3-18E Consider an electrically heated brick house (k = 0.40 Btu/h ft-°F) whose walls are 9 ft high and 1 ft thick. Two of the walls of the house are 50 ft long and the others are 35 ft long. The house ismaintained at 70°F at all times while the temperature of the outdoors varies. On a certain day, the temperature of the inner surface of the walls is measured to be at 55°F while the average temperature ofthe outer surface is observed to remain at 45°F during the day for 10 h and at 35°F at night for 14 h. Determine the amount of heat lost from the house that day. Also determine the cost of that heat loss tothe homeowner for an electricity price of $0.09/kWh.FIGURE P3-18E9 ftTiD=70°F50 ft35 ft

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Asked Sep 20, 2019
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3-18E Consider an electrically heated brick house (k = 0.40 Btu/h ft-°F) whose walls are 9 ft high and 1 ft thick. Two of the walls of the house are 50 ft long and the others are 35 ft long. The house is
maintained at 70°F at all times while the temperature of the outdoors varies. On a certain day, the temperature of the inner surface of the walls is measured to be at 55°F while the average temperature of
the outer surface is observed to remain at 45°F during the day for 10 h and at 35°F at night for 14 h. Determine the amount of heat lost from the house that day. Also determine the cost of that heat loss to
the homeowner for an electricity price of $0.09/kWh.
FIGURE P3-18E
9 ft
TiD=70°F
50 ft
35 ft
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3-18E Consider an electrically heated brick house (k = 0.40 Btu/h ft-°F) whose walls are 9 ft high and 1 ft thick. Two of the walls of the house are 50 ft long and the others are 35 ft long. The house is maintained at 70°F at all times while the temperature of the outdoors varies. On a certain day, the temperature of the inner surface of the walls is measured to be at 55°F while the average temperature of the outer surface is observed to remain at 45°F during the day for 10 h and at 35°F at night for 14 h. Determine the amount of heat lost from the house that day. Also determine the cost of that heat loss to the homeowner for an electricity price of $0.09/kWh. FIGURE P3-18E 9 ft TiD=70°F 50 ft 35 ft

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Expert Answer

Step 1

Determine the total heat transfer area.

 

A 2[ (50 ftx9 ft)+(35 ftx 9 ft)]
A =1530 ft
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A 2[ (50 ftx9 ft)+(35 ftx 9 ft)] A =1530 ft

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Step 2

Determine the rate of heat loss during daytime.

 

Pday = kA|-7T,
Qta0.4 Btu/hr-ft-°FX1530 ft2/ 55°F-45°F
1 ft
Qdgy = 6120 Btu/hr
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Pday = kA|-7T, Qta0.4 Btu/hr-ft-°FX1530 ft2/ 55°F-45°F 1 ft Qdgy = 6120 Btu/hr

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Step 3

Determine the rate of heat loss dur...

2nigjt = kA|-T
Opteit 0.4 Btu/hr ft.°FX1530 ft2/ 55°F -35°F
1 ft
Qntgit 12240 Btu/hr
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2nigjt = kA|-T Opteit 0.4 Btu/hr ft.°FX1530 ft2/ 55°F -35°F 1 ft Qntgit 12240 Btu/hr

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