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General: Steady State Heat Loss

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    Hi guys

    OK, although a novice, I have been appointed to calculate the Steady State Heat Loss of an old University Building, which I have done. The dimensions are 117m x 17m x 18m. The fabric is 60% glazing (single, U-value 5.6), 40% low insulated brick/block wall (U-value 1.5) with a flat, insulated roof (1989m2, U-value 0.5) and the floor is the same area, U-value 0.3.

    Allowing for an air change of 1 per hour and the total volume being 35802m3. I allowed for a design temp of +22 degrees and -1degree.

    So, on this analysis I have worked out that the SSHL is 723,637 W/K.

    But... and here is my dilemma... Being a novice, I'm not really sure what this means in practice! I know this is the worst case scenario that the heating system is designed for, but is this over a time period? Per degree day? I think I'm getting in trouble with units... I'm really confused here, I hope someone can explain the meaning of the figure I have come up with!

    Also, from examing the buildings energy consumption, typically 200,000kW/h a month in winter, my SSHL calculation seems pretty high!!

    Please guys, any help or comments would be extremely appreciated. Thanks!
    • CommentAuthortony
    • CommentTimeDec 12th 2007
     
    Are you sure about your units? I think you got 723,637 W = 72 kW. This is heat loss when it is -1 outside.

    You air changes seem optimistically low at one per hour to me.

    Having no insulation in the roof is insane!!!

    The floor will loose far less heat than the roof so something is wrong with having about the same U value for them both.

    I would expect the building to have a very high heat consumption not something that should be happening where the brains of the country are seated.

    You could work backwards from the heat consumption, temperature settings and weather data to find its overall heat loss.
  3.  
    Right, that sounds right! So 72 kW is the heat loss per second? hour?

    The flat roof has insulation but it is limited. This is a 1960s university building incidently. Does 72 kW sound like a reasonable sum? I mean, reasonable as in pretty damn bad but not impossible...

    Thanks for your input, as i said, I am a novice at this
  4.  
    Posted By: tonyYou air changes seem optimistically low at one per hour to me.


    Actually, they seem rather high. When we had a blower door test done pre-renovation, the figure was 12.7ACH@50Pa - that's equivalent to 0.64 air changes per hour - and the person doing the testing said it was one of the worst he'd ever measured.

    As for units, Watts already have the dimensions of seconds. If you have a heat loss of 72kW, that is 72,000 Joules per second. If you run the heating at this rate for an hour, you will have consumed 72kWh. By the way, 723637W is 723kW, not 72kW - so it's ten times worse than you're suggesting. 200,000kWh per month would give a heat loss (assuming the heat is on constantly) of 277kW - so your figure of 723kW at an outdoor temperature of -1C may not be far off the mark, though it seems somewhat high. 200kWh a month with an outdoor temperature averaging 8C would give a figure 426kW for the heatloss at -1C (with your 22C indoor design temperature). I would imagine if you reduce the air change per hour figure to somewhere around 0.5 your calculations should be closer to your measured values.

    Paul in Montreal

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