**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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