In your home there are likely all kinds of vent caps for exhaust fans. Whether installed on the roof, on side walls, or on soffits, there are many different styles and brands that are available for whatever the application is. There are caps for dryer vents. There are caps for bathroom/ laundry exhaust fans. There are caps for kitchen exhaust fans. Many of these caps come with screens because in most instances screens are required.
Today I want to talk about the use of these caps where the screen should be removed—-in particular dryer vent caps and also kitchen range hood vent caps. Screens are actually not allowed on dryer vent caps because we know how quickly lint can clog the vent caps. In my opinion any application where the screen is likely to become clogged with debris is a good place to NOT have a screen when there is a back draft damper. Screens in dryer vents can quickly become clogged with lint making the dryer work very hard and take a long time to dry the clothes. It may cause a fire.
Since the codes require that Kitchen exhaust fan vent caps have screens, it is important that they be maintained clean. The air that is being exhausted from the kitchen is often dust and grease-laden—depending on what is being cooked. Doughnuts, French-fries, and bacon can make for some very greasy air. The grease collects on the screen over time and becomes a sticky (and yucky) dust magnet. As air is restricted less air is moved out of the duct and more grease builds up inside the ductwork and in the exhaust fan itself—and sometimes even runs down the outside of the pipe as well.
Everyone knows that grease fires can be very dangerous and these ducting systems are not designed for chimney fires. In this next picture we can see where the build-up of grease inside the pipe and cap is leaking out of the connections and running down the sides of the pipe in the attic. The inside of the pipe is always going to be even worse.
Kitchen exhaust ducts should be cleaned often and be only smooth wall metal pipe. They should also never be terminated at ordinary Attic vent caps. These roof vents have screens in them. When the duct is attached over them in the attic, there is no way to inspect or clean the screen and there is no back draft damper. The only proper type of cap for the kitchen range hood is one with a back draft damper and a screen at the exterior.
In my opinion this is something that should be changed in the building codes. The reality is that these screens do not get maintained, and the benefits of not having a screen outweigh the benefits of having one. Of course getting people on board with proper maintenance of many things around the home would be the best solution.
The readily available corrugated metal and flexible foil ducts should NOT be used because they create many nooks and crannies where grease can collect. The foil type ducts also should not be used because they are essentially plastic and are vulnerable to damage by vermin (mice and rats love a good source of grease) and would likely not be able to be cleaned when greasy.
Save the nooks and crannies for the English Muffins.
Charles Buell, Real Estate Inspections in Seattle
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