The heat loss is divided into two groups:

(i) the heat transmission losses through the confining walls, floor, ceiling, glass, or other surfaces, and

(ii) the infiltration losses through cracks and openings, or heat required to warm outdoor air used for ventilation.

As a basis for design, the most unfavourable but economical combination of temperature and wind speed is chosen. The wind speed has great effect on high infiltration loss and on outside surface resistance in conduction heat transfer.

Normally, the heating load is estimated for winter design temperature usually occurring at night, therefore, internal heat gain is neglected except for theaters, assembly halls, industrial plant and commercial buildings. Internal heat gain is the sensible and latent heat emitted within an internal space by the occupants, lighting, electric motors, electronic equipment, etc.

3.1 Heat Transmission Loss

Heat loss by conduction and convection heat transfer through any surface is given by:

(2)

where Q = heat transfer through walls, roof, glass, etc.

A = surface areas

U = air-to-air heat transfer coefficient

Ti = indoor air temperature

To = outdoor air temperature

Heat transfer through basement walls and floors to the ground depends on:

(i) difference between room air temperature and ground temperature/outdoor air temperature,

(ii) materials of walls and floor of the basement, and

(iii) conductivity of the surrounding earth.

These portion of heat transmission is neglected in Hong Kong because of the fact that the weather in winter is not so severe and the values are very small in comparison with other forms of heat transmission.

3.2 Infiltration and Ventilation Loss

The heat loss due to infiltration and controlled natural ventilation is divided into sensible and latent losses.

3.2.1 Sensible Heat Loss, Qsb

The energy associated with having to raise the temperature of infiltrating or ventilating air up to indoor air temperature is the sensible heat loss which is estimated by:

(3)

where r = air density

V = volumetric air flow rate

Cpa = specific heat capacity of air at constant pressure

Ti = indoor air temperature

To = outdoor air temperature

3.2.2 Latent Heat Loss, Qla

The energy quantity associated with net loss of moisture from the space is latent heat loss which is given by:

(4)

where r = air density

V = volumetric air flow rate

wi = humidity ratio of indoor air

wo = humidity ratio of outdoor air