There are several factors to consider when laying out an exhaust fan for a kitchen. The ideal kitchen exhaust fan is affordable, powerful, quiet, and energy efficient. In practice, you might have to make some trade-offs based on which of these qualities is most important. The current layout of your building is another important design consideration. Your choice of ventilation system will depend on whether your cooktop is on top of an island or adjacent to the wall. The accessibility of the attic, roof and exterior wall will impact the final design.
Choose a Hood or Grille
If you have a traditional freestanding oven and range you will most likely install some type of range hood. This hood should sit 18 to 24 inches above the cooking area and overlap it by 3 inches on each side if possible. If you have an island cooktop you will can use a filter grille in the ceiling above it. Select a unit that matches the aesthetics of your kitchen and fits in your budget. You can obtain high quality steel or glass hoods, or low-profile pop-up vents, but these can cost over $1000. Over-the-range microwave vents are cheaper, but less appealing.
Plan Duct Run
Some hood vents have only a fan and filter, circulating the air inside the kitchen. This has limited benefits. Venting to the exterior of the building removes excess heat and moisture. Measure the shortest possible duct run with the least number of elbows or transitions. The duct can run vertically through cabinets, ceilings, and out the roof; or it can run horizontally to an exterior wall. Be aware that installing a new exterior vent can modify the balance of air pressure in your building.
Your exhaust fan should change the air in the kitchen 5 to 15 times an hour. There are several rules of thumb to calculate the necessary airflow in cubic feet per minute. Two of the simplest are multiplying the hood’s area by 100 feet per minute or dividing the kitchen’s volume by 4 minutes.
Choose a fan that is rated for the airflow you just calculated. Inline fans are installed in the duct run. Installing them further from the hood reduces noise. Exterior fans can be mounted on the roof or wall; some of these will have a built-in cap.
If you are venting the exhaust outside of the building, place a one-way damper in the duct run. The damper prevents back drafts of cold air.
If noise is a big concern, consider installing a muffler in the duct run.
Pick Duct Type
Flexible corrugated ducting is suitable for short runs, but rigid metal duct work is probably necessary if you are going through an attic. The diameter of the duct should be the same as the diameter of the fan.
Use Vent Cap
Finally, select a vertically or horizontally mounted cap for the exterior vent opening. The cap protects your building by preventing back flows of cold air and precipitation.