Steady State Heat Loss

Heat Loss Equations

The CIBSE (Chartered Institute of Building Services Engineers) Guide A (2006) explains that: “The thermal transmittance (U-value) of the building envelope is the principal factor in the determination of the steady-state heat losses/gains. Hence, the capacity of the heating or cooling system required to maintain specified inside design conditions under design external conditions.” (2006).

Heat is lost from buildings in two ways:

1) By transmission through the building fabric
Transmission through the building fabric is given by CIBSE Guide A – Environmental Design, as the Equation:

Ht = Σ (U A DT ) + Σ ( L Ψ )

where
Ht is the heat loss through building elements;
U is the U-value of the building components;
A is the affected area;
DT is the temperature difference between inside and outside;
L is the affected length of the linear thermal bridge;
Ψ is the Psi-value of the detail.

2) By ventilation and air infiltration
The ventilation heat loss rate is given by CIBSE Guide B – Heating, ventilating, air conditioning and refrigeration, as the Equation:

φv = ( N V / 3 ) ( tai – tao )

where
φv is the heat loss through ventilation and infiltration;
N is the number of air changes per hour;
V is the volume of the building;
tai is the inside air temperature;
tao is the outside air temperature.

The overall heat loss (OHL) is the sum of these two factors, given as the Equation:

OHL = Ht + φv

where
OHL is the overall heat loss;
Ht is the heat loss through building elements;
φv is the heat loss through ventilation and infiltration.

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