Q:

I’ve been given conflicting advice on the size of whole-house fan that I need. Most sources recommend a fan that replaces the air in a house every 1 min. to 2 min.; others suggest that turnover in 20 min. to 30 min. is adequate. If I just want to cool the house in the evening after the outdoor temperature drops, do I really need a fan that turns over the entire volume of air in my house every 2 min.?

Mark Oltermann, via email, None

A:

Peter Pfeiffer, an architect and green-building expert from Austin, Texas, replies: Our firm has had quite a bit of experience specifying, detailing and installing whole-house fans in the South as well as the North. Although the speed of air turnover is a subjective matter, the proper sealing of the fan opening when the fan is not in operation is a critical issue.

Let’s address air turnover first. If you want to be able to turn over all the air in your house quickly, a fan sized to do this in less than 5 min. is great. This strategy is often used in the South to take advantage of short-lived cold fronts that pass through during spring or fall, or to flush out a home after a big party or a meal preparation that goes awry. We’ve had success with Triangle Engineering’s Comfort Cooler fans (800- 255-9014; www.trianglefans.com).

If you’re looking to take advantage of cooler nighttime air to flush out the home’s interior heat overnight, a smaller fan with automatic motorized doors works well (Tamarack Technologies Inc.; 800-222- 5932; www.tamtech.com). These fans typically take much longer to turn over the air than their larger cousins. However, they are quiet, and they form a good seal to keep out unwanted outside air and to minimize heat loss when not in operation.

No matter what size fan you choose, make sure the enclosure and door assembly are well weatherstripped and insulated. Otherwise, warm inside air can leak out through the fan assembly in the winter, or warm, humid air can leak into the house during the summer. Leaking warm air means running your furnace or boiler more than is necessary, and infiltration of humid air can cause your air conditioner to work overtime, thereby offsetting any energy savings and comfort benefits you get from using the fan.

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