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UK diy (uk.d-i-y) For the discussion of all topics related to diy (do-it-yourself) in the UK. All levels of experience and proficency are welcome to join in to ask questions or offer solutions. |
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#1
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U-values
I'm trying to work out the losses in my new (1970) bungalow. ATM I
have a spreadsheet that tells me that I need 16 kW with an external -1C. Strangely, the boiler, a Worcester HeatSlave Oil combi, is a 15-19kW model - perhaps the engineer that spec-ed it back in 1991 could add up too :-) Now I can find U-values for most things, but how do I work out the effect of adding another 150mm to the existing 100mm loft insulation. 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. Thanks, R. Happy Christmas and a Prosperous New year to all. |
#2
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U-values
On Wed, 21 Dec 2005 17:38:44 +0000, Richard A Downing
wrote: I'm trying to work out the losses in my new (1970) bungalow. ATM I have a spreadsheet that tells me that I need 16 kW with an external -1C. Strangely, the boiler, a Worcester HeatSlave Oil combi, is a 15-19kW model - perhaps the engineer that spec-ed it back in 1991 could add up too :-) Now I can find U-values for most things, but how do I work out the effect of adding another 150mm to the existing 100mm loft insulation. Look up the value for 100mm - call it U1 Look up the value for 150mm - call it U2 Resultant U = 1((1/U1) + (1/U2)) This ignores the surface effects etc. but is good enough for this exercise 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. Find the U value of the insulation from the manufacturer. Calculate the internal surface area by multiplying the circuference of the end (pi x Dia) by the length. Use pipe diameters and Make sure you end up with sq. metres. Treat it as having a temperature drop of 85 degrees and see what you get. It should not be more than a few tens of watts and can then be ignored for boiler size purposes If it's more, then thicker lagging is needed.. Thickness equivalent to dia of pipe is typical. -- ..andy |
#3
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U-values
In an earlier contribution to this discussion,
Andy Hall wrote: Look up the value for 100mm - call it U1 Look up the value for 150mm - call it U2 Resultant U = 1((1/U1) + (1/U2)) I think you must have needed a slash in the middle of typing that last line. g Putting it back in gives: U = 1/((1/U1) + (1/U2)) or, in simpler form: U = (U1 x U2)/(U1 + U2) -- Cheers, Set Square ______ Please reply to newsgroup. Reply address is invalid. |
#4
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U-values
On Wed, 21 Dec 2005 19:36:56 -0000, "Set Square"
wrote: In an earlier contribution to this discussion, Andy Hall wrote: Look up the value for 100mm - call it U1 Look up the value for 150mm - call it U2 Resultant U = 1((1/U1) + (1/U2)) I think you must have needed a slash in the middle of typing that last line. g Ho Ho Ho. Time to dust off my Victor Meldrew starter kit in preparation for the carol singers showing up. Putting it back in gives: U = 1/((1/U1) + (1/U2)) or, in simpler form: U = (U1 x U2)/(U1 + U2) -- ..andy |
#5
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U-values
Set Square wrote:
Putting it back in gives: U = 1/((1/U1) + (1/U2)) Although, as Andy said, that will underestimate the combined U value somewhat, since the boundary layer resistances are being included twice. More accurately: U = 1/((1/U1) + R2) where R2 is the R value of the added insulation. The R value is found from the thickness of insulation (in meters) divided by the thermal conductivity of the material (k). Example: existing U value say 0.5 W/m^2K, add 150 mm of fibreglass or Rockwool with k value of about 0.034 W/mK. R2 in this case is 0.15/0.034, i.e. about 4.4 m^2K/W and the new U value is 1/(1/0.5 + 4.4) which is about 0.16 W/m^2K. -- Andy |
#6
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U-values
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. |
#7
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U-values
On Wed, 21 Dec 2005 17:38:44 +0000
Richard A Downing wrote: I'm trying to work out the losses in my new (1970) bungalow. Many thanks to everyone who helped with this or the previous question about underfloor heating. My spreadsheet (not guaranteed correct, or even sensible) is on my website, if anyone is interested: http://www.langside.org.uk/private.html There are three scenarios: 1) the original almost uninsulated house (July 2005) 2) what I hope to have by mid January after loft insulation to 250mm and low-e 28mm double glazing (January 2006) 3) and my plan for the next phase with Cavity Wall insulation and some other improvements (Next Phase) Of course, the key thing to realise is that U-values have to be added up as reciprocals..... Thanks again. R. |
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