Question
For a poorly thermally insulated building the conductance for the walls, including windows, is 48 W/K, the roof 32 W/K and for the floor 24 W/K. Calculate the heat flow through the building envelope for a indoor temperature of 20 degree C and an outdoor one of 0 degree C. The conductances are including the surface heat transfer coefficients. After a retrofitting, the thermal resistance of the building envelope components are unproved. The thermal resistances, including surface resistances, become: The dimensions of the building is 8mx12mx2.4m. The windows and door area represents 15% of the wall area. Calculate the heat flow through the building envelope for this improved house, using the same temperatures as in the previous case. Concrete: 1500 W, light-weight concrete: 100 W. cellular plastic: 50 W, wood: 1 000 W Before retrofitting: 2 080 W, after: 1702 W 40.1 W, -4.3 degree C Temperatures from the outside to the inside, boundary temperatures, surface temperatures and temperatures at material interfaces: -10 -8.7 -6.6 + 19.6 20 degree C 5.15 W/m a) 110 W/K b) 134.8 W/K c) 70 W/K d) 13.2 W/K e) 2.5 W/K H.7: 370 W, the same Heat flow with solar radiation: 512.6 W, without: 1 584 W, temperature with solar radiation: 14.95 degree C. without: 0.22 degree C 1-glass: -196 W, 2-glasses: -348 W, 3-glasses: -346 W, Temperature on roof: 40.7 degree C, south facade: 35.5 degree C, other facades: 25.1 degree C, the internal surface temperatures are the same since the thermal resistance of the metal structure can be neglected a) 443 W b) 106 W c) 60W From 93 to 351 W -7.4 degree C -22 degree C With low emissivity coating: 1.54 W/m2K, without: 2.98 W/m2K From 0.34 W/m2K to 0.24 W/m2K Day time: -0.009 W/m2K, regular U-value: 0.24 W/m2K Vertical: 0.164 m2K/W. Horizontal: 0.174 W/m2K With solar radiation: -14.64 W/m2K, heat flow -392 W, without; 2.96 W/m2K, heat flow 66.7 W L67 W, 5 glasses (4 air gaps)
Explanation / Answer
Q=Conductance*T
before retrofitting conductance=48+32+24=104w/k
Q=104*20=2080W
After retrofitting conductance=39.1+26+20=85.1w/k
Q=85.1*20=1702W
