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Housing - New Build: Heat Loss Calculations

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    Hi Guys,

    I've been asked to provide a quote for a heat pump for a very modern and energy efficient home, but due to the design being far from normal my usual software and a meers calculator are not accurate enough. I've been given U values for the floor, walls and ceiling but don't have the system to turn this info into a heat loss figure.

    If any of you guys have any idea how I could do this your help would be greatly appreciated

    Kind regards

    Paul
    • CommentAuthortony
    • CommentTimeDec 18th 2007
     
    Tell us what these U values are? I cant fathom why you would need a heating system for a very modern and energy efficient home?
  3.  
    Hi - I’m not going to lecture you, but you should learn what your software is doing. That way you will understand the answer and/or be able to replicate it if the parameters are outside the normal. Software just does things quicker than a slide-rule used to. I expect you have very low U values.

    The u value is the thermal conductivity; it’s the inverse of resistance or R value. The individual U value for materials are usually quoted for a nominal thickness of material so reduce them to the actual size, tot up all the inverses (that would be R’s) and invert the answers to get back to an all up U value. It’s quoted in Watts per meter2 per deg K – that’s a clue. If it’s already a composite figure as above, that someone has worked out for you, then this is simply applied to the areas and the temperature differentials. So if U =1 that means 1 watt flows through 1 m2 for every 1 deg.

    Thus if you have the U values and you have the sizes you can calculate the heat loss / power for a given temp difference eg -1 outside 20deg inside or whatever you choose.

    Cheers, Mike up North
  4.  
    Thanks Mike that's an excellent and easily understood answer to my problem, It's great to be able to put some logic behind the calculations.

    I'll see if I can crunch some numbers and get a result.

    Thanks again

    Paul
    • CommentAuthorCWatters
    • CommentTimeDec 18th 2007
     
    Might be of interest. Also includes explanation of losses due to ventilation and examples...

    http://www.plumbingpages.com/featurepages/Heatloss.cfm
  6.  
    Thanks CWatters, that's a great website that will no doubt come in very handy.

    Kind regards

    Paul
  7.  
    Hi Guys,

    Using the information gained from the above posts I've crunched a few figures and come up with a rather surprisingly low total for what is a rather large house. With a U value of 0.16, an area of 214.75m2 and a heat differential of 21 degrees C I have calculated a loss of 722w for the floor. The walls with a U value of 0.16 and an area of 300m2 ( rounded up a little ) and again a 21 degree C heat differential come out at a loss of just 1008W, with 214.75m2 of ceiling ( flat roof single story ) and a U value of just 0.13 I get a loss of 586W, the external glazing of which there is quite a lot , with most units having a U value in the region of 1.8, comes out at 2593W. This gives a total of a little under 5kW Heat loss. Iv'e based the internal space at a temperature at 20 degrees and an external of -1 degree throughout the entire building, I've not taken internal walls into account, is that ok to do? And does that calculation sound right to you chaps?


    When my heating engineer worked it out using his meers calculator ( could it be this isn't much use with an energy efficient building? ) he came out with a loss of over 20kW, so 4 times more than me using the actual U values. However, with such a gulf in results have I dropped a clanger?

    Thanks very much!

    Paul

    PS. I've not yet taken ventilation losses into account due to the building incorporating a heat recovery system, which I understand can recover in the region of 80 to 90% of the ventilation losses.
  8.  
    Paul,

    you cannot easily just add up the heat losses in the way you did. There's thermal bridging to take into account, there's also internal and solar gains that have to be accounted for as well. By ignoring ventilation loses, you're ignoring possibly 30-40% of the overal heatloss. Even though MHRV recovers some heat from the controlled ventilation, without knowing what your actual air leakage is you have no idea what the loses outside of the MHRV would be. MHRV systems are only effective in air tight houses.

    You might want to use a real simulation program to perform your calculation. I live in Canada and so use hot2000 - this will perform a simulation of an entire year (and there are weather files available for UK locations). I've measured my consumption for the past 2 1/12 years and it is within 5% of that calculated by hot2000.

    See http://www.sbc.nrcan.gc.ca/software_and_tools/hot2000_e.asp for more information.

    Paul in Montreal
    • CommentAuthortony
    • CommentTimeDec 21st 2007
     
    Sounds dead impressive to me. All apart from the flat roof which is mad! What glass in the windows? could it be triple glazed to 0.8?

    You need to add in the heat lost through ventilation (not the heat saved). Also you will loose some heat through air leakage indeed this could well be your biggest heat loss so be careful there. What measures will be taken to stop air leaks? Will a pressure test be done on the house? Would be interested in both answers.
  10.  
    <blockquote><cite>Posted By: Paul in Montreal</cite>Paul,

    you cannot easily just add up the heat losses in the way you did. There's thermal bridging to take into account, there's also internal and solar gains that have to be accounted for as well. By ignoring ventilation loses, you're ignoring possibly 30-40% of the overall heat loss. Even though MHRV recovers some heat from the controlled ventilation, without knowing what your actual air leakage is you have no idea what the loses outside of the MHRV would be. MHRV systems are only effective in air tight houses.

    You might want to use a real simulation program to perform your calculation. I live in Canada and so use hot2000 - this will perform a simulation of an entire year (and there are weather files available for UK locations). I've measured my consumption for the past 2 1/12 years and it is within 5% of that calculated by hot2000.

    See<a href="http://www.sbc.nrcan.gc.ca/software_and_tools/hot2000_e.asp" rel="nofollow">http://www.sbc.nrcan.gc.ca/software_and_tools/hot2000_e.asp</a>for more information.

    Paul in Montreal</blockquote>


    Thanks Paul,

    I've downloaded the HOT2000 software so will have a play this afternoon. You mention thermal bridging between internal walls, but if the internal temperature is presumed to be uniform throughout the entire property then is this still an issue?
    I understand the property has been designed to be airtight and therefore the reason behind the inclusion of the heat recovery ventilation system, however I understand there has to be some leakage, so should I calculate in an air change of 1.5 as surely I have to size the heating system to cover a worse case scenario heat requirement.

    Thanks again and Merry Christmas!

    Paul
  11.  
    <blockquote><cite>Posted By: tony</cite>Sounds dead impressive to me. All apart from the flat roof which is mad! What glass in the windows? could it be triple glazed to 0.8?

    You need to add in the heat lost through ventilation (not the heat saved). Also you will loose some heat through air leakage indeed this could well be your biggest heat loss so be careful there. What measures will be taken to stop air leaks? Will a pressure test be done on the house? Would be interested in both answers.</blockquote>

    Hi Tony,

    I cant answer either question I'm afraid as I have limited info from the architect, but I gather the building has been designed to be airtight and presume the flat roof could be due to height restrictions.

    The glazing in all external doors and windows is double glazed 26mm 4-16-4mm with argon filled cavities and have a low E-coated internal pane, which according to the info I've received gives the windows and frames a U value ranging from 1.5 to 1.9.

    Thank you and Merry Christmas!

    Paul
  12.  
    I've completed a quick ventilation heat loss which was around 7500W, so as suggested was greater than the fabric losses. This gives a total of around 12500W heat loss, which would be great as It makes it possible to install a single phase heat pump.

    Paul

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