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Q.Can a dehumidifier be added to a forced-air system? If so, what should a contractor know about specifying this?

A.It is possible to add a dehumidifier to a forced-air system, but there is really no good reason to do so. Moisture vapor moves freely throughout the house, so humidity control in one place will work for the entire house. This means that a portable dehumidifier is usually enough to control humidity when you can’t do so by eliminating the moisture source or by using an exhaust fan. (These should always be your first choices for controlling moisture.) A dehumidifier with a 40-pint capacity can handle most houses.

A dehumidifier is essentially an air conditioner that discharges the warm air back into the house rather than exhausting it outside. Therefore, an air conditioner can also be used to dehumidify a house. But since an air conditioner only dehumidifies when it is running, most systems usually cool the house before the humidity is brought under control. If little cooling is desired, but you want to dehumidify the house, it’s possible to size a central air conditioner to a quarter or half the size needed for normal cooling and run the system continuously without overcooling the house.

In superinsulated houses with very low cooling loads, we often recommend installing a small (5,000 Btu) window air conditioner somewhere in the house to control humidity, and a central cooling system to control temperature. The window unit can be removed during the winter when dehumidification is not needed.