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Question:

What size round duct is required to handle 300 cfm of air?

For exhaust fan from kitchen microwave.approx8 feet long

Answer:

Your first resource should be the book that came with the machine (manufacturer installation instructions) If not, then next you can check the machine itself and see what fitment is available Usually a 4-inch tube is recommended, but often a 3-inch tube will workBefore considering any other options check the space that you have in the walls or ceiling, and try to see if any nails protrude into the space from any side, as they reduce the available/useable space Sometimes this simple check will dictate what size tubes you must use Note that the smaller tube will tend to make the fan work harder, and thus be more noisy Important note: You should use only metal tubing, either aluminum or steel, but do N O T use a flexible hose type of tubeThe flexible ones invariably collect dirt and grease in only a short period of time and that makes them a potential fire hazard, as well as smelling badThe time this takes depends on how much frying you do, as well as how good the duct filter happens to be, plus how often you clean or change the filterIn this case, more changes or more cleaning are better.
Your first resource should be the book that came with the machine (manufacturer installation instructions) If not, then next you can check the machine itself and see what fitment is available Usually a 4-inch tube is recommended, but often a 3-inch tube will workBefore considering any other options check the space that you have in the walls or ceiling, and try to see if any nails protrude into the space from any side, as they reduce the available/useable space Sometimes this simple check will dictate what size tubes you must use Note that the smaller tube will tend to make the fan work harder, and thus be more noisy Important note: You should use only metal tubing, either aluminum or steel, but do N O T use a flexible hose type of tubeThe flexible ones invariably collect dirt and grease in only a short period of time and that makes them a potential fire hazard, as well as smelling badThe time this takes depends on how much frying you do, as well as how good the duct filter happens to be, plus how often you clean or change the filterIn this case, more changes or more cleaning are better.

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