I've searched google and can't find an answer to this on the various forums out there.

Is it up to code to tee into a kitchen exhaust vent in an attic if you want to add a second range hood somewhere under a roof in the home? it's a 'standard' 12"-ish wide rigid exhaust line that is going through the drywall roof at the current range hood, into the attic, and then into the wood/tar paper/tiles above to outside.

Tee-ing into the existing exhaust hood vent line before it exits my roof/tiles would be much easier than cutting and sealing a new hole in my tiled roof/moving tiles/etc.

Any idea?

Tough question.  I could not find much online either.  And don't really want to dig out by old building code books.  I did find that some inspectors only want one exhaust fan per roof jack. So I dunno.

One thing to check first is if the exhaust conduit is even large enough to handle both range hoods running at max at the same time.   I know some require a 10" pipe just for one.

You may want to search some home inspector forums.  They ask each other questions like this frequently.

Unless you run them both at the same time, part of the exhaust from one vent will come back in through the other.

Yeah the 'CFM' and thoroughput ratings of the metal piping size is what first came to mind...but again I couldnt find any info.

I thought of this as well and assumed there was some sort of check valve or wye-type fitting available. Maybe that was wishful thinking.

http://www.ductanddampers.com/catalog/Duct-Dampers.html

One suggestion would be to find an in-duct fan that could be installed after the T, drawing from both hoods at once.  That would be nice because it would be quiet.

ETA: One like this puts the motor outside of the duct.  That would keep it from getting all greasy and catching on fire.

http://www.ductanddampers.com/products/Duct-Installed-Equipment/Duct-Booster-Fans/Duct-Air-Booster-Fans/DUCT-BOOSTER-FAN-Round-or-Flat-Air-Duct-SKU2117.html

Unless the motor is after the intersection (and larger) you are going to need automatic dampers.

I still prefer exterior blowers to get the fan noise as far away as possible.

You may still hear some air noise in the hood but it is markedly less than the fan noise.



Watch out for back drafting issues if you depressurize the house from inadequate make-up air.

Gas water heaters and other gas fired appliances can cause trouble.

Not just no, but fuck no. Can you say grease Fire in Duct?

Can you enlighten me as to how teeing into a duct could cause a grease fire in the duct?

Grease from the kitchen will end up in unwanted places.

Yeah... I'm still not seeing the point here... unless I'm missing something it's really not super complicated.

Fire needs 3 things to exist, fuel, oxygen, and a source of heat/ignition. How does adding a tee into the duct change either of those within the confines of the ductwork.

The fuel and oxygen already existed in the duct before the tee; does the tee somehow add the heat/ignition source to the space inside the duct?

Without adequate dampers the greasy smoke from the kitchen can end up in every portion of the system.

Kitchen grease in a bathroom fan vent?

Now all you need is a source of ignition.

Residential systems often use a fans at the inlets to the system to push air into the ducts.
This puts the system under a positive pressure.

Any leaks in the duct work drive greasy kitchen smoke OUT of the system.

Commercial systems routinely use rooftop blowers that pull air.

Leaks go INTO the ducts that are under a slight vacuum.

And while kitchen exhausts are required to be metal (even in residential) no such restriction exists for bathroom  duct in residential.

RIF.

lol, this. I'm not teeing into a bathroom PVC or corrugated 4" exhaust line...I would be teeing into the already present kitchen range with a second kitchen range

I do not know your state code on this but one hood on one penetration is how it is here. Commercial we do see multiple on one main roof mounted exhaust but the fan is sized for each hood. Ducting is sized for a specific cfm, unless your ducting was oversized from the beginning this would not work of would work poorly now with just the one hood.

Code says the hood shall have a duct, and the duct shall be independent of all other exhaust systems.

It's a little vague but if you went by the letter it says the hood (singular, not plural) shall have a duct which is independent of all other exhaust systems (which would include the exhaust system from another hood).

You may have a different code... or you may have an inspector who sees it differently.

Code also says you have to follow manufacturer's instructions.. You can call them and see what they say.
I think they'll say no, since whatever testing they did to get approvals from whatever listing agencies were almost certainly done with a single hood attached to a single duct and single exhaust vent.

Less fire risk but you are still going to need dampers to prevent one hood from blowing out in the other.

Yes, I understand ALL of that, but it doesn't answer the question.

Let me remind you to the post I was replying:
Lets establish a few things up front, the OP either has a positive pressure or negative pressure duct, we can't change that. The "system" that we are talking about is within the duct (established by the above quote); in the context of my reply to him we are not concerned with anything outside of the duct. Grease will be present in the duct no matter what, positive pressure or negative pressure. Oxygen is also there no matter what, ignition source is either present or not. If he has all 3 he will have a grease fire no matter what type of duct he has.

So the OP wants to tee into the duct. This will not add nor remove the grease, nor will it add or remove the oxygen, nor should it add or remove an ignition source. Adding a tee doesn't change ANY of those concerning the confines of the duct. I'm curious what the post was referring to in regards to his response. How can adding a tee CAUSE a fire WITHIN the duct?