Here’s a rundown of how it works. As with most passive solar heating systems that don’t require the use of mechanical pumps or fans, heat is invited indoors through a series of south-facing windows. It is then absorbed through building materials with thermal mass, or the natural capacity to absorb heat. These materials include masonry walls or floors. When placed in a container, water can be even more effective than masonry or concrete in that it exchanges heat more quickly. Water walls and roof ponds are examples of how water containers can be used for indirect solar gain. These are described below.
Indirect solar gain uses the same strategies of heat absorption and convection as direct gain. With direct gain systems, however, the living space itself acts as a solar collector. Indirect solar gain places the heat collection system outside the living space, such as between the south-facing windows and the living spaces. A Trombe wall, for instance, is commonly located in this space. This wall, made of 8- to 16-inch masonry, with a frontal glass layer, absorbs heat from the sun from the late morning to the early evening. At night, all the warm air it generates begins to travel through the wall and into the living space. It also travels out of the air space between the wall and window through built-in vents. Trombe walls have no moving parts and require no maintenance or on-going costs, making them a popular alternative to active solar heating systems.
Water walls can be located in the same area as the Trombe wall, but the term can also apply to water containers in other building configurations. Along the wall, the water is stored in a container, which can be accented or hidden completely. Roof ponds are located on flat roofs, heating water that warms the room directly beneath the roof pond during the colder evening hours.
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