Rangehoods and bath fans aren’t hard to install, they’re just easy to install wrong. The goals for both ventilation types are the same: remove only the necessary air and contaminants, and exhaust them safely outside the house. Here are tips for properly sizing and installing ventilation fans to achieve circulation goals and avoid negative pressure.


Right-size your canopy. “If possible, the canopy should overlap the cooking surface by 3 inches on either side,” says Rudy Rodriguez, training manager for Vent-A-Hood. This isn’t always possible in undercabinet or wall-mount applications, but is critical over islands.

Select the right blower. Size blowers according to the range’s highest heat output, accounting for accessories such as woks and griddles. Broan senior product manager Brian Wellnitz suggests:

  • For “conventional cooking” under 60,000 BTU, choose a blower that offers 100 cfm per 12 inches of cooktop width (e.g., 300 cfm for a 36-inch range). In between blower sizes? Round up.
  • For appliances capable of more than 60,000 BTU, opt for 1 cfm for every 100 BTU.

Don’t over-do it. Oversized blowers could be hazardous. “Too large a blower will create negative pressure, which not only exhausts the gases off the cooking surface, but also from your HVAC,” Rodriguez says. This draws conditioned air out of the home, replaces it with unconditioned air, and in the worst cases could extinguish a furnace pilot light or bring in unwanted fumes from the garage or elsewhere. Check local codes regarding use of make-up air appliances to compensate.


Size matters. In rooms with 8-foot ceilings, choose bath fans on the order of 1 cfm per square foot. For large bathrooms or higher ceilings, “count up the fixtures,” says Pat Nielsen, ventilation fan marketing manager for Broan. “The Home Ventilating Institute recommends 50 cfm per shower, bathtub, and toilet, and 100 cfm for a jetted tub.”

Separate your cfms. In large bathrooms or those with separate water closets, use two fans. “Put a 50 cfm fan in the water closet and use a larger fan to handle the rest of the bathroom,” suggests Brendan Loughrey, Panasonic regional sales manager. “Ideally the other fan would be inside or very close to the shower to pull away the most moisture.”

Duct carefully. Always exhaust moisture outside the building envelope and make the ductwork smooth. “Having a good wall cap or roof cap termination is essential to ensuring that moisture doesn’t accumulate in the attic or the ductwork,” Nielsen says. “Always make sure the termination is appropriately sized for the duct.” Another consideration is to use insulated ducts, he adds. “If the attic is very cold and you have hot, humid air inside the ducting, it will condense on that cold surface and drip back down.” Though nebulous, air moves like water, so any elbows in ducting will cause inefficiencies.

Loughrey and Nielsen recommend minimizing elbows, and refer to “equivalent duct.” Ducting runs should be no more than 50 feet of equivalent duct, in which an elbow in ducting causes the same amount of static pressure as 11 feet of straight duct.

—Lauren Hunter, associate editor, REMODELING.