Microwave ovens are an efficient, modern way to heat food quickly. They save space and time, but what if you have a convection microwave oven? Do you need to vent it?
Convection microwave ovens need to be vented if they are built-in to your kitchen cabinetry to improve the air quality in your home. However, ovens that are freestanding or placed on a countertop do not need to be vented separately.
A manufacturer’s guide will usually detail exactly how much clearance any passive vents require for a freestanding, countertop convection microwave.
Venting an Over-the-Range or Built-in Microwave Oven
Convection microwave ovens have an internal fan to circulate heat and improve functionality, but the fan is not designed to vent the oven.
Venting is a sensible decision if you have an over-the-range or built-in microwave.
The fan would normally vent the microwave into the kitchen above the door to the appliance. In this situation, the oven will send moisture-laden air and cooking smells directly into the kitchen.
Apart from making the kitchen uncomfortable, it might also eventually cause a problem with condensation and mold inside the oven (source). As you can see, venting is as essential as keeping your microwave on its own breaker to keep your appliance functional and energy-efficient.
The good news is that these appliances are relatively easy to vent to the outdoors in most homes.
Installing a vent pipe should be straightforward for most seasoned home renovation enthusiasts. Most of the time, you won’t require any specialist tools to do the work.
What You Should Think About First
In thirty years on the tools and running domestic and commercial construction sites, I have found that it’s always best to go for the shortest venting route out of a building. The further you have to go, the more trouble it will be.
Whatever way you fit a vent, the golden rule is always to keep it as simple as possible. I favor going direct through a wall to any other route if at all possible, but in one kitchen, I had to build a box right around three sides of a ceiling to get to the outside. It was unavoidable!
There’s nothing wrong with spending thinking time on a project. After all, it’s free.
How Do I Fit a Vent for My Microwave Oven?
As I said, the first thing to do is identify the route for the extract duct to take. If your property is a single-story dwelling, venting directly through the attic space to the roof is very simple.
If you live in a two-story property or an apartment, this can get a little more complicated, although not impossible. You can run a duct within the ceiling to an exterior wall or along the top of your wall cabinets.
I would always choose the shortest route to the outside in any situation. If my microwave were already set on an exterior wall, that would be my favored route, whatever the circumstances.
What Size Vent Will I Need?
To help you choose which type of duct to fit, I would take the microwave out of its housing in the wall cabinets and check the plate size through which the fan will vent.
If the microwave has been used in this position for some years, do not be surprised if you find grease and staining inside the cabinetry. This is normal. Now you have a chance to clean!
Measure the dimensions of the fan outlet carefully so that the hole you cut at the top of the cabinet will be in the right place and will be big enough to fit the ductwork. I tend to measure everything at least twice to ensure I set things out correctly.
Can I Use the Same Fan?
The existing fan should be perfectly adequate to vent the microwave externally. It will only need to be replaced if it has stopped working.
The most complicated part is to remove the existing fan from its housing within the microwave and turn it through ninety degrees so that it vents vertically out of the appliance.
Before you do this, ensure it is disconnected from the power outlet. If the fan is broken, this is the perfect time to fit its replacement.
You will need a transition piece like Deflect-O Galvanized Transition (link to Amazon) to change the shape of the vent from the opening in the appliance to the shape of the ductwork.
Going Through the Roof
If you are going straight up through the roof, always remove any insulation between the ceiling joists. Loose-fill cellulose will make a mess, and fiberglass will do nothing for the kitchen’s air quality if you are cutting into it.
Fitting a waterproof duct cover like Builder’s Best Roof Vent Cap (link to Amazon) is pretty simple with asphalt shingles.
But with clay tiles, be prepared to line your duct up in the attic so the vent cap fits between the tiles. Or, you may have to cut them with an angle grinder.
In either case, using a flexible duct rather than a solid box duct will help you to get around any obstacles.
Going Through a Wall
If you run a duct along the top of your kitchen cabinets to an outside wall, take the duct to the nearest external wall and cut a hole between wall studs.
Use a masonry hole saw like GSTK Concrete Hole Saw Set (link to Amazon) to make access through concrete blocks or brickwork.
Check that no services are running through the wall at that point. A kitchen will have multiple water pipes and electrical conduits to be aware of so you don’t accidentally cut into them.
Going Through the Ceiling Void
If there is no room to fit ductwork on top of the cabinets, your duct could be put through the ceiling void. It is straightforward if your joists are running the right way. I would avoid cutting through ceiling timbers for duct access if possible.
If you need to remove more than a third of the depth of ceiling timber, then that is too much.
If the microwave is meant to be vented to the exterior, another fan will be fitted within the housing designed to remove cooking smells and moisture.
A countertop oven will require space around it for venting but will not require venting directly to the outside. If there is a separate vented cooking hood in the kitchen, perhaps installing the microwave nearby will be sufficient.
A built-in or over-the-range oven will always benefit from being vented to the outside. Just be certain the chosen route for the duct is feasible.
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Simon Kellow is a master carpenter with over thirty-five years of experience in carrying out home renovations while running construction projects large and small. When he’s not in the office or fixing something in the workshop you can find him enjoying the great outdoors with his family.