Don't Overlook the Kitchen Exhaust

Q: DEAR TIM: The plans for my new kitchen call for a range hood exhaust fan . Is one fan more effective than another? Years ago, the downdraft exhaust fans were popular. What kind of fan is in your kitchen? How do I make sure the one I select will adequately ventilate my kitchen? Where does the replacement air enter the house? -- Kathleen K., Exeter, N.H.

A: DEAR KATHLEEN: You're asking all the right questions about your new kitchen exhaust fan. All too often, I see builders and remodelers fall down here. Either the fan is not powerful enough for the size of the kitchen or the installer fails to vent it properly or overlooks the need for makeup air.

You need a good kitchen exhaust fan system if you cook with grease or boil foods. Cooking often creates visible particles and an invisible mist of grease and smoke that can coat the surfaces of your kitchen if these residues are not exhausted to the outside. Even with a great exhaust fan, light fixtures, cabinets, walls and ceilings can still develop a fine coating of grease. This is the voice of experience talking.

I prefer the overhead kitchen exhaust fans to the downdraft ones simply because hot air rises. Why not use that axiom to your advantage and collect the cooking vapors with a hood?

My kitchen exhaust fan, a powerful three-speed model with brilliant halogen bulbs built in, is matched to the size of the kitchen. There are three removable grease-collector screens that we take out regularly and put through the dishwasher. When the fan is on the highest speed, it sucks 1,100 cubic feet of air per minute (CFM) from above the cooktop and pushes it outside.

The fan is connected to metal ductwork that extends all the way to the roof of the house. Each joint was carefully taped with special metal-foil duct tape by my ventilation contractor. It is important that no air seep from the duct to other parts of the house, where hidden spaces could become covered with grease, posing a significant fire hazard.

The exhaust from my fan exits the roof through a special roof cap that is made to handle that much airflow without letting rain enter the house.

Sizing a kitchen exhaust fan is fairly easy. Many experts simply measure the square footage of the kitchen floor and double that to arrive at the cubic feet per minute of output for the fan. For example, because my kitchen has 350 square feet, I need a fan that can exhaust at least 700 CFM. My fan can do that on its middle speed.

Large kitchen exhaust fans like mine have a voracious appetite for air. In modern homes, sucking that much air out of a house can cause serious problems if a makeup-air inlet is not installed, including deadly carbon monoxide being drawn back down a chimney or metal vent pipe and smoke or smoke odors from fireplaces.

Newer homes are so airtight that when air is sucked out by a powerful fan, it is replaced with outdoor air through the path of least resistance. That path could be a vent for a furnace or water heater or a chimney. Installing a makeup-air vent solves this problem in almost all cases, allowing outside air to flow easily -- and safely -- into the home.

Tim Carter can be contacted via his Web site, http://www.askthebuilder.com/printer_Submit_Question.shtml.

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