Saturday, 11 February 2012

Energy saving achievements and shock ventilation

A couple of posts ago when I complained about how difficult it is to control central heating systems sensibly  (see How much of your house do you heat?) I did mention that UK energy use for home heating has dropped a bit in recent years, reversing a long term upward trend. In fact, weather adjusted heating energy use came down by 20% between 2004 and 2008. How have we achieved this?


The graph below is derived from data in the Great Britain's Housing Energy Fact File from DECC. I have divided heating energy per household by the degree days* in each year to allow for changes in the weather.


A 20% drop in 4 years is far too large to have come only from house improvements. In most cases, even very extensive upgrades to insulation and heating systems will only achieve about 30% reduction in energy use, as seems to be the case for my house. Back in 2008 only 20% of houses had condensing boilers. It seems much of these savings must be due to behaviour change. The 60% increase in gas prices over that period has obviously been quite a motivation.

My friend Ray has done a lot of research into home energy use in Germany and yesterday he showed me similar figures from Germany. German households started saving energy earlier than here - around 2000 - and they are slightly ahead of us on total savings. Also, the German data included energy use by household size. The single person households have reduced their usage by 26% whereas the larger households are only down by 21%. We surmised this is at least partly because in single person households there is one person who has:
  • motivation - because they pay the bills
  • control - because they don't have to persuade anyone else
  • responsibility - because there is no-one else to blame
It's a lot harder when there is more than one of you to dilute the control and spread the blame. Still, at least we know it can be done. I asked Ray what changes people have made but we don't know. We do know what advice they have been getting. That includes turning down thermostats and adjusting radiators, closing curtains at night etc., the same as here, and also something called Stoß-Lüften or 'shock ventilation'.

Even in winter, you do need some ventilation and most people keep some windows a little bit open all the time. However, that does waste a lot of heat, and it causes draughts and cold patches in floors and walls. Shock ventilation means keeping all the windows completely closed most of the time but a couple of times a day you open all the windows at once for just a few minutes. This is enough time to exchange the air in the rooms but not enough to cool down the walls and floors significantly.

It doesn't make sense for our house because we have mechanical ventilation with heat recovery. However, if you give it a go please let me how you get on.

* Degree days are a measure of heating need depending on the weather. If the average temperature on a particular day is below the reference temperature (typically 15.5 °C) then the degree days is the difference. You add the value for each day over a period to get the total. You can find monthly degree days for different regions from various sources on the web. They are useful for understanding your energy bills. There is a longer explanation of degree days in my book.

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