Heating Degree Days:4:Three numbers you need to know about your home

Friends, after the previous three posts (1, 2, 3) about Heating Degree Days, you may be wondering:

  • Is Michael OK? He seems to be obsessed with Heating Degree Days?
  • Hasn’t he been keeping an eye on the COVID figures?

Well, I have indeed been focussed on Heating Degree Days, and in this short (!) article I would like to summarise why.

The Heating Degree Day (HDD) concept enables two calculations for numbers you really should know about your dwelling:

  • It’s thermal leakiness: technically its heat transfer coefficient (HTC)
  • The size of heat pump your dwelling requires.

When combined with an estimate for how good the insulation is, you will be in a great position to make rational choices about improving the thermal performance of your dwelling.

Here are the three calculations:

#1:Heat Transfer Coefficient.

How much does heating power does it take to make your dwelling 1 °C warmer?

The answer to this question is known as the Heat Transfer Coefficient (HTC) for a dwelling.

A first estimate of your HTC can be made by dividing your annual gas consumption (in kWh) by 57.3:

Note: This formula was revised on 21/3/2022 due to a typo in the original text.

This assumes your dwelling (flat or house) is in the southern half of the UK (i.e. South of Manchester) and that you set your thermostat to 20 °C.

  • If you live between Manchester and Edinburgh, reduce your estimate of HTC by 10%.
  • For each 1 °C above 20 °C that you set your thermostat, reduce the first estimate of HTC by 10%.
  • For each 1 °C below 20 °C that you set your thermostat, increase the first estimate of HTC by 10%.

#2:Heat Pump Size.

How big a heat pump do I need?

It’s the question everyone wants an answer to!

A first estimate of the size of heat pump you require can be made by dividing your annual gas consumption (in kWh) by 2,900.

This assumes your dwelling (flat or house) is in the southern half of the UK (i.e. South of Manchester) and that you set your thermostat to 20 °C.

  • If you live between Manchester and Edinburgh, increase your estimate of heat pump power by 10%.
  • For each 1 °C above 20 °C that you set your thermostat, increase your estimate of heat pump power by 10%.


Do I need more insulation?

If your home is a house (rather than a flat), then you can assess how good your home is compared to the best possible as follows.

Divide your annual gas consumption (in kWh) by the floor area of all the floors in your home that live in i.e. include the loft if its part of the domestic space but not if it’s just used for storage.

  • The best possible is < 15 kWh/m^2/year: this is the Passivhaus standard
  • The best possible retrofit is < 25 kWh/m^2/year: this is the Enerphit retrofit standard
  • The AECB retrofit standard is < 50 kWh/m^2/year.

My house was ~ 90 kWh/m^2/year before external wall insulation and triple-glazing reduced it to around 45 kWh/m^2/year. The only way to significantly improve on this would be with underfloor insulation and air-tightness work.

If the figure for your home is very much above 100 kWh/m^2/year then I would suggest you consider insulation work.


If you know these numbers – even approximately – for your home, then you will be in a position to make reasonable choices about what to do next.

Please bear in mind that all the figures are approximate. I can see ways in which they could be wrong by 10%, but I would be surprised if they were 20% wrong.

4 Responses to “Heating Degree Days:4:Three numbers you need to know about your home”

  1. Jack Chidley Says:

    Using your articles, the degree days website, and my data, this is how I calculated my own numbers:
    1) Because I have a smart meter, I was able to use the ‘regression’ ‘data type’ to work our historic ‘base temperature’. It is 17C.
    2) I worked out my HTC (I have a combination gas boiler which I have been operating at 90% efficiency). Last year I burnt 16,940 kWh of gas, so that is 15,246 kWh of heat delivered. My local HDD (Northolt is the nearest suitable station) was 2062. The HTC is 308.
    3) Maximum heating power required is 6,316 kWh for 20.5C of expected maximum demand.
    4) The total floor area of my house is 190m^2 so my house’s ‘figure of merit’ is 80. My home is a 1930s built semi-detached house. I have made some improvements (loft insulation, double and triple glazing) but have yet to bite the bullet on EWI (external wall insulation – my house has solid walls).

    This year I have upgraded my radiators (and thus will lower the flow temp from 60C to 45C), I have plugged a very large ventilation problem and I will not be setting my living room temperature above 22C (from 25C today!). I will see what effect that has on my heating needs. Hopefully at least 10%.


    • protonsforbreakfast Says:


      Your actions are inspiring. I hope you have a nice warm winter. Two comments

      Your new flow temperature is so low that this may not apply to you, but another thing you might consider is ‘Weather Compensation’. This is standard with heat pumps but can be applied to some boilers. It means that when the outside temperature is (say) 10 °C your flow temperature would be 30 °C but when the outside temperature falls to (say) 0 °C the flow temperature would increase to match the higher heat loss.

      If you have a Figure of Merit of 80 kWh/m^2/year, then I would not particularly recommend EWI. It does work, but it is expensive and disruptive and your home is already pretty good by UK standards. It’s important not to let the perfect be the enemy of the adequate.

      Best wishes


  2. Stephen Kelly Says:

    Hi Michael.

    I read your articles with great interest. I had a go at generating numbers for my house and arrived at a ‘Figure of Merit’ in the order of 120 kWh/m2. My house is quite a large detached 4 bed built in 1980 and has cavity wall insulation and full double glazing. The loft is not fully insulated to the latest standard and I am in the process of upgrading this. However I’m not sure what I am doing will reduce my Figure of Merit to below 100 which still seems high?

    A couple of questions if I may:

    1. Why do you use the total area of living spaces? With a 2 storey house the ground floor loses heat to the floor above not to the outside, although I appreciate there are losses through the walls etc. However, if I did not use the full living area my Figure of Merit would be even higher! I estimated my house at 170m2 which seems about right for a detached 4 bed house?

    2. Some of my gas is used to heat hot water so shouldn’t I deduct some of my gas usage for this before doing the calculations?


    • protonsforbreakfast Says:


      Very good points.

      The metric of “kWh per metre squared per year” is not my invention. I first saw it on the specification pages for the Passivhaus project. It specified that a Passivhaus must use less than “15 kWh of space heating per metre squared per year” and their retrofit standard (called Enerphit) was 25 kWh of space heating per metre squared per year. I don’t think it even specified what the internal temperature had to be!

      And these numbers apply to the UK as a whole and there is a wide variety of climates even within the UK so its clear these figures are ‘indicative’ rather than precise. Indeed it might better if there were a ‘category label’ rather than a number associated with the metric. So coming to you questions.

      1. That’s just what the metric says: the living area of the house – so unless the loft is converted, it wouldn’t count. My guess is that with 120 kWh/m^2/year there probably are some steps you can take but I’d need to know more before I could suggest anything.

      2. Yes, you should deduct water heating and cooking – I am sorry if I didn’t make that clear. But that will probably only shift the figure by ~10% and since it’s an indicative figure anyway it may not make much difference.

      Best wishes


Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s

%d bloggers like this: