Ideas needed for new build

Jan 15, 2005 207 Replies
R

The message from Andy Wade contains these words:

I may be venturing into the unknown (or at least somewhere I haven't been for the best part of 40 years) but the difference between say 290 K and 287.6 K is just about significant at the 4th power but it pales into insignificance when the temperature difference is say 15 K.

No. The warm body in question is the human body that dIMM maintains loses heat very much faster when the ambient temperature is raised to compensate for the discomfort caused by having a poorly insulated house.

Well for a start a poorly heated room needs much more heat to keep it up to temperature than a well insulated room and if that heat is supplied by a radiator some 40% is probably radient heat. If supplied by a fire it will of course be a much higher percentage.

Secondly the radiator (if that is the heat source) being considerably hotter is a much better radiator than the cool walls. If the walls are say at 16 C, the warm body at 39 C and the radiator at say 60 C I will leave you to work out what those proportions would mean in degrees K at the 4th power.

Vote
Twitter Facebook LinkedIn
J

Na, just the absence of wind chill since he will be gas tight apparently....

Vote
Twitter Facebook LinkedIn
A

For small temperature differences (relative to the absolute temperature) you can forget the fourth-power law and assume that the net heat flow is proportional to the difference in temperature between the bodies. So a ~2.5 K change in 15 K difference makes about 15% difference in the heat loss from the warmer body.

OK (penny drops) - I see what you were saying now. Yes that does work in the opposite direction to the radiation argument. I need to go away and think about this some more...

Yes, but the source will, in those cases, have a much smaller surface area than the walls. To work out the net heat flow to any point you'd need to integrate the radiant flux from all the surfaces 'visible' over the whole 4*pi of solid angle. This (AIUI) is what leads to the concept of mean radiant temperature.

That's easy for plane facing surfaces, otherwise far more tricky:

  • 60 C --> 39 C is a net flux of 160 W/m^2 (assuming emissivities of 1)
  • 16 C --> 39 C is a net flux of -142 W/m^2.
Vote
Twitter Facebook LinkedIn
R

The message from Andy Wade contains these words:

This is not an area I am familiar with but where do you draw the line? Reliance on the 4th power term (as your figures below show) suggests that there is the best part of 25% difference per degree between the range 16-39 C and the range 39-60 C.

Likewise mean radiant temperature is not a concept I am familiar with but how do you get round the fact that a poorly insulated room needs several times as much heat in order to be maintained at the same temperature as a well insulated room.

Vote
Twitter Facebook LinkedIn
R

The message from Andy Wade contains these words:

And?

Vote
Twitter Facebook LinkedIn
R

The message from Roger contains these words:

I will take it that you now agree with me then.

Vote
Twitter Facebook LinkedIn

Join the Discussion

Have something to add? Share your thoughts — no account required.

Didn't find your answer?

Ask the community — no account required