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Microwave oven vent installation suggestions:
This article describes the venting options for built-in microwave ovens and microwave oven-vent systems typically installed above a stovetop or range. We discuss the importance of venting to the exterior, choice of vent components and materials, and we refer to microwave oven manufacturers' installation instructions.
Reader Question: how can I stop cold drafts from the microwave oven vent I routed into the attic?
Sir, I have a installed microwave oven that is vented by an 8 inch
sheet metal pipe into my attic space (no cap). how can I insulate
it to stop letting my micro wave from getting all cold ?
Thank you in advance for any help offered. - D.M. 1/28/2014
Reply: microwave oven vent to exterior wall: vent kits
I don't think what you described sounds like a good design - you are venting moisture into an enclosed attic space, asking for condensation, mold, lost insulation value and related trouble. Examples of this sort of problem are at
Our photo at left illustrates a side-wall vent termination for a kitchen exhaust fan. This same duct arrangement would suit a microwave oven - vent system.
[Click to enlarge any image]
If your microwave oven requires venting to the outdoors (I'd review the manufacturer's installation requirements) you would be better actually route it outside using the materials and sizes and routing recommended by the microwave appliance manufacturer.
For example a typical installation guide for a GE brand microwave describes mounting the appliance to a building wall (securing its mounting support to wall studs). The company gives three types of microwave installations:
Outside top exhaust using a vertical duct vented microwave.
Outside back exhaust using a horizontal duct vented microwave
Recirculating, non-vented or ductless microwave installation. This installation requires the addition of a manufacturer-supplied charcoal filter accessory kit.
None of these recommended installations would approve what I'd call "pseudo-venting" of the microwave via a vertical duct but terminating inside the structure.
But other than the two very general parts sketches provided by the manufacturer (adapted and shown here) the installation instructions are silent about the duct materials, sizing, length, and routing.
One could guess that's because it's pretty obvious that while the microwave may have come set up for top exhaust venting (there are instructions for converting various parts to vent out of the back of the microwave), in a retrofit installation it is going to be hell to vent a microwave out through a first floor kitchen, up through a building, and outside through a roof, soffit or gable wall. Obviously in new construction it's easier to install vent ducting before the ceilings and walls have been finished.
Actually the microwave installation instructions I reviewed presumed that the vertical duct was already in place before the microwave was mounted. The instructions simply state
Extend the house duct down to connect to the exhaust adapter on the microwave.
Seal the duct joints using [duct] tape.
If that vent has to pass through your attic but terminates outdoors, perhaps down through a soffit or out through a gable end wall to avoid having to make a hole in the roof, the exit opening will need an exhaust-air-operated vent closure (as is used on dryer vents) not only to keep cold drafts from backing down into the occupied space but also to keep critters out of the vent system.
The top-vented microwave example given assumes that the vent runs straight up through the roof (see the roof cap in the sketch above). But an alternative worth considering if it does not make the vent run too long is to vent across the attic floor and down and out through a soffit or out through a gable end wall.
For any exhaust vent that is routed through a cold or cool space (like your attic) you will want to pay attention to these considerations as well:
Un-vented microwave exhaust fans (and other re-circulating range hood fans) typically require an additional filter specified by the manufacturer
The vent run length and diameter cannot exceed the capacity of the vent fan in the vented appliance or it not only won't work, it may be unsafe. Bliss points out that for kitchen vent systems
For good performance, the total
equivalent duct length, not counting the wall or roof cap,
should not exceed about 30 feet. Equivalent duct lengths
for common fittings are shown in Table 6-15.
The vent materials must be approved for the appliance. I doubt that you'd want to nor be permitted to use plastic flex vent for a microwave oven. And we want to avoid a vent installation like the one shown in our photo.
The vent must terminate outside the building, not in an attic and not in a crawl space (photo at left).
The vent should be sloped so that moisture condensing in the duct drains out of the duct to the exterior, not back into the building where it may create a mold problem or water damage
The vent termination should prevent animal entry as well as back-drafts.
