More Than One Way to Vent a Range

By Paul Bianchina

For many kitchens where the traditional overhead exhaust fan is either not desired or simply not practical - as in the case of an island-mounted cooktop - downdraft venting offers a great alternative.

Downdraft exhaust systems, of which the brand Jenn-Aire is probably the most widely known, utilize one of two methods to exhaust air from the room. Blower-motor systems utilize a blower motor mounted in the cabinet directly underneath the cooking surface. When activated, the high-CFM (cubic feet per minute) motor blows a high volume of air through a duct to the outside, creating a suction effect behind it, which draws in air at the cooktop.

Remote blower systems utilize a large suction fan that is mounted outside the house, either on a wall or on the roof. When the motor is operating, it sucks air through a duct that is connected directly to the cooktop, drawing the odors and moisture in with it. Both systems utilize some type of back-draft damper which is spring-loaded to remain in the closed position when the motor is not running, keeping outside air from coming back into the house.

Remote blower systems have the advantage of quieter operation. Since the motor is located outside the house rather than inside the cabinet, the noise is considerably lessened, especially if the unit is mounted up on the roof. The disadvantage is cost - the motors are larger to provide the necessary suction over long distances, and are enclosed in a weatherproof aluminum or stainless steel housing to protect it against the elements. Because of the larger volumes of air it has to handle, remote systems also typically utilize larger sizes of duct pipe.

Installation

The specific installation methods vary from unit to unit, but they all have some things in common which are important to keep in mind at the planning stage of your kitchen. The more elements of the installation that you can plan for early on, the easier the installation will be.

If you are using a blower motor that is mounted at the cooktop, the motor unit will either already be attached to the cooktop vent, or else provisions will have been made by the manufacturer to provide space under the range for the blower to be installed. With a remote unit, you'll need to decide on where the motor unit will be located, and either install it or allow for it at the time that the siding or roofing is done (depending on if it's a wall-mount or a roof-mount).

With either type of system, the connection from the cooktop vent to the outside is done using ducting that is provided by the installer, not the cooktop manufacturer. All of the ducting must be smooth-wall galvanized sheet metal - corrugated flex duct is not allowed. Some manufacturers provide small lengths of flexible duct for making the connection between the vent and an interior blower motor, and as long as it's factory-supplied and engineered for use with the ventilation unit, it should meet all applicable building code regulations.

From the cooktop vent, there are three ways you can get outside. The advantages and disadvantages of each need to be studied and a decision made early in the planning, since some installation may require that ducting be installed in walls or floor joist cavities that may be inaccessible later.

If the cabinet housing the cooking appliance is located on an exterior wall, the most direct venting method is to go directly through the wall behind the cabinet. This simplifies all of the duct work and keeps the length of the duct run to a minimum, which increases the efficiency of the fan's operation. This method is particularly advantageous with remote-motor installations, since the motor can be mounted directly to the exterior wall with a minimum of hassle.

The second method, and probably the most common, is to run the duct down through the bottom of the cabinet and through the floor, into the crawl space below. From there, the duct can be turned in any direction and vented to the outside. In some instances, the vent pipe can be run through one of the existing foundation vents to simplify installation, but you need to be certain that blocking off one of the foundation vents with the exhaust pipe does not reduce the amount of underfloor ventilation enough to place it below the code requirements.

The third method is to route the duct pipe into a wall cavity, and from there take it up through the attic and out through the roof. This is typically the most difficult installation, and requires careful planning of the duct layout.

No matter which installation method is used, remember that all of the duct pipe needs to be of the correct gauge and material, and that all ducts must exit completely from the building - not just dump into the attic or crawl space - and must terminate in an approved type of cap. Finally, remember that the CFM of the motor will determine how long a run of duct pipe can be used, and that each elbow you use reduces the overall length of the allowable run - read the manufacturer's installation instructions carefully for details on how to calculate the allowable size and length of the ducts.

Copyright 2002-2006 Inman News Features. Distributed by Inman News Features.