The vent hood over our kitchen range lets in lots of cold air in the winter, and gets pretty cold itself. I have to block the vent’s intake with boards, and then remove them when I turn on the fan. Have you got a better solution?
—Dave Henseler, Maplewood, MN
Pro Tip: If your range hood exhausts to the outside, consider installing a makeup-air duct to balance the pressure. As explained by This Old House experts, one approach is to connect the makeup-air inlet to the main return duct on your HVAC system, which will moderate the temperature and humidity of the incoming air before it enters your living space. Another option is to lead the duct to an enclosed utility room where the mechanical systems are located. Your furnace and AC may work a bit harder when the exhaust hood is on high, but that’s a small price to pay for keeping the air pressure in proper balance.
Kevin O’Conner replies: That sounds very annoying, not to mention uncomfortable. I got in touch with Brian Wellnitz, senior global category manager for kitchen ventilation at Broan. He’s based in Wisconsin, and so has firsthand experience with cold winters like yours. Here’s what he had to say:
“A properly installed range hood has two metal dampers to stop outside air from coming in. One is at the top of the hood, where it connects to the duct, and the other is at the wall or roof cap where the duct exhausts its air. These thin metal flaps don’t form airtight seals because they have to open when the hood’s fan is working at low speed, but they should keep the cold drafts, and the hot ones, from coming in through your hood.
Pro Tip: As Tom Silva has pointed out on This Old House, “The shorter the run and the fewer elbows, the better.” That’s why the ideal setup is a hood mounted right against, and ducted through, an outside wall. Longer duct runs with multiple bends create more opportunities for cold air to pool and seep back through those thin metal damper flaps.
A damper’s job is straightforward—it prevents outside air from coming in when the fan is off. But as This Old House Magazine has noted, vent hoods over 400 cfm require that makeup air be brought in from the outside when the unit is on, typically from a supply fan, a motorized damper, or another source. If your makeup air system isn’t properly balanced, it can contribute to unwanted drafts even when the hood isn’t running.
So the first order of business is to inspect the dampers and make sure they’re operating properly. Start with the outside cap. If the hood’s duct has a wall cap, it should have a spring holding its damper shut when the fan is off. Also, see if the damper itself is bent or being blocked from closing completely. Do the same thing with a roof cap, although it may not need a spring due to its more horizontal orientation. In either case, if a cap isn’t closing the way it’s supposed to, consider replacing it.
Don’t forget that the wall or roof cap on the exterior end of the duct run also has a damper or flap. A cap in a more horizontal orientation may need a spring to close properly. If a cap isn’t closing the way it’s supposed to, consider replacing it—cold air can pour in through even a small gap in a stuck-open exterior cap.
Now take a peek at the hood’s damper. That will involve removing the section of the duct it’s connected to. In many cases, you’ll find a butterfly damper, with two metal flaps pivoting on a horizontal bar. Although it’s unlikely that there’s a buildup of grease on the flaps, the bar, or its housing—the hood’s grease screens should be capturing most of it—give them a spritz of degreaser and gently wipe them clean. Then turn the fan on and off to check that the damper opens freely and shuts completely. Sometimes a small piece of foam stuck to the top of the flaps adds enough weight to ensure a no-gap fit when the fan is off.
Also, make sure no duct screws interfere with the flaps’ movement. Installers may inadvertently plant a few in the wrong spot. Again, if the damper is damaged, put in a new one, carefully, before reconnecting the duct to the hood.
Your tweaked or new dampers should take care of your hood’s problem, and allow you to take down those boards.”
