A 200 square foot composite building wall is comprised of the following componen
ID: 234685 • Letter: A
Question
A 200 square foot composite building wall is comprised of the following components:
• 30% windows; U value = 1.06; SC = 0.70
• 20% hollow wood door; U = 0.46
• 50% wall; wall is constructed of 6” lightweight concrete (resistance of 0.60 per inch of thickness) and R-11 insulation.
(Don’t forget to add surface film resistances!!)
If the inside temperature is desired to be maintained at 70F and the outside temperature is15F; what is the heat transfer in Btu/hr for each component of the wall? What is the heat transfer for the entire wall?
Explanation / Answer
Formula for transfer of heat through a wall
Q = A x U x ( T2 - T1 )
For window
A = 30% × 200 = 60
U = 1.06
T2 - T1 ( T ) = 70 - 15 = 55
Q a = 60 * 1.06 * 55 = 3498 Btu / hr
For door
A = 20 % × 200 = 40
U = 0.46
T = 55
Q b = 40 * 0.46 * 55 = 1012 Btu / hr
For wall
A = 50 % × 200 = 100
For U of wall with a coating of R - 11 Material
1/ U = 1/ U 1 + 1/ U2
Given resistance of wall is 0.6 / inch
Thickness = 6 "
U1 = 0.6 * 6 = 3.6
U2 = 5.5 ( for R - 11 )
So U = U1 * U2 /( U1 + U2 )
= 3.6 * 5.5 / 9.1 = 2.18
Q c = 100 * 2.18* 55
= 11990 Btu / hr
Total heat transferred
Q = Qa + Qb + Qc
= 16500 Btu / hr
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