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Green Building Bible, Fourth Edition
Green Building Bible, fourth edition (both books)
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Fundamentals: Is heat loss absolute or relative?

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    • CommentAuthortony
    • CommentTimeMay 1st 2010
     
    All buildings consume energy and loose heat.

    Should we measure this in KWh or £ or tonnes of carbon?

    To me energy consumption should only be measured in kWh

    I recently saw an interesting unit, W/K this a bit similar to degree days used for calculating fabric losses and I liked it but it is rarely used.

    To measure in anything other than kW over a period of time turns it into a relative rather than an absolute measure.
    •  
      CommentAuthorSteamyTea
    • CommentTimeMay 1st 2010
     
    Tony

    Heat loss is relative to absolute zero. But that is not what you wanted to know.

    Best to measure it in kWh and then you can convert to historic cost, cost or future estimated cost, can do the same with CO2 as the energy mix changed.

    If you plotted kWh against time of day then you would get an idea of real time usage.

    W/K could be written as Js^-1K^-1, so could be rearranged.

    It is usual to measure heat loss in a steady state condition, so one assumes that there is a constant differential between inside and outside. Instantaneous losses are very complicated to work out and not really worth the bother. It is what I do when analysis my solar stuff.
    • CommentAuthorCWatters
    • CommentTimeMay 1st 2010
     
    Why is W/K interesting? Isn't it rather ordinary?
    • CommentAuthortony
    • CommentTimeMay 1st 2010
     
    I means the number of Watts lost (through the fabric) per degree Kelvin (same as Centigrade or Celsius)

    May we could talk W/K /m2 possibly or an average of this over the whole building?
    •  
      CommentAuthorSteamyTea
    • CommentTimeMay 1st 2010
     
    Or U-Value even
    • CommentAuthormbh
    • CommentTimeMay 1st 2010 edited
     
    You can think of the heat losses by conduction of a building as

    Building part (how leaky is your house) x location part (how cold is it where you live, and for how long) x conversion factor to make the number come out in kWh

    The first part is in W/K, summed over all parts of the skin : sum over parts: U of the part x area of the part : how hard does your heating system have to work to maintain the inside temperature, if the outide is 1 K colder than the inside.

    The second part is in Degree days. The colder it is (degrees difference between inside and outside), the harder your heating system has to work, ie the more W it needs to deliver, and the longer the time (days) for which it is cold, the longer your heating system needs to deliver those W, so the more energy (W x days) you have to pay for. But we dont pay in W days, we pay in kWh. Thus we have the third part : 24/1000 : 24 to convert days to hours; 1000 to convert W to kW.
    • CommentAuthormbh
    • CommentTimeMay 1st 2010 edited
     
    Measuring energy in kWh is real measure. A given building under given conditions will require x amount of energy to keep it at a given temperature, and we can use whatever unit we want to record that, as long as it is energy we are recording. We might as well, though, use a unit that is widely understood within the context, so let's use use kWh. If however we convert that to £ cost or kg CO2 emitted per year, these measures are not so much relative as changeable due to factors that have nothing to do with the house. If the tariff changes, then the £ changes, and if our heating is electrically driven, then the CO2 emissions will change if a bit more or less coal is shovelled into the power stations.
    Hence, kWh is a good measure of the building, but £ and kg Co2 are useful indicators to the occupants of how their occupancy of that building is affecting themselves and others, right now.
    • CommentAuthorCWatters
    • CommentTimeMay 3rd 2010 edited
     
    Posted By: tonyI means the number of Watts lost (through the fabric) per degree Kelvin (same as Centigrade or Celsius)


    I know what it means. Just interested to know why you thought W/K more interesting than other units? New Scientist has a long running interest un unusual and interesting units of measure but I don't think W/K would qualify. :bigsmile:

    To measure in anything other than kW over a period of time turns it into a relative rather than an absolute measure.


    True but turning it into a relative value can help people comprehend the scale of a unit. That's why people sometimes choose units such as weight in "elephants".
    •  
      CommentAuthorSteamyTea
    • CommentTimeMay 3rd 2010 edited
     
    Forests per year, sacks of coal equivalent, garden sheds per day. Something like that. Trouble is we are generally talking burning here so cant get too insensity.

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