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Difference Between Ducted & Non Ducted Installation for Microwaves

Ducting for microwaves conforms to all the same rules and logic as ducting for range hoods; it is perhaps easier to visualize the relevance of advice offered for range hoods as they are visibly extracting palpable odors, smoke, heat and heated gasses such as steam. Ducted and non-ducted installations both have their pros and cons; in domestic applications the decision whether to duct or not is typically left to the home owner.

Non-Ducted Microwave Installation

Only metal ducts should be used for venting hot gasses.

A non-ducted microwave installation, also referred to as “duct free,” simply means setting the machine on a worktop or shelf in accordance with the manufacturer’s clearance and support instructions.  The small integral fan built into the machine itself is relied upon to vent gasses from the interior of the machine into its surroundings. So long as clearance around the machine is adequate, the microwave should not overheat and cross-contamination of odors from one dish to another should not take place.  While this scenario is by far the most common, largely due both simplicity of installation and ease of removal, it has one major failing: all the cooking byproducts are vented directly into the room where the microwave is situated.

Ducted Microwave Installation

Microwave ducting is intended to clear the cooking area of the byproducts of use; smells, smoke, particulate grease and moisture are all channeled to the outside of the building and vented into the atmosphere.  A ducted installation is vastly better at protecting the home or commercial kitchen from contamination, grease build-up and intrusive odors. In commercial applications, the force of the extraction may also help to mediate the risk of fire spreading if the microwave catches alight.  However, installation is much more complex and costly than simply setting the machine on a shelf and plugging it in The ducts must be routed through the building to a safe exterior location; municipal authorities almost invariably mandate where ducting may vent for commercial operations. Ducting must never vent into enclosed spaces, such as garages or loft voids, because a subsequent buildup of condensated particulate grease is likely to be flammable.  An electrical supply must be made available to the duct to power an extractor device; hot air rising cannot be relied upon to successfully vent the microwave, so forced air must be used.

Air Moving Devices in Ducting

Two types of air moving devices are commonly available for ductwork.  A bladed rotary fan, the traditional fan seen in most venting and cooling applications, is relatively cheap to buy and operate. A barrel-shaped centrifugal fan is more effective and quieter in operation, but is also more costly.  Variable speed control devices, similar to dimmer switches for lights, are available to control both types of air mover The perceived loudness of the operating device is measured in sones; the louder the fan noise, the higher the sone rating will be on the packaging. The amount of air the device can move in one minute is measured in cubic feet per minute, universally abbreviated to CFM. 

Other Considerations for a Ducted System

For more complex commercial installations, heat sensors can be used to automatically increase the fan’s speed -- or operate an alarm -- if decreased flow or increased heat is detected.  The air removed from the room after it passes through the microwave must be replaced; this is properly called “make-up air” A room with a ducted installation must have sufficient inward ventilation to provide the make-up air.

About the Author

John Cagney Nash began composing press releases and event reviews for British nightclubs in 1982. His material was first published in the "Eastern Daily Press." Nash's work focuses on American life, travel and the music industry. In 1998 he earned an OxBridge doctorate in philosophy and immediately emigrated to America.

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