Direct gain is a passive heating technique generally used in cold climates. It is the
most common, simple and effective approach. The basic principle is that sunlight
is admitted into the living spaces, directly through openings or glazed windows,
to heat the walls and floors and thereby the air inside. The requirements of a
direct gain system are glazed windows and thermal storage. The glazed windows
are generally located facing south to receive maximum sunlight during winter (in
northern hemisphere). They are generally double glazed, with insulating
curtains, to reduce heat loss during night time.
During the day, the affected parts of the living spaces tend to get very hot, and hence, thermal storage mass is provided in the form of bare walls or floors of either masonry, concrete or water filled drums to arrest the increase in room temperature. The heat thus stored is released during night for space heating. The direct gain system with its large glazed area and storage facilities could result in undesirable winter losses and summer overheating. Various controls such as roof overhangs, shutters and reflectors can be used for decreasing or increasing solar heat gain. Exhausts and vents can be used to cool the interior spaces through ventilation when summer temperature rises.
Glazing materials may be glass in different layers (i.e. single, double, or triple glazing; the gap between the panes is generally 12 mm) and located in the wall or roof so that radiation falls directly on the thermal storage mass. Direct gain can be achieved by various forms of openings like clerestories, skylights, green houses or glass curtain walls designed for the required heating.
The direct gain approach causes large temperature swings (typically 10 degree Celsius) because of large variations in the input of energy in the room (in the form of solar radiation).
Different types of thermal storage walls are discussed in this section.