Exhaust fan depends on room size

Ask The Builder

June 22, 2008|By Tim Carter

My new kitchen plans call for a new exhaust fan. To be more precise, a kitchen hood exhaust fan has been suggested. Is one fan more effective than another? Years ago, the downdraft exhaust fans were popular. How do I make sure the one I select will adequately ventilate my kitchen? Where does the replacement air enter the house?

You're asking all of the right questions. You need a good kitchen exhaust fan system if you cook greasy foods and boil foods. The cooking process often creates both visible particles and an invisible aerosol mist of grease and smoke that can coat the surfaces of a kitchen if they are not vacuumed and exhausted to the exterior of the home. Even with a great exhaust fan, you can still develop a fine coating of grease on light fixtures, cabinets, walls and ceilings.

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

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

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

The exhaust from my fan exits the roof through a roof cap made to handle that much airflow.

Sizing a kitchen exhaust fan is fairly easy. Many experts simply measure the square footage of the kitchen floor and multiply that by two to arrive at the cubic-feet-per-minute of output for the fan. For example, since my kitchen is 350 square feet, I would need a fan that must exhaust at least 700 CFM of airflow. My fan can do that on its middle speed, and the highest speed produces the massive 1,100 CFM of air movement.

Tim Carter has 20 years of experience in the house-building industry. If you have a question, go to askthebuilder.com and click on "Ask Tim."

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