Can someone tell me if it is OK, by codes, to use flex duct in a range hood exhaust? I’m in SoCal if that makes a difference. I’m thinking it’s not OK, that it should be rigid duct only. It’s for a micro wave range hood combo if that means anything.
They put in new cabinets and it looks like they’re preparing to use flex through the upper cabinet to the existing rigid in the attic.
Join some of the most experienced and recognized building professionals for two days of presentations, panel discussions, networking, and more.
"I have learned so much thanks to the searchable articles on the FHB website. I can confidently say that I expect to be a life-long subscriber." - M.K.
Dig into cutting-edge approaches and decades of proven solutions with total access to our experts and tradespeople.
Start Free Trial NowGet instant access to the latest developments in green building, research, and reports from the field.
Start Free Trial Now
Replies
Are you talking about the corrugated aluminumn tube that is accordianed down to a smaller size that can bee made longer by bending it, or are you talking about the HVAC duct that has the fiberglass insulation?
All metal flex, yes, not the vinyl stuff.
Cannot use it
no matter. I have never seen a mechanical code that allowed flexible duct of any material allowed in a domestic kitchen exhaust. I have seen many that specifically forbid it. Do you know what codes applies where you live?
California code. Someone gave me a reference and it says something like 'smooth wall' and 'air tight'. A friend said he used flex to get his house to sell and it passed inspection scrutiny (not code inspector) by a guy knows as 'the deal breaker'. Go figure.
Even if it's not a code violation, I'd say it's a bad idea. Way more resistance, which range hoods hate, and a higher likelihood of grease buildup.
The duct
"shall have smooth inner walls and be air tight..." according the Code I have. No flex.
I found two references that say that ... IRC and Calif Title 24 Mech Code.
Pretty much what I thought, but I wanted some feedback before I begin to to to the contractor or BO.
Not just 'no' but "EFF
Not just 'no' but "EFF NO."
Flex is delicate and prone to tears. It does not remain connected very well. I don't even like it for clothes dryers.
A duct for a range hood should be smooth, solid, and double-walled. Like a stove chimney. It should also NEVER contact anything flammable - including the cabinet shelves. And the vent shuld project substantially above the roof.
Now ... I'll let you know if I ever find a residential range hood that's actually done right!
Looks like the code says 'smooth wall' and air tight, but I think is silent on the clearance to combustibles.
The contractor installed rigid/smooth wall, but half the air was blowing into the upper cabinet, so I'm asking that be corrected.
I was hasty ...
I got my start in the working world performing the actual "industry standard" fire tests on all manner of building assemblies. That has colored my ideas as to how things should be done.
I also overlooked the simple fact that codes are a lot less stringent (in most instances) for consumer items than for 'commercial' applications. Why? Well, one respected writer (Patten) on fire protection issues has flatly statd that codes are driven by insurance companies, insurance companies are driven by losses - and it doesn't cost them much when a house burns down. Thus, warehouses have always had tightr rules than bedrooms - though bedrooms are where people die.
In testing a range hood, it is assumes that there WILL be a grease build-up and that there WILL be a fire. So, they pack it with grease and light it up.
It becomes quickly clear that the smallest pinhole will shoot a lance of flame at the nearest combustible, and that the piping will get red-hot in a heartbeat. Thus, the desire for double-wall ducting, a "light test," and clearance from combustibles. Commercial hoods will often have the "double wall" made with the outer duct in contact with framing, while the inner, air-tight duct will hang free with about 2" of clearance all around. Oddly enough, this 'dead air' around the duct seems to reduce the rate at which grease collects.
Then there's the matter of cleaning. Just how well do you think a length of flex will stand up to a stiff brush and harsh chemicals being shoved through it?
Cleaning can be an issue, especially if the cooktop has a 'grill' or 'griddle' section, as opposed to the usual pot-burners. AN heaven help you if there's a wok ring!
That most have 'never heard' of such being done for a hood in a home does not mean we should not do it. This is about FINE homebuilding, after all. It's not about doing the least possible.
Then again, I'm the only guy I know with an extinguisher in the kitchen .... never mind over the range or in the hood.
I would prefer smooth ductwork over flex duct.
with flex duct you will accumulate fat deposits easier = danger of ductfire
If you want to use flexduct because you do not want to install too many elbows it would be good to evaluate if there is a better way to vent. Too many elbows will restrict the airflow
Ductless venting might be more work to clean the filters but I would prefer this over a "stuffed" ductwork
Code #?
I am looking for the code # that refers to this. Noone in this thread quotes the exact code #. Do you have the code reference #
I know I'm late to the party but I'm hoping it's ok to piggyback off this post and maybe someone can shed some light: is there a difference between the ductwork between the hood and the ceiling, and that which extends to the roof vent?
My contractor installed flex from the brand new roof vent which he had a roofer put in (presumably/hopefully directly above where the new hood is going to be located) up to the ceiling joists, which still have to be cut and reinforced so the ductwork can enter through the ceiling. I haven't complained yet because I want some legitimacy to my argument, and there doesn't seem to be anything in Ontario's building code that suggests flexible ducting isn't allowed. My parents, who are lay people but have been closely involved with lots of home renovation and construction, suggested that the ductwork from the ceiling down is what matters, not what extends to the roof, and that they've never seen a rigid exhaust duct go all the way to the roofline through the attic.
Looking forward to hearing some of your knowledgeable perspectives :)