How to Replace the Ductwork Connected to the Over the Range Microwave

What You'll Need
Tape measure
Pencil and paper
New ductwork: 3-foot straight pipes, wall cap and 90 degree elbow(s)
Drywall stud finder
Drywall cutting knife
Duct tape
Power drill and screwdriver bits
Metal screws
Drywall repair putty
An assistant

When you purchase a new over the range microwave, you may need to relocate or replace the ductwork that vents the microwave oven. Follow these tips to replace ductwork connected to an over the range microwave.

Step 1: Remove the Existing Over the Range Microwave

Set and climb the stepladder in front of the microwave support shelf. Slide the microwave unit slightly forward on the shelf and unplug it from its power source. With your assistant, lift the microwave out and detach the existing duct pipe. Leave the duct pipe in the wall for the present. Move the old microwave out of the kitchen.

Step 2: Check the New Microwave for Where External Vent Attaches

If your new microwave has a vent in a different location than the old one, you will need to make a new wall hole for the vent, and replace the ductwork. It is best to replace the ductwork when installing a new microwave, as the old ducts might be clogged or have leaks from exposure to heat and moisture.

Step 3: Measure the Location for the New Wall Vent

Get the dimensions of the back of your new microwave and draw a sketch of the new vent location on paper. Draw the new vent location onto the rear wall of the microwave support shelf. Make sure the new vent is located beside, not through, a drywall support stud. Measure the distance from the old wall vent to the new one, and purchase another 90-degree elbow and straight piping as needed to connect the new ductwork to the existing pipes.

Step 4: Cut the New Inside Wall Vent

Use the drywall knife to cut the hole for the new wall vent. Sand the edges to make a smooth entry for the new ductwork.

Step 5: Inspect the Wall Cap and Duct from Outside

Check the wall cap and attached duct from outside the house. If it is in excellent shape, with no cracks, corrosion or blockage, you can use it again. If it has any damage, remove it from the wall and replace it with new ductwork.

Step 6: Connect the Ductwork to the New Inside Wall Vent

Cut a short 3-inch long piece of square duct to attach to the back of the microwave. Attach a 90-degree elbow to this duct, and run it inside the wall to the existing exhaust ductwork. Fasten the new pieces of ductwork together with fresh duct tape.

Step 7: Attach the New Microwave to the Wall Vent Duct and Plug It In

Lift the new microwave oven up to the support shelf. Plug in the microwave oven and ensure it is functioning correctly, then fasten the wall vent duct to the back of the oven. Run the microwave for 1 minute to heat up a cup of water, while your assistant goes outside to the exhaust vent wall cap to check that the vent is channeling exhaust heat out of the vent.