DEAR TIM: My new kitchen plans call for a new kitchen exhaust fan. To be more precise, a kitchen hood exhaust fan has been suggested. Is one kitchen stove exhaust fan more effective than another? Years ago the down-draft exhaust fans were popular. What exhaust fan is in your kitchen if you don't mind me asking? How do I make sure the kitchen exhaust fan I select will adequately ventilate my kitchen? Where does the replacement air enter the house? Kathleen K., Exeter, NH

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

You really need a good kitchen exhaust-fan system if you cook greasy foods and boil foods. The cooking process often creates both visible particles as well as an invisible aerosol mist of grease and smoke that can coat the surfaces of your kitchen if they are not vacuumed and exhausted to the exterior of your home. Even with a great exhaust fan, you can still develop a fine coating of grease on light fixtures, cabinets, walls and ceilings. This is the voice of experience talking.

PHOTO CAPTION: This high-powered kitchen exhaust fan is tucked up under a decorative hood. It is sized properly for the large kitchen. IMAGE CREDIT: Tim Carter

I prefer the overhead kitchen exhaust fans rather than the down-draft ones simply 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 that has brilliant halogen bulbs that are built-in to the fan. 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 grease-covered 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 air flow. It was easy to install so that rain does not enter the house.

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 air flow. My fan can do that on its middle speed, and the highest speed produces the massive 1,100 CFM of air movement.

You are really observant to recognize that large kitchen exhaust fans like these have a voracious appetite for air. In today's modern homes sucking that much air out of a house can cause serious back drafting issues if a makeup air inlet is not installed. Back drafting can cause deadly carbon monoxide to be drawn back down a chimney or metal vent pipe and/or smoke or smoke odors from fireplaces.

Newer homes are so airtight that when air is sucked from a house by a powerful fan, it replaces that air with air from outdoors through the path of least resistance. That path could be a furnace or water-heater vent, a chimney, or other vent that is open to the atmosphere. Installing a makeup-air vent solves this problem in almost all cases as outside air can easily flow through this device into the home.

Before you buy a kitchen exhaust fan, it is always a good idea to get the written installation instructions from the manufacturer. These documents will often contain sizing guidelines as well as detailed step-by-step methods the manufacturer wants you to follow to keep the warranty in force. Reading these ensures that the fan you are considering is the right size and that you can satisfy the minimum installation requirements.

Resist the temptation to use smaller ducting for the fan. Some people think that the size of the exhaust piping is not that important. Believe me, you must use the exact pipe as called for, and be sure that you do not exceed the maximum length of pipe allowed.

Pay particular attention to the bends in the exhaust piping. The written instructions will almost always tell you to avoid 90-degree bends, and how many can be put in the exhaust piping. These hard bends in the pipe create significant restrictions that make it hard for the fan to exhaust the air from your kitchen.