Insulation or insulated vent material may be appropriate for some installations to improve performance and to reduce condensation.
Take a look at our clothes dryer venting discussion for a very similar topic. DRYER VENTING for additional suggestions
Watch out: Vertically-vented appliances from kitches may be a fire hazard, pointed out by Steve Bliss at KITCHEN VENTILATION DESIGN
Follow the laundry dryer equipment manufacturer’s guidelines and safety standards such as those published by the National Fire Protection association (NFPA), the American Society of Heating, Refrigeration, and Air Conditioning Engineers (ASHRAE) and the local code authorities.
Kitchen Exhaust Fan Companies
Pop-up and traditional range hoods
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Commercial-style, stainless-steel pop-up and overhead
range hoods; hood liners for custom canopies
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in stainless-steel, aluminum, and glass
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Pop-up, under-cabinet, wall, soffit, island, and fans and
hood liners for custom canopies
Updraft, downdraft, and island-range hoods
Wolf Appliance Co.
Pop-up, chimney-style, wall, and island-range hoods
in stainless steel
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 "Overheated Clothes Dryers Can Cause Fires, CPSC Document # 5022 Updated June 2003", U.S. Consumer Product Safety Commission, www.cpsc.gov., websearch 11/25/2011, original source: cpsc.gov/cpscpub/pubs/5022.html
 "Electric and Gas Clothes Dryers, Staff Evaluation and Contractor Report", US CPSC Memorandum, 25 Feb 2000
"Report on Electric and Gas Clothes Dryers", U.S. CPSC Memorandum, 25 Feb 1999, the Clothes Dryer Project, March 1999
 UL Standard 2158, voluntary standard for electric clothes dryers
 ANSI Z21.5 1 (CGA 7.1) voluntary safety standard for gas powered clothes dryers
 "Healthy Indoor Air for America's Homes, Indoor Air Hazards Every Homeowner Should Know About - room by room assessment", U.S. Government Publications, web search, 11/30/2011, original source: http://publications.usa.gov/epublications/indoorair-hazards/assessment.htm More about this information source, quoting the US Government website: ... Federal Citizen Information Center (FCIC) has been a trusted one-stop source for answers to questions about consumer problems and government services. FCIC, part of the General Services Administration's Office of Citizen Services and Innovative Technologies, has traditionally provided publications to consumers via the publications distribution center in Pueblo, Colorado. The Pueblo.GSA.gov website was where consumers could go to find information and order publications on a variety of topics from the federal government. Publications.USA.gov replaces the former Pueblo.GSA.gov.
 "The Facts About Clothes Dryer Exhaust Systems", John Cranor, the ASHI Reporter, April 2005,American Society of Home Inspectors,® Inc., 932 Lee Street, Suite 101, Des Plaines, Illinois, 60016, Tel: 847-759-2820, website: ashi.org, original source: ashireporter.org/articles/articles.aspx?id=161
 Tjernlund Residential Capacity Dryer Duct Booster®, "Dryer Duct Booster Fan Model LB1, Installation Instructions" [PDF], Tjernlund Products, 1601 9th Street
White Bear Lake, MN 55110-6794 , (800) 255-4208, web search 01/06/2012, original source: tjernlund.com/dryer_booster.htm Quoting:
The Dryer Duct Booster®, Model LB1, has been specifically designed to boost residential capacity clothes dryer duct exhaust velocities
where dryer duct runs exceed 25 equivalent feet. Proper exhaust velocities will reduce drying times, save energy and prevent lint
buildup in the dryer duct. The LB1 is controlled by an electronic Pressure Response Control (PRC) for automatic operation. The LB1
utilizes galvanized steel construction, a reverse inclined, particulate handling impeller that is guaranteed not to clog with lint and
an externally mounted PSC motor for trouble-free operation.
"About the House - Bathroom Vents", Henri deMarne, New England Builder, November 1985
"Bathroom Vent Fan Beats Open Window", James Dulley, Poughkeepsie Journal, 11/4/1987 p. 12D.
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