Calculating Heat Loss From a Room of 125m3 - Confusing Results?

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SUMMARY

The discussion centers on calculating heat loss in a room of 125 m³, maintained at 30°C, while external air is at 20°C. The user calculates the heat required to maintain the temperature using the equation \dot Q = V \cdot C_P \cdot \rho \cdot \dot T, resulting in 144W. However, using the heat transfer equation \dot Q = U \cdot A \cdot \Delta T yields a loss of 300W, leading to confusion about the discrepancy of 156W. The discussion suggests potential inaccuracies in constants used and the neglect of radiation heat transfer as possible sources of error.

PREREQUISITES
  • Understanding of thermodynamics principles, specifically heat transfer.
  • Familiarity with the equations for heat transfer and heat capacity.
  • Knowledge of material properties such as thermal conductivity (U-value).
  • Basic understanding of temperature gradients and their effects on heat loss.
NEXT STEPS
  • Review the calculation of heat transfer coefficients (U-values) for different materials.
  • Study the impact of radiation heat transfer in thermal dynamics.
  • Learn about the specific heat capacity (C_P) and density (\rho) of common building materials.
  • Explore methods for measuring and verifying thermal performance in real-world applications.
USEFUL FOR

This discussion is beneficial for engineers, architects, and HVAC professionals involved in thermal management and energy efficiency in building design.

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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...

[tex] \dot Q = V \cdot C_P \cdot \rho \cdot \dot T[/tex]

...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...

[tex] \dot Q = U \cdot A \cdot \Delta T[/tex]

...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.
 
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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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