Getting outside air to furnace helps prevent condensation

We have severe condensation on the windows of our home. One of your readers solved a similar problem in her tightly insulated Montana house by installing a flue which carried outside air directly to the forced-air furnace plenum. What do you say?

Blake W.H. Smith East Lansing, Mich.

All fossil-fuel-burning furnaces require combustion air. If the air is taken from the inside of a tightly sealed space, the oxygen supply is reduced.

One of the following methods might be used to improve the situation:

* If your furnace is in a confined space, acquire combustion air from outside by running a suitably-sized pipe directly from the room to the attic. The pipe, unconnected to the furnace, simply allows outside air to be brought into the room where the furnace takes the fresh air required for combustion.

* It is usually possible and somewhat simpler to bring a small fresh-air pipe directly from outside into the return-air system. For a furnace that ranges from 80,000 btu up to 120,000 btu, a four-inch pipe would suffice. The pipe should be equipped with a damper to control air volume.

This method allows air to be drawn from out-side directly into the return-air system and then distributed throughout the house. Actually, this slightly pressurizes the home and allows the moisture-laden air to be extracted.

If your furnace has a humdifier controlled by a humidistat, drop it back to the 20 percent range. Otherwise, condensate will form inside the house.

Plaskolite Inc., P.O. Box 1497, Columbus, Ohio 43216, makes a kit to install on windows to prevent condensation.

How about an attic fan or dehumidifier?