Heat Loss from Buildings

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 norm-heating 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 (m²)

U = overall heat transmission coefficient (W/m²K)

t _i = inside air temperature (^oC)

t _o = outside air temperature (^oC)

• Online Heat Loss Calculator

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 (^oC)

Overall Heat Transmission Coefficient

The overall of heat transmission coefficient - U - can be calculated as

U = 1 / (1 / C _i + x₁ / k₁ + x₂/ k₂+ x₃ / k₃ + .. + 1 / C _o ) (3)

where

C _i = surface conductance for inside wall (W/m²K)

x = thickness of material (m)

k = thermal conductivity of material (W/mK)

C _o = surface conductance for outside wall (W/m²K)

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/m²K)

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 (m²K/W)

With (4) and (5), (3) can be modified to

1 / U = R _i + R₁ + R₂+ R₃ + .. + R _o (6)

where

R _i = thermal resistivity surface inside wall (m²K/W)

R _1.. = thermal resistivity in the separate wall/construction layers (m²K/W)

R _o = thermal resistivity surface outside wall (m²K/W)

For walls and floors against earth (6) - can be modified to

1 / U = R _i + R₁ + R₂+ R₃ + .. + R _o + R _e (6b)

where

R _e = thermal resistivity of earth (m²K/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/m³ )

q_v = air volume flow (m³ /s)

t _i = inside air temperature (^oC)

t _o = outside air temperature (^oC)

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/m³ )

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 (m³ )

t _i = inside air temperature (^oC)

t _o = outside air temperature (^oC)

Temperature ^oC
K
^oF

Length m
km
in
ft
yards
miles
naut miles

Area m²
km²
in²
ft²
miles²
acres

Volume m³
liters
in³
ft³
us gal

Weight kg_f
N
lbf

Velocity m/s
km/h
ft/min
ft/s
mph
knots

Pressure Pa
bar
mm H₂O
kg/cm²
psi
inches H₂O

Flow m³/s
m³/h
US gpm
cfm