What to do on the coldest day of the year?

Friends, the coldest day of the year affords a rare opportunity to find out the key number that describes the thermal performance of your dwelling: the Heat Transfer Coefficient (HTC).

This is particularly relevant if you heat your home with a gas boiler because, thrillingly, it also allows you to estimate the size of heat pump your dwelling will require when it’s time to switch.

The coldest day is probably still a couple of months away, and that gives you time to prepare and practice for your day of measurement.

Let me explain.

The Coldest Day?

Let me begin with the profound philosophical question: “How do we know which day is going to be the coldest?”. As the Zen master said “Even a very cold day may be followed by colder days.”

Fortunately, we don’t the need the very coldest day of the day – any reasonably cold day will do, and the weather forecast should alert you to its arrival. Ideally it would be a day with an average temperature close to 0 °C, perhaps with a nighttime minimum temperature well below 0 °C.

And seeing how the results compare on a couple of similarly cold days will help you assess the likely uncertainty in your estimates.

A Very Cold day

On this cold day you need to use your electricity and gas appliances as you would normally so that your home is as warm as you would like.

Then you need to read your electricity and gas meters before the coldest night – and then at exactly the same time the next day. Alternatively a smart meter might well give you the information more conveniently.

Finally you need to know the average temperature inside and outside your home.

I’ll explain how to do the calculation below but with these readings you can estimate the Heat Transfer Coefficient (HTC) for your dwelling and – when the time comes to change – the size of heat pump you require.

The General Idea

Let’s say you used 20 kWh of electricity and 100 kWh of gas over a period of 24 hours. Then a simple first guess would be that the total energy used for heating was 120 kWh. Over 24 hours this would correspond to an average heating power of (120 kWh)/(24 h) = 5 kW. We’ll make a more sophisticated estimate below but this would be a good first estimate of the size of heat pump you require.

If the internal temperature through the day was 20 °C and the average outside temperature was 0 °C, then you can estimate the HTC by dividing the average heating power (5 kW) by 20 °C i.e. 5 kW/20 °C = 0.25 kW/°C.

A more sophisticated estimate

Electricity. Unless you are charging large batteries or directly heating hot water with an immersion heater, all the electricity you use – for televisions, lighting etc – ends up heating your home. So the 20 kWh of electricity used would all end up as heat.

Gas. In a typical boiler, 15% of the energy in the gas that went through the meter is lost out the flue. In older boilers losses could be as much as 25%. If you don’t know better, a good first estimate would be that your boiler efficiency was 85%. So if 100 kWh of gas was metered, I would estimate that only 85 kWh  actually entered the dwelling.

Cooking with gas. The heating power of gas used for cooking is generally small (a few kWh/day) and most of the heat ends up in the home any way.

Domestic Hot water. Gas or electricity used to heat water doesn’t generally heat the home (much) and needs to be subtracted from the estimate of heat supplied to the house. The industry guideline is that each adult uses about 3 kWh/day of hot water, so if there are two adults in the house you need to subtract 2 x 3 = 6 kWh/day from the estimate of gas used for heating the house.

People. People are actually a source of heat, releasing around 2.4 kWh/day. If there were two adults in the dwelling all the time then add 2 x 2.4  = 4.8 kWh to the heating energy.

So the total heating in the dwelling would be 20 + 85 – 6 + 4.8 = 103.8 ± 5 kWh. The uncertainties are such that this can be conveniently rounded to 104 kWh rather than 120 kWh in the simple estimate.

So over 24 hours this allows you to estimate of the size of heat pump you require as being (104 kWh)/(24 h) = 4.3 kW.

If the internal temperature through the day was 20 °C and the average outside temperature was 0 °C, then the HTC is estimated as 4.3 kW/20 °C = 0.22 kW/°C.

