convert this to Kilowatts. It came out at 150kWh.
This would indicate a heat pump requirement of 6.25kW. My estimate was 6kW based on our annualised gas usage and a COP of 3. When I tried the usual survey methods it came out out at 15kW. No wonder there are problems with sizing.
I have a fairly large 4 bedroom detached house in a semi rural location on a western facing slope 100 meters above sea level. So it is not sheltered in any way. I have good levels of cavity wall insulation and the loft is also insulated adequately. The house also has double glazing.
I think the traditional methods for surveys make assumptions we are perhaps not made aware of. How do we know the assumed ‘U’ values are correct? How do we know the predicted air changes per hour are correct? There are many variables and as we know every house has its own unique characteristics.
I can’t thank you enough for your help in getting this right. I am now in a good position to argue my case if any spurious estimates are produced by a survey.
Regards
Les
January 18, 2024 at 11:00 pm |
Les, Good Evening,
Well that is an extraordinarily large difference! It is not the largest difference I have come across, but it is very large!
Just to make sure it’s really real, please let me ask you some questions. Most importantly, did you keep your home a reasonably uniform temperature, or did you switch the heating off for a significant period? Why do I ask? Well if you heated your home for only 12 hours, and then switched off the heating, then the 150 kWh would per day i.e. 150 kWh/24 = 6.3 kW should more properly be thought of as 150 kW/12 (where the 12 is the hours of actual heating) i.e. 12.5 kW – much closer to the survey estimate.
I would also recommend repeating the measurement just to check.
Do let me know how it goes.
M
January 18, 2024 at 8:17 pm |
I live in Montreal in a home with over 3000 sf (basement excluded) and well insulated. I currently have 3.5 heat pump with heating back up (same as baseboard/wall heaters) which kicks in when the temperature outside is -8c. I do not use gas but wondering whether I can use your formula for electrical appliances with slight modification to size my new heat pump system which I need to buy. Three contractors are recommending 3 ton and I suspect it should be 4 ton. Luckily, my utility company has calculated that I used 11,000 kWh for my heat pump. They can’t provide the breakdown between the heat pump kWh and heating element kWh. I figure that my heating element is 20% of the total kWh so the net use for the heat pump should be 8,800kwh. If I divide this by 2900 I get 3.03 kw. Does this make sense to you. Can I apply this formula to already existing heat pump using electricity or am I completely off track. I am concerned about under sizing and do not trust the contractors. Thank you in advance for your help.
January 18, 2024 at 11:30 pm |
Alan, Good Evening.
I am having trouble understanding your problem, so please let me repeat it back to you and you can let me know whether I have got hold of the right end of the stick. But the short answer is no – the rule of thumb would work for gas or direct electrical heating, but not for electricity used to run a heat pump.
1. You have a heat pump: Is it air to water and radiators? or Air to Air?
2. The heat pump is 3.5 what? Kilowatts?
3. Heating back up – you mean just electrical heaters?
4. I don’t understand teh references to 3 ton or 4 ton? Over here a ton is a unit of mass!
5. You used 11,000 kWh of electricity for your heat pump. If it has a seasonally-averaged COP of 2 – as it might when it’s cold then it delivered around 22,000 kWh of heat to your home. If it has a seasonally-averaged COP of 3 then it delivered around 33,000 kWh of heat to your home. Let’s guess 27,000 kWh ± 5,000 kWh of heat required per year.
6. I looked up the degree days for Montreal airport for the last 3 years and the data told me that you have had a relatively mild winter this year so far. But for an internal temperature of 20 °C there were something like 3,500 °C-days of heating demand. Using the basic formula this suggests your home has a HTC of about 300 °C. i.e. it takes 300 W to raise the temperature of your home 1 °C. This is typical of a UK home and is *not* a well-insulated home that I would expect to encounter in Canada – perhaps the estimate is wrong?
7. If the HTC is 300 °C and you want to maintain 20 °C when it is -10 °C outside then your will need 30 °C x 300 W/°C i.e. 9 kW of heating power. That’s my guess for your heat pump sizing requirement. There are European heat pumps that can provide this level of heating with no additional direct heating required. I am unsure about the North American market.
Does this sort of make sense to you?
Best wishes
Michael
January 19, 2024 at 3:58 pm |
Michael
With regard to the “rule of thumb” and the coldest night of the year.
You asked if the heating was switched off for a time during the 24 hour period. Yes it was! Our heating is timed to go off at 22.30 hours and come back on at 06.30 in the morning. So, in the 24 hour period our CH was switched off for 8 hours. I understand how this affects the figures. However, as the boiler has to warm up the house from a lower temperature, I would have thought more gas would have been used at least from 06.30 onwards until the thermostat kicks in at 21 degree’s C. We are retired so the heating is never off during the day. I really feel our house is not “leaky” and is generally quite easy to heat.
I think the initial figure I originally calculated (15 KwH) was somewhat flawed in some way. Hence, the big difference between the theoretical and the actual numbers. For example, the house has been extended over the double garage and utility room. The on-line tool did not allow the inclusion of a garage. I use it as a workshop but Included it as an unheated storage space under the two upstairs bedrooms. The space between the garage and bedrooms above has been highly insulated, so I do not think there is a problem there.
Regards
Les
January 19, 2024 at 4:20 pm |
Les, Good Afternoon.
Teh suggested heat pump size is based on the average temperature in a dwelling over a period.
As you say the boiler – typically with 20 kW to 30 kW heating capacity, can rapidly re-warm the house. However a heat will probably not have that much ‘headroom’ for rapidly warming the house. Personally we just leave the heating on at 20 °C all the time, but others prefer a modest set-back temperature – 2 °C to 3 °C cooler overnight is a common choice.
