Stove hood microwave fan vent

Can I get away with running 4" flexible dryer vent in my attic from the microwave to the vent at the peak about 40' away? I don't want to go through the roof and it's on an inside wall and the eves are very hard to get to and very narrow.
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No First of all, it is flammable. A vent over a stove should be metal. If you have a grease fire on the stove you may end up with a fire in the attic. Next, it is too small. The vent of the microwave is probably 4 x 8 or so. Next, it is much to small for the distance
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*Well the fan probably won't be that effective because of the duct's reduced size. The fan motor may be more noisy due to the air resistance. Grease will probably accumulate inside of the duct and the attic. I think the microwave usually has something like a 10" x 3 1/4" duct opening. That will fit in a wall. You can hire a roofer or someone else to install the proper roof cap close to the microwave. Try calling the factory for guidance.
A few years ago a customer asked me about replacing her ancient hood which had a duct going through the cabinet upwards. I went up in the crawl space above the stove to see the duct so I could figure on the proper fittings. I couldn't see it. I went outside and looked on the roof for the cap and could not find one. So I asked her to cook something smoky and turn on the exhaust fan. When she did the smoke starting coming out of the eaves. Not through vent holes, but through gaps in the wood over a wide area. The duct through the cabinet was just poked through the ceiling and stopped. Needless to say after about 30 years it was ugly up there. Thank goodness she never had a fire.
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That's the way it used to be done.
The same for bathroom fans.
The primary reason to have it vented directly outside is to keep moisture from condensing on the underside of the roof. That's the potential of the "downside." In most climates, most of the time, it's a non-problem.
Most of the grease should be stopped by those metal mech "filters" which can be cleaned in a dishwasher (somewhat, anyway).

Unlikely. Even in busy restaurants, such fires are rare.
The hood vent piping isn't installed like a flue; it often is directly against combustible surfaces. If a stove fire gets out of control to the point where the vent is hot enough to damage your 4" alluminum dryer vent material, you have other problems.
The "flex vent (metal)" shouldn't be used mainly because it picks up crap from the air flowing through. Sections of galvanized vent are dirt cheap and are better for longer runs.
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As a plumber and someone who doe's this for a living to code , please consult the fire dept or your dept which handles code in your area . Never use flexable alum pipe it is for dryers and not for stove fans , should not even be used there. Always use solid pipe as it last much better in a fire , flexable is for lazy not good work . Restraunts usally will have fire supression in the hood here they have to and they have to clean more often hence less fires.
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On Feb 16, 11:43am, snipped-for-privacy@gmail.com wrote:

The hood vent piping isn't installed like a flue; it often is directly against combustible surfaces. If a stove fire gets out of control to the point where the vent is hot enough to damage your 4" alluminum dryer vent material, you have other problems
Yes you do consult the fool that did this install , your fiberglass insulation takes more heat then the flexable dryer duct to melt and hence is allowed to be around your galvanized pipe or vent . Still new code calls for the vent to go down and across basement and out as it stops attic fires . here the pratice of attic vents are on there way out .
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Buerste wrote:

Hi, Isn't that against code?
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