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Q.How do I size the exhaust fan for a range hood above a gas cooktop?

A.Paul Raymer, president of Tamarack Technologies in West Wareham, Mass., responds: The Home Ventilating Institute (847/394-0150;www.hvi.org) recommends that a range hood exhaust fan should have a flow rate of 40 to 100 cfm per foot of range width. For example, a 30-inch-wide range should have a minimum flow rate of 100 cfm and a maximum rate of 250 cfm.

A gas cooktop is a combustion appliance, and it requires an exhaust fan to remove not only cooking odors and smoke, but also combustion byproducts from the gas burner. The range hood exhaust fan should be thought of as one component of a house’s ventilation system. Whenever air is exhausted from a house, an equal volume of air must be able to enter the house. The house’s exhaust system includes all appliances that exhaust air, including the range hood, bathroom fans, clothes dryer, central vacuum cleaner, and any combustion appliance that is not a sealed-combustion unit (including a furnace, boiler, water heater, or fireplace).

Usually these devices are not all operating simultaneously, and most older houses are so leaky that negative pressure is not a problem. But in a tightly built house, a range hood fan — even a 100-cfm unit — may not operate effectively unless a window is opened.