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We had a house built recently and there's a vent on the wall where the microwave is on the exterior of the house, but it doesn't seem like anything is coming out of this vent when we turn it on while cooking food on our stove top (which is directly below the microwave). We leave the fan on for twenty minutes and it makes a loud noise and doesn't really do anything. And in the winter, I don't feel any cold air coming from the bottom of the microwave.

While the house was still being built, we noticed that there was no hole cut out in the wall for the vent (they said they would do that when the microwave is installed). Is there an easy way to know if that was done or could there be another issue at play here?

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    I've never heard of a microwave vent. Are you sure you don't mean a range vent, perhaps to support a range that is below the microwave? I have seen cases where the range vent and range light are built into the bottom of the microwave. – Harper Feb 17 '17 at 3:58
  • Yes, sorry: range vent that is attached to our microwave. – thecoolmacdude Feb 17 '17 at 4:04
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    This still seems odd (picture) but the easiest way is to look from the outside - if the wall is unbroken there's no vent. But it could just be a cavity cooling fan (though this should be automatic) – Chris H Feb 17 '17 at 7:45
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    check above the microwave in the attic or above the roof to see if they ran the vent up and through the roof. – d.george Feb 17 '17 at 10:31
  • I will check outside. There are cabinets above the microwave and nothing inside of there resembles a vent. – thecoolmacdude Feb 17 '17 at 12:54
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Properly installed range hood vent, including those combined with microwaves, almost always use an external vent hood that has a flap to prevent backdrafts. The flap is pushed out by the increased air pressure created by the vent fan.

While there are sometimes internal backdraft flaps, there is almost always an external one as well.

Look at the exterior hood and have someone turn on the vent fan. You should see the flap moving. If it does not, you likely have a problem.

You can remove the external hood by removing mounting screws and cutting the caulk seal around the edges. You can then look into the hole to see where (if) it is connected.

Many vent hoods can be set up for interior or exterior venting. This usually involves changing the orientation of the fan itself during setup. The installer may have left the fan pushing the exhaust into the kitchen. If so, you should be able to feel a draft at the top of the unit when the fan is on.

  • Looking at the exterior opening was the easiest solution--didn't think of that. – thecoolmacdude Feb 18 '17 at 21:04
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It's fairly rare for microwave range hoods to be vented outside in newer homes unless specifically requested by the initial owner (at least in my part of the world, where any exhaust vent wastes expensive heated interior air). They're set up from the factory to be recirculatory, running air through charcoal filters and/or wire mesh grease traps, then out the front louvre. By reversing the fan unit housing they can be configured to vent to the exterior.

Your microwave is probably supported by two screws through the floor of the cabinet above. These are very long and can be loosened up to an inch or more without the microwave coming unattached. They typically fit a #3 Phillips or large flat-blade driver. (Using a #2 driver can result in screw damage.)

With an assistant, loosen one of the screws an inch. Verify that it hasn't come completely out of the threaded socket it runs into. Now, while your assistant supports the front of the microwave, loosen the second screw. Lower the front of the microwave and have a look over the top, under the cabinet. The unit will be supported at the lower rear by a bracket flange.

Your microwave's vent either exhausts upward or rearward, but from right at the top rear of the unit. You'll be able to see whether there's a duct opening at that location. If necessary, completely remove the screws to lower the unit further.

Snug the screws up when you're finished. They don't need to be overly tight.

  • Thanks for the very detailed answer--if I was unable to tell by looking at it from the outside, this would have been very, very useful! – thecoolmacdude Feb 18 '17 at 21:04
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If there is a cooktop below the micro, we require the micro vent to be vented to exterior. They come with standard "charcoal recirculating vent", but there is a plate that can be removed and a duct connected... Only sure way to confirm if it's vented is to see it, which means dropping it down and inspecting it.

  • Yes, there could be "another issue". If it's vented it must be vented "directly to the exterior." That is to say it can't be vented to attic, crawl space, etc. it must go to exterior. So, if you've dropped the unit down and seen the vent, but there is no exterior vent grille cover, then it's illegal. (The vent grille cover could be small...4" x 4" or so.) – Lee Sam Feb 18 '17 at 6:52
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Microwaves don't need venting to the outside. You are making this thing too difficult. Venting for the outside is for smoke and moisture and unless you have an industrial microwave (you don't) there won't be smoke or moisture going out of the microwave.

What people use venting for on microwaves is to get rid of smell. This isn't overly important and not needed for code purposes. You can vent smells anywhere - anywhere were others won't mind them.

To answer the question - there is no reason someone would set an outside vent for a microwave for a home. So unless this used to house an oven I would assume it does not vent to the outside.

  • Doesn't answer the question. I didn't ask if I needed a vent or not. Please re-read the question. – thecoolmacdude Feb 17 '17 at 17:28
  • I will expand on my answer but thought it was apparent. – DMoore Feb 17 '17 at 17:52
  • There is a stove top below the microwave. I thought the tag made it apparent enough. Added that piece to the question. – thecoolmacdude Feb 17 '17 at 18:28
  • Stoves are a top source of indoor air pollution and should be vented externally: well.blogs.nytimes.com/2013/07/22/… – Ben McCann Jun 24 '18 at 19:18
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recirulating over the range microwaves could be checked by placing a paper towel at grease filter, if fan is in right position it should be sucked up, depending on fan cfm

  • Does that indicate that the fan is venting to the exterior? – Paul Price Sep 18 '18 at 23:32

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