On Wed, 21 Dec 2005 17:38:44 +0000, Richard A Downing
wrote:

Also, my heating pipes run through the loft, and are lagged, but I
think I should allow a loss per meter run, any ideas how to do that?
The house is L-shapped and some of the runs are very long.

What sort of insulation and how thick is it? If it's old sacking I'd
expect it's well worth replacing with thick foam, especially if adding
loft insulation makes freezing temps more likely in the loft.

There's a formula for pipe insulation heat loss in the following
paper:

http://www.anotherurl.com/therm/howmuchenergy.htm

Rcyl = ln(Outer radius / Inner radius)/ (2 * PI * length *
conductivity)

Q = dt / Rcyl

and the thermal conductivity of pipe insulation he

http://www.nmc.be/iso/upload/world_092004.pdf

l = 0.038 W/mK

Using them for 25mm insulation on 15mm pipes with a 60C temp
difference gives a heat loss of ~10W per metre.

Rcyl = ln(32.5/7.5)/(2 * PI * 1 * 0.038) = 6.14 C/W

Q = 60 / 6.14 = ~10W/m

Suprisingly 9mm foam insulation gives ~18W/m, so adding lots more gets
you well into diminishing returns. But 25mm will freeze almost as
twice as slowly...

cheers,
Pete.