Heat Loss from Buildings
Overall heat transfer loss from buildings  transmission, ventilation and infiltration.
The overall heat loss from a building can be calculated as
H = H _{ t } + H_{v} + H _{ i } (1)
where
H = overall heat loss (W)
H _{ t } = heat loss due to transmission through walls, windows, doors, floors and more (W)
H_{v} = heat loss caused by ventilation (W)
H _{ i } = heat loss caused by infiltration (W)
1. Heat loss through walls, windows, doors, ceilings, floors, etc.>
The heat loss, or normheating load, through walls, windows, doors, ceilings, floors etc. can be calculated as
H _{ t } = A U (t _{ i }  t _{ o } ) (2)
where
H _{ t } = transmission heat loss (W)
A = area of exposed surface (m^{2})
U = overall heat transmission coefficient (W/m^{2}K)
t _{ i } = inside air temperature (^{o}C)
t _{ o } = outside air temperature (^{o}C)
Heat loss through roofs should be added 15% extra because of radiation to space. (2) can be modified to:
H = 1.15 A U (t _{ i }  t _{ o } ) (2b)
For walls and floors against earth (2) should be modified with the earth temperature:
H = A U (t _{ i }  t _{ e } ) (2c)
where
t _{ e } = earth temperature (^{o}C)
Overall Heat Transmission Coefficient
The overall of heat transmission coefficient  U  can be calculated as
U = 1 / (1 / C _{ i } + x_{1} / k_{1} + x_{2}/ k_{2}+ x_{3} / k_{3} + .. + 1 / C _{ o } ) (3)
where
C _{ i } = surface conductance for inside wall (W/m^{2}K)
x = thickness of material (m)
k = thermal conductivity of material (W/mK)
C _{ o } = surface conductance for outside wall (W/m^{2}K)
The conductance of a building element can be expressed as:
C = k / x (4)
where
C = conductance, heat flow through unit area in unit time (W/m^{2}K)
Thermal resistivity of a building element is the inverse of the conductance and can be expressed as:
R = x / k = 1 / C (5)
where
R = thermal resistivity (m^{2}K/W)
With (4) and (5), (3) can be modified to
1 / U = R _{ i } + R_{1} + R_{2}+ R_{3} + .. + R _{ o } (6)
where
R _{ i } = thermal resistivity surface inside wall (m^{2}K/W)
R _{ 1.. } = thermal resistivity in the separate wall/construction layers (m^{2}K/W)
R _{ o } = thermal resistivity surface outside wall (m^{2}K/W)
For walls and floors against earth (6)  can be modified to
1 / U = R _{ i } + R_{1} + R_{2}+ R_{3} + .. + R _{ o } + R _{ e } (6b)
where
R _{ e } = thermal resistivity of earth (m^{2}K/W)
2. Heat loss by ventilation
The heat loss due to ventilation without heat recovery can be expressed as:
H_{v} = c_{p} ρ q_{v} (t _{ i }  t _{ o } ) (7)
where
H_{v} = ventilation heat loss (W)
c_{p} = specific heat air (J/kg K)
ρ = density of air (kg/m^{3} )
q_{v} = air volume flow (m^{3} /s)
t _{ i } = inside air temperature (^{o}C)
t _{ o } = outside air temperature (^{o}C)
The heat loss due to ventilation with heat recovery can be expressed as:
H_{v} = (1  β/100) c_{p} ρ q_{v} (t _{ i }  t _{ o } ) (8)
where
β = heat recovery efficiency (%)
An heat recovery efficiency of approximately 50% is common for a normal cross flow heat exchanger. For a rotating heat exchanger the efficiency may exceed 80% .
3. Heat loss by infiltration
Due to leakages in the building construction, opening and closing of windows, etc. the air in the building shifts. As a rule of thumb the number of air shifts is often set to 0.5 per hour. The value is hard to predict and depend of several variables  wind speed, difference between outside and inside temperatures, the quality of the building construction etc.
The heat loss caused by infiltration can be calculated as
H _{ i } = c_{p} ρ n V (t _{ i }  t _{ o } ) (9)
where
H _{ i } = heat loss infiltration (W)
c_{p} = specific heat air (J/kg/K)
ρ = density of air (kg/m^{3} )
n = number of air shifts, how many times the air is replaced in the room per second (1/s) (0.5 1/hr = 1.4 10^{4} 1/s as a rule of thumb)
V = volume of room (m^{3} )
t _{ i } = inside air temperature (^{o}C)
t _{ o } = outside air temperature (^{o}C)
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