Installing an Over-the-Range Microwave Oven With Vent Fan

Over-the-range microwave
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MIcrowave Ovens for Over-the-Range Installation

Interiors of a kitchen
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A microwave oven designed to be mounted over a cooking range typically has a built-in vent fan and light fixture designed to illuminate the stovetop. These microwaves usually fit between adjoining wall cabinets and below a specialty upper cabinet that hides the vent connections and electrical outlet. When added to an existing kitchen, this type of microwave replaces the existing range hood and vent fan. 

While some microwaves of this type use simple recirculating fans that do little more than filter cooking vapors and return the air into the room, the better types have external vents that exhaust to the outdoors via ductwork and a vent opening mounted in the roof or on an exterior wall.

Every microwave oven will have specific instructions for installation, but all follow a basic nine-step process:

  1. Installing an electrical outlet
  2. Cutting holes for the ductwork and vent cover
  3. Installing the ductwork and vent cap
  4. Mounting the wall bracket
  5. Drilling holes in the cabinet
  6. Preparing the microwave vent
  7. Installing the microwave
  8. Connecting the ductwork
  9. Finishing the project

Tools and Materials You Will Need

  • Microwave oven for over-range installation
  • Metal ductwork sections
  • Transition fitting for microwave vent (if needed)
  • Vent cover (for roof or side wall)
  • Tape measure
  • Drill with bits
  • Drywall saw
  • Reciprocating saw or jigsaw
  • Sheet metal screws
  • Metal shears (if needed)
  • Hammer
  • Caulk gun and exterior caulk or roofing cement
  • Electrical supplies: 20-amp circuit breaker, 20-amp receptacle, wall box, and 12-gauge NM cable
  • Screwdrivers
  • Level
  • Stud finder
  • Wood screws
  • Spade bit or hole saw
  • Metal duct tape

Note: Installing an over-the-range microwave oven requires both moderately good carpentry skills and a good knowledge and experience working with electrical circuits. If you are not confident in your abilities, you may want to hire a handyman, carpenter, or electrician for some of this work. 

Installing an Electrical Outlet

Building codes require that a microwave oven have a dedicated 20-amp circuit that serves no other outlets of appliances. This likely means that you'll need to add a new electrical circuit and outlet within reach of the oven's power cord. Most microwaves are configured so the power cord extends up through the floor of the upper cabinet above the microwave, so the best place for this wall outlet is inside the upper cabinet.

It is good to make this the first step of your project, as it often involves opening up walls to run cables. 

  • Caution: this is a complicated job that requires you to run a new electrical cable through walls to the main circuit breaker panel, to install and connect an electrical outlet, and to finalize the circuit hookup at the circuit breaker panel. Unless you are experienced and very confident in your skills, this kind of electrical work is best left to a professional electrician. 

Note: Microwaves typically are rated for 15 amps, but they should be fed by a dedicated 120-volt, 20-amp circuit. This is because the high wattage of a full-size microwave plus a vent fan can exceed the safe capacity of a 15-amp circuit. 

Cutting Holes for the Ductwork and Vent Cover

As you begin the installation, you'll need to determine whether to vent your microwave through the back of the unit, with the ductwork running straight back and through the exterior wall; or through the top of the unit, with the ductwork running up through the cabinet and ceiling to the roof. This is sometimes a matter of simple convenience and accessibility. Either method will work, but your vent fan will work most efficiently with a short, straight run of ductwork. Usually, this means that exterior wall installation of the vent is the easiest option, but where it is impossible to do this, or where you don't want to mar the exterior siding with a vent cap, you can run ductwork up through to the roof, attaching it with roof cap fitted into the shingles. 

Use the template provided with the microwave to mark the hole for the vent duct on the wall or the bottom of the upper cabinet.

If Running the Ductwork Through the Wall

Note: If, when cutting a hole for the microwave vent, you run into wall studs, you will need to cut away the studs and frame out the opening in order to maintain the structural integrity of the wall. Alternately, you could adjust the pathway of the vent—for example, run the ductwork up and to one side before venting it out the exterior wall. 