If you wanted the heat pump to keep your home at 20 °C when it was (say) -5 °C outside, then you can use the HTC to estimate the size of heat pump required. You multiply the temperature difference (20 – (-5)) = 25 °C) by the HTC (0.22 kW/°C) to give 25 °C  x 0.22 kW/°C = 5.4 kW.

I have prepared a spreadsheet that does the calculations for you:

Why You Need a Cold Day

You can make these measurements on any day of the year, but except on the coldest days, the uncertainty in the estimate can be large.

By making the measurements on a cold day, the main heating component – the gas consumption – can be estimated modestly well, and all the corrections are relatively small. My guess is that the answers should be within about 10% of the right answer.

Reading the Meters 

If you have a smart meter with an in-home display, then one of the settings will tell you how much energy you have used in the last day. Typically, they show data for gas and electricity separately, each for a 24-hour period starting at midnight. If the weather stays cold for two days, it might be better to record energy usage over a 48 period so as to include a complete cold night.

If you don’t have a smart meter, then you will have to find out where your energy meters are in your home and read them manually. If you don’t know how to read an energy meter there is help available from:

But there is one difference between reading the meter for an energy company and reading it for yourself. When reading the meter for the energy company they tell you to miss off the last digits. This is because they want to minimise the chance of mis-reading and transcription errors. And they know that what you don’t pay for this month you will pay for next month!

But there is information in these digits which can be useful, especially if your usage is low. So record all the digits from your gas meter.

Click image for a larger version. The left-hand image shows a gas meter reading in cubic metres and the right-hand image shows a gas meter reading in cubic feet.

Gas meters record your gas usage by measuring the volume of gas passing through them in cubic metres or cubic feet. To estimate the energy contained in that gas you need to subtract the volume readings made at the start and end of your chosen 24-hour period, and then multiply by a factor which tells you the energy content per unit volume of the gas.

Click image for a larger version. Spreadsheet excerpt showing how to subtract two readings to obtain the volume of gas used in one day, and multiply them by the energy density to find the energy contained in the gas that flowed through the meter.

Older gas meters sometimes confusingly read in units of hundreds of cubic feet rather than cubic feet. An example of this is given in the illustration above. If you are unsure you can check that you have the right units because for any reasonable home heated primarily by gas, the gas used on the coldest day of the year will be somewhere between 10 kWh (a well-insulated flat) and 200 kWh (a large poorly insulated house).

Fortunately electricity meters read directly in kWh.

A spreadsheet that does the calculations for you can be downloaded here:

Click for larger version. Graphic showing the spreadsheet that will do the calculations for you.


Reading your meters can be tricky, but working the average temperature inside and outside your house can be trickier.

The best way to do this is to measure it yourself with thermometers and weather stations. For most people that’s not possible.

If you don’t have an internal thermometer, then I have been told that the average household temperature is likely to be approximately 2 °C colder than the thermostat setting. So if your thermostat is set for 20 °C, then the average temperature of the dwelling is likely to be around 18 °C.

To estimate the average external temperature you might try this web site which allows you view historical weather data in your location. This link is for London, but you can choose other locations.

Alternatively use the Weather Underground’s Wundermap to find a local weather station. You can zoom in to a local level and click on an individual weather station and then its weather station ID to get its local daily and weekly average temperatures.

Last thoughts

Friends, the essential and expensive energy which flows into and out of our homes is sadly invisible. And this makes it difficult to assess the thermal properties of your home.

But the coldest days of the year afford us an opportunity to assess the thermal properties of a home that only comes about on a few days a year.

I urge you to get ready for when the cold days arrive – perhaps by practicing on less cold days – and then you will be able to obtain valuable information about your home.

Good luck!

7 Responses to “What to do on the coldest day of the year?”

  1. Paul Rudman Says:


  2. Simon Duane Says:

    Interesting stuff, as ever, Michael – thanks!

    I declare my interest – I am such a gas boiler owner who aims to use it to do exactly as you describe, to gather information that would reliably inform the specification of a heat pump that could replace it.

