Heating Degree Days:3: How do they vary?

Friends, having read the previous two posts (1, 2) about Heating Degree Days, you may be wondering.

  • How carefully do I need to be in choosing the baseline temperature?
  • How do heating degree days vary around the UK?
  • How do heating degree days vary from year-to-year?

If you were wondering things, then the text below should provide the answers you seek.

Seek on!

Choice of Base Temperature

Click on Image for a larger version. The graph shows annual running average of the number of heating degree days for the London St. James Park Weather station. Each curve corresponds to the number of heating degree days with a difference base-temperature. For each degree Celsius increase in the internal temperature, the heating demand increase by approximately 260 °C-days.

The choice of the base temperature is important when estimating heating demand.

The evidence in the previous article is that a ‘rule of thumb’ for choosing a base temperature is to pick a value 3.5 °C below the internal thermostat setting is probably OK.

  • 19 °C thermostat setting: use a base temperature of 15.5
  • 20 °C thermostat setting: use a base temperature of 16.5
  • 21 °C thermostat setting: use a base temperature of 17.5

Using data from St James Park in London, each 1 °C change in base temperature changes the annual degree-day estimate by roughly 260 °C-days/year. So if we estimate an average value of HDD(17.5 °C) is ~ 2,100, then turning down the thermostat by 1 °C would reduce heating demand (and hence gas consumption) by ~260/2,100 = 12.4%.

Variability over Time

Click on Image for a larger version. The graph shows annual running average of the number of heating degree days based on a 16.5 °C base temperature for London Heathrow Airport (in black). The dotted lines show the 20-year average and ± 1 standard deviation. Also shown are the monthly degree day totals (in purple) from which the annual averages are derived.

Looking at the data from Heathrow – which has a longer HDD record than most stations

  • The average number of HDD(16.5)s is 2053 °C-days/year and the standard deviation is roughly 8%.
  • The average number of HDDs(15.5)s is 1778 °C-days/year and the standard deviation is roughly 9%.

First we notice that the difference between HDD(15.5) and HDD(16.5) is 275 °C-days/year, similar to the 260 °C-days/year that we deduced from looking at the St James’s Park data.

Considering the variability, a standard deviation of 8% or 9% suggests that once in 20 years or so one might expect winters which have 16% or 18% more heating demand.

Variability with Location

The number of HDDs varies from place to place. The figure and table below show the number of HDDs based on a 16.5 °C base temperature averaged over the last 3 years.

  • A wide swathe of southern England, from Manchester southward, has heating demand within approximately 16% of the heating demand at Heathrow.
    • i.e. in the range 2,150 ± 150 °C-days/year
  • In Yorkshire, the North East, and Central Scotland, heating demand is about 25% greater than Heathrow.
    • i.e. ~ 2,500 °C-days/year

Click on Image for a larger version. The annual number of heating degree days based on a 16.5 °C base temperature for various UK locations averaged over the 3-year period from 1/3/2019 – 28/2/2022. The data are also shown as deviations from the number of HDDs(16.5 °C) at Heathrow Airport.

Click on Image for a larger version. Summary of the results in the previous figure.

In addition to large scale variations across the UK, there are smaller variations due to local factors, notably elevation and the city heating effect.

Based on the typical decline in temperature with height (typically 6.5 °C/km) then each 100 m of additional elevation would be approximately 0.65 °C colder. This will result in additional HDDs roughly equivalent to 275 x 0.65 °C or 165 °C-days/year.

To look at the urban heat island effect, I downloaded data from 4 locations around London.

Click on Image for a larger version. The annual number of heating degree days based on a 15.5 °C base temperature 4 locations around London.

Compared to data at Heathrow, there are significant changes in heating demand, with the centre of London being significantly warmer, and Gatwick Airport – just 38 km from the centre of London – being significantly colder.

Variability Summary

The heating demand at Heathrow Airport with base temperature of 16.5 °C (i.e. a likely thermostat temperature of 20 °C) is very roughly 2,000 °C-days per year.

This 2000 °C-day/year varies by typically:

  • 10% in nearby locations depending on more or less urban heating.
  • 12% to 15% per °C change in base temperature
  • -3% to + 15% over England and Wales south of the latitude of Manchester.
  • Up to 30% as far north as Aberdeen
  • Year to year variability of ±9% with occasional excursions to ±18%

So if one could not look up the number of degree days for a particular location (which one can easily at Degree Days!) one could characterise heating demand against a base temperature of 16.5 °C as likely to be within 15% of 2,300 °C-days per year almost anywhere in the  UK.

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