So how will this affect your estimates for the heat pump size? Well the extreme case would be that one would multiply daily gas consumption by 24/16 = 1.5. This would suggest a daily gas consumption of 225 kWh and a heat pump size of about 9.4 kW. But as you say, the gas ‘boost’ in the morning somewhat compensates for that.
So your original estimate was 6.2 kW and this maximum value is 9.4 kW. This suggests that the appropriate sizing is somewhere in between those two numbers. A nominal 7 kW Vaillant Arotherm plus can output up to 10 kW when configured for low flow temperatures through the radiators. SO that looks be *about* right.
Click to access arotherm-plus-spec-sheet-1892564.pdf
In any case: keep warm and best wishes
Michael
April 18, 2024 at 11:38 am |
Have you received the post about estimating heat pump size from the EPC I made on 17/4/24? Maybe it is still in moderation.
ATB, podesta5ea167cbf1
April 18, 2024 at 2:10 pm |
Hi. I’m afraid I don’t have any comments pending from 17th April. But it sounds very interesting!
Michael
April 18, 2024 at 3:36 pm |
Your method Michael for estimating heat pump size involves dividing measured annual gas input in kW by 2900.
The EPC on the other hand estimates the heat required and has an entry like
Estimated energy needed in this property is:
• 29,200 kWh per year for heating
• 2,800 kWh per year for hot water
But the Podesta method assumes (I think) that the heat came from a gas boiler with an efficiency of 85%. So get the amount of gas you need to buy you need to divide by 0.85.
Hence the heat pump size is (Total EPC energy requirement in kWh)/(0.85*2900)
Now 2900*0.85 is 2465 which we can round to 2500 without significant loss of accuracy, which is easy to remember and easy to calculate with in your head.
So my final formula for you is (Total EPC energy requirement in MWh) * 0.4
In my case the total is 32MWh and so the recommended HP is 12.8kW which agrees closely with the result from room-by-room heat loss calculations.
April 18, 2024 at 6:32 pm |
Good Evening. And thank you for your interesting idea.
Firstly, regarding the “Podesta” rule of thumb, the key feature is not the numerology, but the fact that it is based on actual gas consumption. This is a well measured quantity derived when the dwelling owner was comfortable in their home.
The EPC estimate of heat loss is – if I understand it correctly – barely more than a guess, based on building type, floor area and levels of insulation. So like the heat loss survey it is based on assumptions about the property which may or may not be accurate. So the agreement between the heat loss survey and the EPC is interesting, and comforting. But neither estimate is based on any actual thermal measurements pertaining to a dwelling.
Secondly, I think (if I remember correctly) I assumed a 90% efficient boiler. This is an optimistic assumption about boiler efficiency, but it adds a little “buffer” to the estimate and tends to slightly overestimate heat pump size. It’s just a rule of thumb after all.
Do you know your actual annual gas consumption? Or do you know your daily consumption on the coldest day of the year? I would be very curious to see how well these measured quantities tied in with the two survey estimates from the EPC and the heat loss survey.
Best wishes in your endeavours.
Michael
April 18, 2024 at 7:49 pm |
In a word, no, we have an oil boiler anyway and some of the radiators are too small even for that, so parts of the house are too cold and we hope installing the HP and bigger rads will sort out both.
Of course, the EPC is only a rough estimate, I have tried to find out what the algorithm is but it is proprietary so I don’t know what variables and scoring system it uses.
However before you can make an application for the BUS grant the EPC has at least to exist, so at that point both the customer and prospective installers can get a rough idea of the size of HP they will need – by predicting the gas consumption as an implicit step.
Adapting the formula to suit a boiler efficiency of 90% results in the exact multiplier being 0.383 not .406, which is even less memorable and sacrifices your implied buffer Michael, so for both reasons I would suggest 0.4 is the figure to use!
Estimated Heat Pump size is (Total EPC energy requirement in MWh) * 0.4
I hope people might find this a useful result.
April 18, 2024 at 10:06 pm |
If you have an oil boiler then if your heating requirement was 32 MWh/year a rough conversion suggests you used 3,200 litres per year.
Given that oil boilers are (I have been told) less efficient that gas boilers, then if the heating requirement was 32 MWh/year I would expect you needed between 3,000 and 4,000 litres of heating oil. Does that sound about right?
April 18, 2024 at 10:29 pm |
It might come in that range if we were to heat the entire house all the time (like you would with an HP) and we lived there all the time, and didn’t have a solar diverter to heat the water, or an oil-fired AGA of unknown efficiency as a roomheater. Too much to try and unpick.
April 19, 2024 at 7:05 pm |
Ahh. OK. Now I see your situation. As you say, too much to try and unpick.
If you want to resolve some of the uncertainties about the house, then you might wish to consider fitting an Open Energy Monitor system. This gives you independent meter quality data about how the heat pump is performing and uses a heat-meter to tell you how much heat is being supplied to your home. Getting someone to fit it will cost about £1,000.
You can either keep this data for yourself, or share it (anonymously) on the Heat Pump Monitor Web site.
https://heatpumpmonitor.org
If you look through this site you can see how more than 100 heat pumps are performing.
In any case: Please allow me to wish you all the best with your endeavours.
Michael
October 29, 2024 at 8:37 am |
Michael
Love this rule of thumb. As you may be aware I am edging towards an ASHP and am very prepared to bin my 3 year old gas boiler which was an emergency replacement.
I just wanted to properly understand the derivation of the number 57.3 (and therefore 2900). Am I right that it simply the result of using the figure of 15000 kWh with a representative HDD(16.5) of 2150 in the overall formula?
Many thanks, Graham
PS – I’ll try not to ask too many questions over the coming months!