Cooktop hoods in a kitchen are designed to vent the heat, steam and smoke from cooking so that your house doesn't fill up with smoke when you make your "famous blackened catfish." Hoods have an optimal efficiency that is affected by the distance from the hood to the stove, the size of the hood, the size of the cooktop and the type of venting used. Always refer to the building codes in your area and the installation instructions for your cooktop and vent hood.
There are several factors that determine the ability of a vent hood to capture the air from a certain area. The size of the hood and the size of the area are important factors, but so is the flow of air created by the fan. Rated in cubic feet per minute, or "CFM," the higher the CFM on a hood, the greater the capture rate will be, but it will also generate more noise since the fan is spinning faster to create more suction. If you require a smaller hood or a larger cooktop you can accommodate for the variance with greater CFM.
Kitchen vent hoods come in two types: ducted and ductless. Ducted vents exhaust the air to the exterior of the house. Ductless vents run the air through a filter then recirculate it through the house. When considering the height of the vent, it is important to know that a ductless vent may be blowing air back in your face if it is mounted lower and you are taller.
The larger the hood, the greater the amount of surface area it will be able to vent effectively, and the closer the bonnet is to the cooktop the more efficiently it will vent the area. As a general rule, if the cooktop is farther away from the hood than 50 per cent of the diameter of the hood, or if the cooktop is more than 50 per cent larger than the diameter of the hood, the capture of air particles will drop to less than 7.5 per cent efficiency according to the U.S. Environmental Protection Agency. So a hood with a diameter of 30 inches should be 15 inches from a cooktop that is no more than 45 inches in diameter for maximum efficiency.
The standard required distance for home installation of hood ventilators is usually between 18 and 24 inches from the top of the stove. High flow systems can be as high as 30 inches. Moving the hood higher reduces the intake capacity, but increases work area, head clearance and sight lines in the kitchen. Professional cooking environments must be more well ventilated and adhere to the EPA guidelines.