I never thought there would be a code for hood venting. A larger
cross-sectional area allows more volume to be moved and more heat
transfer. I'd want to make the size as small as reasonable practical,
and opt for a quality fan with 2 or 3 speeds. If you have a 6 or 8
burner stove, think larger volume movement. A longer pipe, one that
has lots of turns, or thick filters installed should have larger
Check with the manufacturer or installation specifications. They will
give the size and other specifications. Local codes may also have minimum
specifications. Follow both. Different models may have different
Vent hood? Down draft vent? 3&1/4" X 10" is a common rectagler vent or
6" round, but checking with the manufacture instructions for that
venting device is probably best.
Appliance Repair Aid
International Mechanical Code does indeed have a section on kitchen and
The main thing in this case is, what does the hood state to use? If its has
an 8 inch takeoff on the unit, then you use 8 inch hard duct. Hard duct is
required in ALL cases, and if you have a square opening, you CAN change over
to a round, provided that you have the same FLOW, not so much cross section
area, as the opening you have.
I changed from a vent hood over the stove to a Microwave - over the stove. The
man who installed my cabinets did the installation but I had to go to Home
Depot and buy two things - a regular vent pipe (7" I think) and another piece -
round at one end and rectangle at the other. The round piece fitted the vent
pipe already in place that goes up to the roof - and the other end fitted the
microwave. There were about three different sizes at HD and I think they were
6, 7 and 8. I know if I hadn't taken the old vent hood out and measured before
I left, I would have been clueless about what to get.
The most common range hood units I've seen require either a 7" or
5" round duct (although the 7" models can usually accommodate a
rectangular duct as an alternative). The 7" hoods are readily
available at Home Depot; the 5" models usually have to be special
BTW, I have no idea what the applicable code requirements might
Point is, you don't connect the kitchen hood/microwave vent ducting to the
bathroom fart fan ducting.
That's just half-assed jackleg work, typically found only in lazy
"I-did-it-myself and saved a bundle" homeowner specials.
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