  1. Tape the provided paper template onto the interior wall. Mark the location of the vent opening on the wall. 
  2. Cut out the opening in the drywall with a drywall saw or jigsaw. If any structure reframing of studs is necessary (see above), do it now. 
  3. Mark an outline for the vent cap opening on the exterior wall sheathing, and drill marker holes through the sheathing and siding to the outdoors. If your vent cap is rectangular, drill a marker hole at each corner. For a round vent cap, drill a marker hole at the center. 
  4. From the outside, mark an outline of the vent cap on the siding, using the marker holes as a reference.
  5. Cut out the exterior siding and sheathing, using a reciprocating saw or jigsaw. This can be a relatively easy job if you have standard lap siding, but brick or stucco exteriors may require considerable work and additional tools.

If Running the Ductwork up Through the Roof

Venting up through the roof is practical only where the kitchen space lies below an unfinished and accessible attic space. Because this entails cutting a hole in your roof, do not start this step unless you are prepared to finish the installation the same day—preferably a day without any chance of rain.  

  1. Tape the appropriate paper template to the bottom of the upper cabinet. 
  2. Outline the location of the vent opening on the bottom of the upper cabinet. 
  3. Drill holes at the corners of the outline, then cut out the vent opening, using a jigsaw or reciprocating saw. 
  4. Mark and cut out additional openings on the top of the upper cabinet, the ceiling, and the attic floor, where the ductwork will run.  Most metal ductwork is round, so these openings will be round. Note: For best air flow, the ductwork should run as straight as possible from the microwave to the roof line. 
  5. From inside the attic, outline the shape of the roof cap collar on the roof sheathing.
  6. Drill pilot holes then cut a hole in your roof for the vent roof cap. Move to the next step quickly to complete the installation of the ductwork and roof vent cap. 

Installing the Ductwork and Vent Cap

Now you will install the metal ductwork and outdoor vent cap for either the exterior wall or the roof, as applicable. If you are venting through the exterior wall directly behind the microwave, you may need very little, if any, ductwork. In this configuration, the collar on the vent cap may be long enough to slide directly onto the microwave's vent fitting.

Where ductwork is necessary, it should be rigid metal and can be either rectangular or round in shape. Segments of rigid metal ductwork can be assembled by drilling and securing the pieces together with sheet metal screws. You may need to cut segments of ductwork down to size, using metal shears. 

  • Caution: Flexible plastic or corrugated aluminum ductwork, although easy to install, is not recommended, and it may be prohibited by the local building code. These ducts can collect grease on their inside surfaces, creating the potential for fire.

Depending on the shape of the ductwork you choose and the shape of the collar on the vent cap, it's possible you will need a transition fitting to make the connection between the vent cap collar and the first ductwork section. 

  1. From the exterior or roof, insert the collar of the vent cap into the hole you have cut into the siding or roof. 
  2. Secure the vent cap in place following manufacturer's recommendations. This will usually mean applying some type of silicone caulk or roofing cement, then securing the flanges on the vent cap with nails or screws. Roof caps are usually self-flashing, with metal flanges that slide underneath overhanging shingles and are secured with roofing nails. Caulk beneath the flashing with roofing cement to prevent leaking. Wall vent caps should be sealed in place with silicone caulk. 
  3. If you are using ductwork, secure the first section of rigid metal ductwork to the inner collar on the vent cap, using sheet metal screws. Seal the joint with metal duct tape (not plastic tape).  
  4. Continue extending the ductwork with additional sections, securing each joint with sheet metal screws and metal duct tape. 
  5. At the last segment, cut down the section of ductwork to fit, using metal shears, if necessary. The last segment of ductwork should be flush with the wall surface (back venting) or the top of the upper cabinet (top venting). 

