UKC

Ok, if the outside temperature is zero and we ignore the energy requirement to raise the inside temp., why does it take more energy to maintain an inside temp of, say, 25 degrees instead of one at, say, 20 degrees?

Is it because the higher temp has a higher rate of loss?

In reply to Removed User:

Stop thinking about it and go to sleep

In reply to Removed User: why, is it still minus on the prarie?

In reply to Removed User:

Because the rate of heat loss through your walls is determined by the temperature difference across them. So if it's colder outside, or warmer inside, you have to generate heat at a greater rate (more logs on the fire, more oil per day etc) in order to maintain your inside temperature.

In reply to Removed User:

Surely this makes no sense at all. You've just said to "ignore the energy requirement to raise the inside temp" then asked about said energy.

So I'm not entirley sure what you're asking but it has something to do with Kinetic Theory and the fact that temperature is a a measure of the average kinetic energy of the particles in a body.

In reply to Removed User:
And ignore the impossibility of lowering the temperature of the entire universe to absolute zero.

In reply to Removed User:

Yep, exactly that. The rate of conductive (heat) energy loss is proportional to the temperature difference between inside and outside.

So if you start with a case where the outside is 15 degrees and inside 20 degrees then raising the inside to 25 degrees would double the temperature differential and the rate of heat loss

In reply to Removed User:

Correct, and if you have an empty house with no carpets or curtains or anything else in it, and the boiler is calculated to reach a room temperature of 25c with an outside temp of -5 the house will never reach that temperature if the outside temp is -12. Best we could do with it running continually was 17c. Don't undersize your boiler and rads.

In reply to Removed User:

Basically yes. Its all about the surface temperatures on either side of a structure and the two temperatures trying to equalise and the bigger the difference the more energy flows between the two surfaces.

The U value (a measure of Watts per m2) is a measure of how quickly a structure allows the energy to flow, for instance an brick cavity wall with insulation has a much lower U value than say a window.

It takes more energy to maintain 25 degrees because there is a bigger difference between that and 0 degrees and so a bigger flow in energy from inside to outside. The insulation level of the house just slows the flow down.

In reply to Ste Brom:

Thanks all, just what I thought but needed it clarified.

Yes Steve, its still zero on the prairie but better than they are having in the east.

In reply to Removed User: You answered your own question, you're right, it's because of Fourier's law:

http://en.wikipedia.org/wiki/Heat_conduction

Loss through radiation works the same way, you lose more the hotter you are.