If you have less space in your kitchen than you'd like (and who wouldn't like more space in the kitchen?) installing an over-the-range microwave venting kit can be an appealing option. The microwave doubles as your hood vent for the stove, which means you need to decide how it's going to vent that air. There are three over-the-range microwave venting options to choose from.
Recirculated Filtered Air
The simplest option to install is a microwave with a recirculating fan. It requires no outside venting at all, so it's ideal in rentals or places where ducting is impractical. The air is simply drawn through one or more filters, then blown back into the kitchen.
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The pros and cons of the recirculating microwave are straightforward. The positives include low cost and ease of installation. The obvious downside is that air is not actually removed from your kitchen, which means odors and steam may take longer to go away. You can also expect grease buildup over time, which will require cleaning.
Direct Vent Installation
The next-simplest option is to vent horizontally from the back of the microwave through an exterior wall. There are some strong positives to a direct vent installation. It's easier and less costly than vertical installation and provides the shortest and most efficient path for exhaust air. On the downside, it's only a practical option if your range and microwave can be located on an exterior wall. That's not necessarily a deal-breaker, but it limits your options if you're doing a full kitchen remodel.
Vertical Exhaust Installation
The third option is to exhaust your microwave vertically through your cabinetry and wall and continue your duct run vertically from there to your roof or horizontally to an exterior wall. This is the biggest and costliest project, but it gives you more freedom in the positioning of your range. As long as you have a viable route for your ducting and the budget and expertise to make it happen, you can have the venting you need.
Microwave Venting Requirements
Microwaves use standard 3 1/4- by 10-inch rectangular ducting or round ducting at a minimum of 6 inches in diameter. You may need to purchase a microwave vent duct adapter to connect round ducting. For runs approaching or exceeding 140 feet, including elbows and connectors, you may need to step up to 7- or 8-inch round ducting, as GE Appliances advises. Very long runs may also require a booster fan to increase draw.
Some Practical Considerations
As with any installation, be sure you have all the necessary tools, mounting hardware and adapters on hand before you begin. These are sometimes included with the microwave and sometimes sold separately, so double-check to be sure. If the microwave is vented to the outside, you'll need a weatherproof vent for the roof or exterior wall, and need caulking, flashing and possibly siding or shingles to patch the exterior when you're done.
You'll also want to be sure your proposed microwave exhaust vent meets code as well as the requirements of common sense. For example, don't "save time" by venting into a wall or attic, which can create moisture and grease buildup over time and eventually damage your home. You should also verify that you're venting enough air to match your stove. Most microwaves are limited to moving 300 to 400 cubic feet per minute (CFM) of air. A large range may require 600 CFM or more.