Overall Design Considerations Active and Passive Systems

When designing an active or passive system, many design considerations are common to the two systems. For example, some provision for removal of condensation that forms in the exhaust pipe will be required. Routing of the pipes from the basement to the roof must be considered when the house is being designed. Placement of the exhaust is extremely important.

Removal of condensation is an important consideration. Water collecting in an elbow or other low point of the system can effectively block the pipe, and reduce or disable the system. Builders should strive to design a pipe system that will allow condensation to run back through the pipe to the subslab aggregate. This can be accomplished by ensuring that the pipe run is vertical to the entire distance from the basement to the exhaust. A completely vertical pipe run with no bends or elbows will also provide a pipe system with lower static pressure losses that will enhance the effectiveness of both active and passive systems. If elbows or a low point is incorporated into the design, a condensate pump can be used to drain the water away. The use of condensate pumps will increase the cost of the system in both materials and labor; hence the ideal situation is to design a system that does not require pumps.

Pipe routing should be considered when the home is being designed. This will ensure that there is an area reserved for the exhaust pipe and preclude any possibility of having to build the system with numerous elbows and long horizontal pipe runs. Ideally, the pipes should be run through an interior wall of the home or up through closets.

The exhaust should be located above the highest ridge line. Some builders prefer to exhaust their systems out through an attached garage roof, rather than through the main roof. This type of design does require at least one short horizontal run, and will not seriously impact the effectiveness of an active system. When choosing the exhaust point, avoid the reentry of radon-laden soil gas into the home through open windows and doors. Do not exhaust the soil gas in an outdoor occupied area such as a porch or patio. Locating the exhaust close to a chimney that could back-draft and draw the exhausted soil gas back into the home should also be avoided. For a good discussion on the theory of exhaust design, see the ASHRAE Fundamentals Handbook.19

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