Mounting the Wall Bracket

Now comes a step that may require some carpentry skills. Different brands of microwaves differ somewhat in how they are hung from the wall, but most use a bracket that is installed on the wall behind the microwave oven. The oven will hang on this bracket at the back and will be secured at the front by bolts driven down through the top cabinet and into the chassis of the oven. 

If you are lucky enough to have a wall stud positioned at the exact center of the bracket, you can drive the center mounting screw for the bracket directly into this stud. If not, most installation pros suggest removing a section of drywall and installing a strip of plywood across the wall where the oven bracket will be attached. The plywood strip is anchored to wall studs and provides a secure backer base for attaching the mounting bracket. 

The microwave oven should come with paper templates that assist you in attaching the wall bracket, drilling the bottom of the upper cabinet, and attaching the mounting bolts for the oven. 

  1. Draw a vertical line at the center of the cabinet space, using a level. If your oven comes with a paper template, tape this to the wall as a guide.
  2. Use a stud finder to locate the wall studs. If one of the wall studs aligns with the center line, you can continue to the next step. If not, you will first need to remove drywall, then cut and install a plywood backer strip across the wall at the level where the bracket will be installed. Anchor this backer strip securely to wall studs. 
  3. Position the wall-mount bracket on the wall so it is aligned with the centerline. Adjust the height of the bracket based on the cabinet and microwave configuration, following the manufacturer's instructions. Makes sure the bracket is level. Mark hole locations for the mounting screws. 
  4. Remove the bracket, then drill pilot holes at the marked locations. 
  5. Position the mounting bracket on the wall, make sure it is level, then attach the bracket with wood screws driven through the bracket and into the pilot holes or wall studs.  

Drilling Holes in the Cabinet

Now, you'll drill holes in the bottom of the upper cabinet, through which the microwave's power cord and mounting bolts with be threaded.

  1. Tape the paper template provided with the microwave to the bottom of the upper cabinet.
  2. Mark the location for both the mounting holes that will secure the microwave oven and a larger hole for the power cord. 
  3. Drill holes for the mounting bolts using an appropriately sized twist bit. Drill the power cord hole with a spade bit or hole saw. 

Preparing the Microwave Vent

Make any necessary adjustments to the microwave's vent fitting. For example, if the unit is set for back-venting at the factory, you may need to reconfigure the vent system for top-venting. Follow the manufacturer's instructions for all alterations.

The exhaust openings for most microwaves are rectangular in shape, and if you are using round ductwork, you may need to use a transition collar to convert the round shape of the ductwork to a rectangular shape that can be connected to the microwave.

Installing the Microwave

Microwave ovens are heavy, and this step calls for a helper or two as you lift the oven into position and secure it. 

  1. Lift the microwave up into position, and feed the power cord up through the hole in the cabinet.
  2. Hook the lower edge of the microwave onto the wall bracket, then tilt the unit so its back is flush against the wall.
  3. While a helper holds the microwave in position, insert the mounting bolts down through the bottom of the cabinet and into the threaded holes in the top of the unit. Tighten the bolts securely. 

Connecting the Ductwork

Connect the metal ductwork to the vent opening on the microwave.

For Back Venting

If the ductwork installation has been correct, the microwave's vent fitting should slide smoothly into the collar of the exterior vent cap. But it is not uncommon for some minor manipulation to be necessary at this point to complete the connection between microwave vent and the ductwork. 

For Top Venting

You will need to cut and fit the final section of ductwork to run from the ductwork entering the top of the upper cabinet and the microwave's vent fitting at the bottom of the upper cabinet. This is normally a simple short length of rigid metal ductwork. 

Once the connection is complete, seal any joints that are accessible, using metal duct tape. 

Finishing the Project

Your microwave oven installation is nearly complete. 

  1. Install the grease filter on the microwave, as applicable.
  2. Plug the microwave oven into the electrical outlet, and turn on the circuit breaker.
  3. Test the microwave for proper operation by heating up a glass of water. Do not run the microwave empty.