An energy-efficient house has been constructed so tightly that there is no air e

Question

An energy-efficient house has been constructed so tightly that there is no air exchange with the outside. While this airtight construction greatly reduces heating and cooling needs for the house, building codes specify that for the health of the occupants there should be an air exchange rate of 10 m3/min. In order to comply with code yet preserve the energy efficiency of the house, a heat exchanger is installed. (See picture below.) Assume the temperature of the outside air is 0oC and the temperature of the inside is maintained at 21oC and the heat exchanger heats the outside air to 19oC. 1. If the mass flow of incoming air is the same as outgoing air, what is the temperature of the air exiting the heat exchanger to the outside? 2. What is the heat loss rate for the properly-vented house with the heat exchanger installed? 3. What is the heat loss rate if an el-cheapo vent fan were installed in the wall to deliver the exchange rate?

Explanation / Answer

heat is exchanged between the inlet and the outlet air air in the heat exchanger
Q = mC(T2 - T1) must remain the the same. since the mass flow rate is also equal in this case,<br/>

hence T2 - T1 = T2' - T1'<br/>

19 - 0 = 21 - T<br/>

Therefore temperature of the air exiting the heat exchanger = 2 degree celsius<br/>

2) Q = mC( T2' - T2) = 10*1.005*(21 - 19)/60 = 0.335 KJ <br/>

3) Q = 10*1.005*(21 - 0)/60 = 3.5175 KJ

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