What it takes to heat my house: 280 watts per degree Celsius above ambient

Slide1

The climate emergency calls on us to “Think globally and act locally“. So moving on from distressing news about the Climate, I have been looking to reduce energy losses – and hence carbon dioxide emissions – from my home.

One of the problems with doing this is that one is often working ‘blind’ – one makes choices – often expensive choices – but afterwards it can be hard to know precisely what difference that choice has made.

So the first step is to find out the thermal performance of the house as it is now. This is as tedious as it sounds – but the result is really insightful and will help me make rational decisions about how to improve the house.

Using the result from the end of the article I found out that to keep my house comfortable in the winter, for each degree Celsius that the average temperature falls below 20 °C, I currently need to use around 280 W of heating. So when the temperature is 5 °C outside, I need to use 280 × (20 – 5) = 4200 watts of heating.

Is this a lot? Well that depends on the size of my house. By measuring the wall area and window area of the house, this figure allows me to work out the thermal performance of the walls and windows. And then I can estimate how much I could reasonably hope to improve the performance by using extra insulation or replacing windows. These details will be the topic of my next article.

In the rest of this article I describe how I made the estimate for my home which uses gas for heating, hot water, and cooking. My hope is it will help you make similar estimates for your own home.

Overall Thermal Performance

The first step to assessing the thermal performance of the house was to read the gas meter – weekly: I did say it was tedious. I began doing that last November.

One needs to do this in the winter and the summer. Gas consumption in winter is dominated by heating, and the summer reading reveals the background rate of consumption for the other uses.

My meter reads gas consumption in units of ‘hundreds of cubic feet’. This archaic unit can be converted to energy units – kilowatt-hours using the formula below.

Energy used in kilowatt-hours = Gas Consumption in 100’s of cubic feet × 31.4

So if you consume 3 gas units per day i.e. 300 cubic feet of gas, then that corresponds to 3 × 31.4 = 94.2 kilowatt hours of energy per day, and an average power of 94.2 / 24 = 3 925 watts.

The second step is to measure the average external temperature each week. This sounds hard but is surprisingly easy thanks to Weather Underground.

Look up their ‘Wundermap‘ for your location – you can search by UK postcode. They have data from thousands of weather stations available.

To get historical data I clicked on a nearby the weather station (it was actually the one in my garden [ITEDDING4] but any of the neighbouring ones would have done just as well.)  I then selected ‘weekly’ mode and noted down the average weekly temperature for each week in the period from November 2018 to the August 2019.

Slide3

Weather history for my weather station. Any nearby station would have done just as well. Select ‘Weekly Mode’ and then just look at the ‘Average temperature’. You can navigate to any week using the ‘Next’ and ‘Previous’ buttons, or by selecting a date from the drop down menus

Once I had the average weekly temperature, I then worked out the difference between the internal temperature in the house – around 20 °C and the external temperature.

I expected that the gas consumption to be correlated with the difference from 20 °C, but I was surprised by how close the correlation was.

Slide2

Averaging the winter data in the above graph I estimate that it takes approximately 280 watts to keep my house at 20 °C for each 1 °C that the temperature falls below 20 °C.

Discussion

I have ignored many complications in arriving at this estimate.

  • I ignored the variability in the energy content of gas
  • I ignored the fact that less than 100% of the energy of the gas is use in heating

But nonetheless, I think it fairly represents the thermal performance of my house with an uncertainty of around 10%.

In the next article I will show how I used this figure to estimate the thermal performance – the so-called ‘U-values’ – of the walls and windows.

Why this matters

As I end, please let me explain why this arcane and tedious stuff matters.

Assuming that the emissions of CO2 were around 0.2 kg of CO2 per kWh of thermal energy, my meter readings enable me to calculate the carbon dioxide emissions from heating my house last winter.

The graph below shows the cumulative CO2 emissions…

Slide4

Through the winter I emitted 17 kg of CO2 every day – amounting to around 2.5 tonnes of CO2 emissions in total.

2.5 tonnes????!!!!

This is around a factor of 10 more than the waste we dispose of or recycle. I am barely conscious that 2.5 tonnes of ANYTHING have passed through my house!

I am stunned and appalled by this figure.

Without stealing the thunder from the next article, I think I can see a way to reduce this by a factor of three at least – and maybe even six.

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One Response to “What it takes to heat my house: 280 watts per degree Celsius above ambient”

  1. Why does heating my house require 280 watts per degree Celsius above ambient? | Protons for Breakfast Blog Says:

    […] Making sense of science « What it takes to heat my house: 280 watts per degree Celsius above ambient […]

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