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Homework Help: Heat transfer, energy needed to heat house

  1. May 26, 2012 #1
    Hi everyone!
    First time here, but I'm having trouble with a problem. Here goes:

    On a given day, heat losses through the 20cm-thick walls of a warehouse are 0.02 kW/m2 when the outside wall temperature is 5C and the inside wall temperature is 25C. The owner would like to reduce her heating bill by adding urethane insulation to the outside wall. k:urethane = 0.026 W/m.K

    a. If the walls have an area of 200 m2, what is the total energy (in kJ and kWh) needed to heat the house that day (a day has 24 hours) without the insulation (ignore windows and doors)
    b. What is the thermal conductivity of the wall?
    c. Determine the thickness of the insulation needed to reduce the heating bill that day by a factor of 2.

    So the equation I'm looking at is Q=kA(ΔT/Δx)
    I think I can use 0.02 kW/m2 multiplied by area to get a Wattage for power, but I don't think it's right.

    Once I have power, I can do the rest, but this has been driving me nuts all day.

    Your help is much appreciated.
  2. jcsd
  3. May 27, 2012 #2


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    Welcome to Physics Forums!
    Actually that looks like the right approach to me.
  4. May 28, 2012 #3
    Thank you, but what do I do about time? Or is that extraneous information?
  5. May 28, 2012 #4


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    Yes, you'll get the power, but question (a) is asking for energy. So you need to know the time to get the energy from the power that you calculate.
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