How Does a Home Exhaust Fan Work?

Function

• Vapors, smoke and gases can result from household activities such as cooking, cleaning, showering or bathing. Exhaust fans remove these elements by pulling them into a vent and blowing them outside of the house. In bathrooms, exhaust fans remove odors, fumes and humidity or moisture.

Types

• Exhaust fans are available in standard and inline models. Standard exhaust fans are usually easier to install but are limited as to placement. Most are placed just above the ceiling and connect to a roof vent. They can also be wall mounted, where they feed inside air directly to the outside of the house. Weight and space requirements, especially with drop-in ceilings, can require some standard exhaust fans to be suspended from attic joists. Standard exhaust fans are available that combine ventilation with lighting and heat-lamp features.

  The inline exhaust fan does not apply any weight at or near the ceiling level. Instead, it is mounted to a joist in the attic. Ducting runs from the fan to vents in the ceiling and roof. Since an inline fan can be installed farther away from ceiling vents, fan noise is reduced.

Ceiling Vent

• The ceiling vent of an exhaust fan is installed in the room where air needs to be vented or evacuated. The vent is an opening in the ceiling that air is sucked into by the fan motor. With standard exhaust fans, the vent intake is part of the fan unit. With inline exhaust fans, the vent intake is attached to the fan motor by duct or tubing.

Roof Vent

• The roof vent is where an exhaust fan discharges or vents air inside the home to the outside. It is often a small pipe or stack that rises from the roof. The roof vent is connected to the fan motor by a duct.

Fan Operation

• Both standard and inline exhaust fans work off of a push-pull approach. The fan itself is essentially a small blower motor. When air is blown or pushed toward a roof vent, this creates suction or pulls air from the other side of the fan motor. This is similar to how a vacuum works. As air is sucked into the fan, it is blown out the other end to the roof, where it is discharged.

Exhaust Air-Flow Measurements

• The exhaust flow or air movement of an exhaust fan is rated in cubic feet per minute, or CFM. Air movement in a home is measured by ACH, or air changes per hour. To determine what exhaust fan rating you need, multiply the length, width and height of the room. This figure is the room's air volume. For a room that is 300 cubic feet, you will need to ventilate that much air to achieve 1 ACH. Since the Home Ventilating Institute recommends changing the air in small rooms eight times per hour (an ACH of 8), you will need to multiply 300 times 8. The resulting number, 2,400, is the total amount of air that needs to be changed per hour to equal the recommended ACH. Since exhaust fans are rated in minutes, divide 2,400 by 60. The required exhaust fan CFM rating is 40.