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A room's heat loss

  1. Jan 29, 2006 #1
    A room is 5x5x5=125 m3 and holds a temp of 30C. On the other side of all the walls the air is 20C. I measure that the room temperature drops about 0.001C per second. If I now use this equation...

    \dot Q = V \cdot C_P \cdot \rho \cdot \dot T

    ...I get the amount of heat that needs to be added in order for the room to keep this temperature (I think), and it's about 144W. What I don't get is that if I now calculate how much heat goes through the walls using the following equation...

    \dot Q = U \cdot A \cdot \Delta T

    ...this comes to 300W, but that means that I'm actually losing 300-144=156 Watts more than the first equation tells me!

    Am I getting something all wrong here?! This really confuses me, and all attempts to explain this to me will be appreciated.
  2. jcsd
  3. Jan 30, 2006 #2


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    The two equations are valid. I assume you're doing this experiment on a room and finding some discrepencies, is that correct?

    If so, I'd guess that one or more of the constants you're using is off. It would be very difficult to accurately describe the overall [tex] C_P, \rho [/tex] or [tex] U [/tex] accurately.

    Another problem may be overall heat transfer. Are you neglecting radiation heat transfer? Seems to me there would be some significant contributions due to the sun's heating.
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