    As you say, the analysis is a matter of balancing energy input (including what’s pumped in) against energy lost but, as we once discussed, there can be more to the losses than the temperature difference, inside – out.

    If I recall our discussion correctly, you’d looked (in vain) for evidence of wind-speed dependence in your own energy consumption, while I’d found that far and away the biggest peak in the energy consumption in my house was a period of several days of sustained ~30 mph winds, from what one would expect to be the “worst” direction, which is when the wind blows face on to the terrace which includes the house. To boot, we benefit from uninterrupted sea views in that direction 😉

    In short, but in a very building-dependent way, significant energy loss _can_ arise from convection – the exchange of expensively heated air and cool outdoor air.

    Relatedly, a few days ago I stumbled across this old article https://passivehouseplus.ie/magazine/feature/windy-or-cold-weather-when-does-heating-demand-peak
    which also makes for interesting reading.

    They go into some examples where excellent thermal insulation was seriously compromised by these effects.

    Best wishes

    • protonsforbreakfast Says:


      Good Afternoon. The gist of the article is that one finds out experimentally how much heat one uses on an extreme day. For most people, the extreme is simply one related to temperature. For your extremely exposed location the heating demand extreme may come at some combination of cold and windy.

      Note that I don’t think the additional heat loss in a windy state is caused by air speed across the exterior surfaces. I think heat losses at those surfaces are essential instantaneous compared with the rate of heat transfer through a brick wall.

      Rather, the heat loss likely arises from air flow through the interstices of your home driven by the extreme ΔP across your house. Air tightness tests are conducted at ΔP = 50 Pa, but there is large fact – perhaps as much as 30 (if I remember correctly) that relates the number of air changes per hour at ΔP = 50 Pa to what one might expect in a normal domestic setting. On a seafront, it could be your number of air changes per hour is very high.

      But whatever it is, I expect that a measurement of gas consumption on a cold and windy day will allow you to estimate it. Indeed comparing cold and still with cold and windy would be a great experiment!

      All the best


      • Simon Duane Says:

        I’m not sure if you followed that link, but it was, of course, not about my own house but about the housing (and also commercial building) stock in Ireland (rather than the UK), and was making a very general point about the two mechanisms of heat loss.
        Two terms are used (and distinguished)
        – “airtight”, which I understand to be quantified in a blower test, by increasing the pressure indoors by 50 Pa, and
        – “windtight”, which sounds like it should be meaningful, but I would struggle to suggest how to quantify it. (I imagine that the pattern of air flowing over a building creates pressure differentials on windward and leeward sides, Bernoulli effects in chimneys, etc. I suppose that the pattern of winds makes some of the leaks that spoil airtightness more important than others.)
        As you mentioned, smart meters can make these things easier to investigate. Indeed, a “simple” approach is to review daily combined energy consumption over the year (after the fact), to home in on the peaks, and ask – what caused that? before trawling through weather records etc in search of a plausible story.
        All good fun …

  3. Simon Duane Says:

    PS Re-reading the article I linked to, I see there is an interesting hint, which I shall make more explicit:
    if the maximum demand for heat is when it is windy but not sub-zero, I might expect an air source heat pump to come into its own: wouldn’t the wind reduce the tendency for the external unit to get iced up when it’s trying to pump as much heat as possible into the house?

    • protonsforbreakfast Says:

      I don’t think windiness will particularly affect ASHP performance. Why? because the air flow past the evaporator is already very fast. A bit of extra wind will not make much odds.

      The main factors affecting condensation of water – and ice – are around the water content of the air. I analysed this here: https://protonsforbreakfast.wordpress.com/2021/08/30/heat-pumps-power-noise-and-condensation/

      Personally I wouldn’t worry about the de-icing issue. There is nothing you can do about it! and it only affects COP on a few days a year. Better to look for better performance throughout most of the year.

      Best wishes


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