Passive Solar Heating
Thermal mass storage
Think of south facing windows as several small furnaces providing free heat to your house. The only trouble with Mother Nature’s furnaces is they lack a thermostat to control the amount of heat provided during sunny hours. In lieu of a thermostat, to prevent overheating, excessive solar energy needs to be collected and stored for later use within the thermal mass of the building.
When the sun’s rays enter a room, they are partly reflected and partly absorbed by the room’s surfaces. The absorbed radiation heats these room surfaces, which in turn, raises the temperature of the surrounding air to various degrees depending on the density of the material.
A “light density” floor surface, such as sheet vinyl on plywood, would tend to heat up fairly quickly when exposed to the sun. As the surface temperature of the vinyl would be high, it would raise the temperature of the surrounding air and the room environment. If the sun ceased to shine on this floor, because of its light density, the mass would tend to give off its heat to the room quite quickly and cool back to room temperature. This type of room surface has poor thermal mass properties.
 Conversely, a “heavy density” floor surface exposed to the sun, such as ceramic tile on a bed of grout, would tend to heat up fairly slowly. Any solar energy striking the surface would be absorbed downwards into the tile and grout. Because of this energy absorption process, the surface temperature of the ceramic tile would remain relatively low and would not significantly raise the temperature of the surrounding air and room environment. If the sun ceased to shine on this floor surface, it would give off its heat to the room fairly slowly as the absorbed heat deep in the grout worked its way back to the surface of the ceramic tile. This type of room surface has excellent thermal mass properties.
When the ratio of south facing glass to floor area begins to exceed 8%, more thermal mass should be installed to store excess energy and prevent overheating of the room. Adding mass allows the use of more glazing for increased solar contributions without overheating. For passive solar heating to work properly, it is important to charge the mass during sunlight hours and discharge it during the night.
Typical thermal mass building elements would be:
- Thicker or multiple layers of drywall
- Brick or stone feature walls
- Ceramic tile floors on a bed of grout
- Poured concrete floors
- Water
- Hardwood flooring
When adding mass to a floor area, its use must be considered. The addition of a carpet over a floor surface, which acts as insulation, can reduce heat storage by as much as 70%.
Circulation of heat
 Proper air circulation is required to distribute solar gains from the south side of a house to all internal areas of the home. Air circulation can be sustained either by natural convection or be fan driven.
Natural air movement is motivated by two factors: temperature difference and height. Warmer, less dense air will rise, while cooler denser air will fall. The taller the space and the greater the temperature difference between the top and bottom of that space, the faster the air will circulate. To promote natural internal air movement in a passive solar home:
- Keep floor plans compact
- Use large open spaces
- Provide openings for air to flow between floor levels and between north and south zones
- Locate the solar gain windows as low as practical within the air circulation loop
In homes with forced air heating, the furnace fan can be used to move air around all areas of the home. A furnace fan can be set to run continuously to pick up the heated air from rooms on the south side and move it to heat all other areas of the home. The fan will operate in low speed between heating cycles and in high speed on a demand for heat. As this type of air circulation will increase electrical consumption to run the blower motor, a high efficiency DC fan motor should be chosen.
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