I’m confused about the difference between passive solar heating and active solar heating. Don’t they both use fans, making them both “active?” Which system is best?
Becky S. — Portland, Maine
Strictly speaking, passive solar heating systems are not supposed to have any working parts other than mechanical airway baffles that do not need external energy to operate — that’s why they are called “passive.” By contrast, most active solar heating systems use electric pumps (needing external energy) to move fluid that’s been heated by the sun through pipes in the building. However, many “mostly” passive solar heating systems incorporate electric fans which, of course, need external energy; thus the confusion between the two systems.
Learning how each system works will help you understand the difference between them, so let’s consider a some typical designs.
The simplest passive solar heating systems allow sunlight to pass through a building’s windows, glass doors or skylights during the day, warming a “thermal mass,” a heat-retaining material such as a masonry chimney or a concrete or tile floor. At night, the warmed thermal mass releases (radiates) its stored heat back into the room.
Variants on this design include “passive solar greenhouses” — with their own built-in thermal mass — attached to the outside of a building.
Heat that’s collected outside the building must be transferred inside. This is usually accomplished by passing air over the thermal mass to pick up its heat, then moving the warm air into the building. Since warm air rises, it can be moved by natural convection if the greenhouse is on the lower story of the building.
But natural convection is not always the most efficient means of moving air, so many passive solar heating systems incorporate small fans to push the warm air along and, sometimes, to return the cool air from the building back to the greenhouse for reheating.
Active Solar Heating
Active systems are designed from the outset to capture heat in one place and move it to another. A typical design consists of panels on a roof through which a fluid circulates. The sun warms the fluid, which is then moved by an electric pump into the building where it can be used for direct space heating via radiators, or indirect heating through radiant floors or walls. (The warm fluid heats the floor or wall, which in turn radiates its heat into the living space.)
The fluid can be water, so long as antifreeze is added to keep it from freezing in the panels on cold nights or some winter days. Other fluids can store heat more efficiently than water, but can be more expensive and possibly release toxins if there is a spill.
Many factors determine which design will work best for a particular building. However, as a rule of thumb, choose the design with the fewest working parts to reduce maintenance costs and the chances of a breakdown. Also, avoid complex piping systems and toxic materials whenever possible. Finally, a multi-use design such as a passive solar greenhouse which helps heat your house and serves as a three-season room where you can grow plants, dry clothes in winter without a dryer and so on is desirable.
Generally, passive solar heating is the best choice for houses and small buildings.
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The Passive Solar House (The complete guide), James Kachadorian
The Solar House: Passive Heating and Cooling, Dan Chiras
Passive Solar Architecture, David A. Bainbridge
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