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For those who wish to replace the air in a room during a cold day with active heating: Given that the purpose of opening the windows is to exchange as much of the air in the room as possible with the least amount of heat lost – is it more efficient to either:

  1. open all windows wide for a few minutes
  2. crack all or some of the windows open a bit for a somewhat longer period of time

– or are both approaches equally efficient?

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You will probably lose less energy from a very short burst of high-rate ventilation, than slow ventilation over a long period. That's because you'll probably lose more hot air in a long, slow ventilation: it will take some time to notice that the air really is fresh now, whereas with all the windows open for a few seconds, you'll notice the fresher air immediately, and then close all the windows. If you over-ventilate in this case, the heating system will have had very little time to warm the incoming air which then leaves.

It is better to do this when the outside temperatures are warmer, rather than colder, and that's probably about half an hour before the sun goes down. And do turn the heating off some time before you do this, so that the air you expel is at the bottom end of the range of acceptable internal air temperatures.

  • but with a high rate the time between noticing and closing is still going to result in a lot of extra heat loss. – Móż Mar 27 '14 at 23:42
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    @Mσᶎ There will be a lot of extra air change, but not much heat loss, as there won't be time for heat to transfer from the fabric's thermal mass and heating system into the incoming air before it leaves again. – EnergyNumbers Mar 28 '14 at 9:04

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