Archive for August, 2019

Why does heating my house require 280 watts per degree Celsius above ambient?

August 18, 2019

Previously I explained how I learned that for each degree Celsius the outside temperature falls below 20 °C, it takes 280 watts of heating to keep my house at 20 °C.

In order to provide this heating, I burn gas which last winter resulted in the emission of around 17 kg of carbon dioxide per day – around 2.5 tonnes in all.

I would really like to reduce this shameful figure, but I have only finite resources. In order to act I need to know where best to spend my money.

In this article I will explain how I came to understand the relative significance of the windows, roof and walls in this heat loss.

Windows

It is easier to estimate the heat loss from windows than it is from walls.

This is because walls are opaque and (without expert knowledge) it is not obvious what the wall is made of. Moreover, different walls in the house can have different construction and thickness. However, being transparent, one can see directly the type and construction of windows.

The heat flow through a window(or wall) is characterised by a U-value. This states the amount of heat which flows across 1 square metre of the window when there is one degree Celsius of temperature difference across the window.

The units are for U-values are watts per metre squared per degree Celsius (W/m2/°C) or watts per metre squared per kelvin  (W/m2/K). These two units are equal to each other.

Roughly speaking U-values for windows are [Link]:

  • Old single-glazed windows: 6 W/m2/°C
  • Old double-glazed windows: 4 W/m2/°C
  • New double-glazed windows: 1.5 W/m2/°C
  • The best triple-glazed windows: 1.0 W/m2/°C

I proceeded as follows:

  • I made a list of the 21 windows, skylights and glazed doors in in my house.
  • I measured their area – width × height in metres.
  • I multiplied their area by their U-value to get the transmission per degree Celsius through that window.
  • I then added them all up.
Slide5

For each window in the house I multiplied the area by the estimated U-value to get the heat transmitted per degree Celsius of temperature difference. I colour-coded the column to highlight which windows were the worst. Adding up all the windows came to 75.7 watts per degree Celsius. If I replaced all the windows with the best available I might be able to reduce this to 24.0 watts per degree Celsius.

The estimated total transmission through all the windows and doors came to about 76 watts per degree Celsius. I concluded that:

  • Firstly,  I could see which windows lost the most energy – they are colour-coded red, amber, and green in the figure above. There are no surprises – the largest area windows lose the most energy.
  • Secondly, I could see that if I replaced all the old windows with modern ones (U = 1.5 W/m2/°C), I might hope to reduce the window losses by roughly half their current value, to around 36 watts per degree Celsius. If I spent a lot – on triple-glazed windows and used insulating blinds, I might hope to achieve U = 1.0 W/m2/°C and reduce the losses to 24 watts per degree Celsius.
  • Thirdly, since the house as a whole is losing 280 watts per degree Celsius, I could see that windows and doors account for about a quarter of the energy lost from the house.
  • And finally, logically, the remaining 75% of the losses (280 – 76 = 204) must be going the through the roof, walls, and floors or lost in draughts.

Roof and Walls 

By analysing the thermal transmission of the windows and doors (transmission = 76 watts per degree Celsius), I concluded that roof and walls must be transmitting about 204 watts per degree Celsius.

  • Is this estimate reasonable?

To answer this question I embarked on yet another tedious and difficult exercise.

  • The tediousness arises because I need to add up all the areas of the roof and walls, subtract the areas of the windows and skylights, and then estimate the U-value,
  • The difficulty arises because I don’t know the materials from which the walls of the house are constructed!

Most of the walls date from the 1930’s (I think) and are probably solid brick. A 1970’s extension is probably not much better thermally, but I don’t know. However, the extension we built 10 years ago was built to building regulations at the time and I have a pretty good idea of the appropriate U-value.

So I made measurements of the wall areas. And then I assumed (link) that:

  • The old walls had a U-value of 2 W/m2/°C – a value appropriate for a double-skin solid brick wall.
  • The new walls had a U-value of 0.3 W/m2/°C – a value specified by current building regulations.
Slide6

For each wall or roof, I multiplied the area by the estimated U-value to get the heat transmitted per degree Celsius of temperature difference. I colour-coded the column to highlight which were the worst. Adding it up came to about 229 watts per degree Celsius. If I clad all the walls to achieve a U-value of 0.3 watts per metre squared per degree Celsius, I might be able to reduce this to 54 watts per degree Celsius.

With these assumptions I estimated the heat transmission through the roof and walls. As shown in the table above, I arrived at an estimate of 229 watts per degree Celsius. This should be compared with estimate of 204 watts per degree Celsius that I arrived by analysing:

  • My gas meter readings
  • The average weekly temperature
  • The estimated properties of the windows.

Given all the uncertainties, I take this as confirmation that within about 10% uncertainty, I can understand the thermal properties of my house.

Summary

Slide7

Currently my house loses 280 watts for each degree Celsius the external temperature falls below ambient. Of those 280 watts,

  • roughly 76 watts flow through the windows and doors
  • the remaining 204 watts flow through the walls, floors and roof.

With modern double-glazing I could reasonably hope to reduce the glazing losses from 76 watts to around 36 watts, or possibly even lower with triple-glazing and thermal blinds.

Cladding the entire house I could hope to reduce the losses from around 204 watts to around 50 watts.

  • What should I do?

In the next article I will discuss my strategy.

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

August 16, 2019

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.

BAMS State of the Climate 2018

August 14, 2019

Reading the annual ‘State of the Climate’ report in the Bulletin of the American Meteorological Society (BAMS) has done nothing to help with my anxiety.

If you dare, you too can read it here:

Summary 

Imagine learning that your friend was in hospital. You race to the hospital and find your friend hooked up to every conceivable monitoring device.

If your friend is “the Climate”, then reading the BAMS State of the Climate report is like reading their autopsy before they have died.

You can foresee every tiny detail of their future suffering.

And yet the doctors don’t seem to be doing anything. Your friend is on the table, haemorrhaging, and the doctors are in an endless series of meetings!

The alarms on the monitors are beeping and flashing. But nobody comes to attend your friend.

You bang on the windows of the doctors’ meeting room and the doctors turn and glance at you, and then turn back to their conference.

You ask to see the hospital administrator. But they are too busy. An assistant assures you that they understand your distress.

You explain that this is not just A. N. Other Climate. This is the Climate, the one we all depend on for our food and air and water.

And the assistant agrees with you, sympathetically. But they patiently explain that the administrator is busy with IMPORTANT budget meetings right now.

And then you realise that your friend has been on the table for years…

…and that the doctors meeting has been going on all this time.

With each passing year the doctors become more and more certain of the exact manner in which your friend will die. But no treatment has begun.

You begin to feel angry. And depressed. And frustrated. And you consider acting irrationally.

You begin to consider that acting – rationally or irrationally – is the only chance to save the friend you love.


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