crosspost- range hood exhaust duct into crawlspace below

10 years ago

.Anyone have their range hood duct so that it sends the exhaust into the crawlspace beneath the house? (into the dirt)

Our house is pier and beam foundation with about 2 ft high crawlspace beneath. Routing the duct exhaust behind the hood downward instead of horizontal would give us a lot of extra space in the closet where this duct work needs to be installed. (cross-section of duct is 3 X 14) - contractor says that range hood exhaust is "friendly exhaust". Of course a screen would be added where it reaches the area below so little critters couldn't enter. I was mainly worried about whether grease can travel 4 1/2 ft to escape and coat the underside of our house.

Also wondering about maintenance - do these types of ducts need to be unassembled and cleaned periodically?

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Comments (3)

snoonyb
10 years ago
Those that I've done continue the duct to, and through the
rim joist ending in a screened hood with a damper.

There is also an in-line or exterior blower which substantially reduces the noise in the kitchen.

"contractor says that range hood exhaust is "friendly exhaust"."

And his substantiation of grease and water vapor as "friendly" to wood, would be?

Not withstanding, it may be a building code violation.
mxyplx
10 years ago
As I read your situation you'd have the xost going up then making a 180ÃÂº down under then probly another 90ÃÂº to horizontal.Something like that anyway. Be sure to size the duct diameter accordingly because each bend will be equivalent to several feet of straight duct. For example two 3 foot straight sections connected by a 90ÃÂº elbo might be equivalent to 10 feet of straight pipe.

So you'll need to figure the equivalent straight line duct length then size it's diameter to reduce resistance. The instructions with the hood should provide enough info for you to figure it out. A 6 inch dia pipe has 44% more area than a 5 inch dia pipe. 5 inch is pretty much standard so watch it. If the pipe is too small the hood won't work for diddly.

Don't count on the contractor to be able to handle that horribly advanced mathematical concept. Don't.

We have a downdraft type cook top with 5 inch pipe. I shoulda been 6 inch. Total length is about 15 feet but the equivalent straight length is roughly 25 feet. It runs thru the crawl space and xosts to the outside.

Most grease stops in a trap but the fan and housing collect a lot over the years. Not much if any goes into the pipe. It's a centrifugal fan so the grease gets thrown to the housing instead of being impelled along the duct. If your fan is not centrifugal that may not be true.
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renovator8
9 years ago
Your contractor is simply ignorant of common knowledge regarding moisture control in buildings which is a little frightening considering he builds houses for a living.

The only "friendly" air that should be introduced into a crawl space is conditioned air from the house. Supplying air from a range exhaust hood will increase the humidity in the space and increase the risk of fungi damage to wood. It also provides a path for hot gasses from a fire to reach a concealed space: a serious mistake in any building. It is therefore a violation of all residential building codes.

2009 IRC: SECTION M1503 RANGE HOODS
M1503.1 General. Range hoods shall discharge to the outdoors through a single-wall duct. The duct serving the hood shall have a smooth interior surface, shall be air tight and shall be equipped with a backdraft damper. Ducts serving range hoods shall not terminate in an attic or crawl space or areas inside the building.

You should cover the dirt in your crawl space with a vapor retarder to reduce the risk of moisture from the ground feeding fungi that can damage the floor structure.

Building houses is not rocket science but if you are going to make important design decisions you should buy a copy of the applicable residential building code ... and give a copy to your contractor too.