- 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.