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Can I vent a range hood into the water heater stack? (1 Viewer)

Scott H

Supporting Actor
Mar 9, 2000
I have been doing a fair amount of work on a friends condominium. One rather annoying aspect of the place is that the range hood does not vent externally.

I did replace the range hood with a modern grease and charcoal filter equipped model, but as cooking folks the heat and odors and moisture is proving too much.

It would not be possible to vent the hood conventionally because opposite the wall behind it is another unit and the area directly above is living space. However, it would be possible to meander ductwork horizontally through some cabinetry and a wall and join it with the hot water heater exhaust stack, approximately 11' away. The exhaust stack is approximately 3" in diameter and clear and straight with some type of vent cap. The joint would of course be such that both ducts enter in an upward orientation (direction of flow). The duct from the water heater would be totally vertical in travel. The fan driven duct from the range hood would have to make some up and down moves and the range hood has a damper flap and is below the level where the two ducts would join.

I have been told by folks at home supply/hardware stores that this is an okay plan. I have even been told that it is not against residential building code (LA City). However, I am most interested in whether or not it is a sensible idea and would appreciate any comments/suggestions by any knowlegeable folks here.

Edited: Corrected water heater exhaust stack diameter to 3"

Lee L

Supporting Actor
Oct 26, 2000
I've never seen a water heater exhaust duct that is 3' in any dimension but I would say emphatically, DO NOT CONNECT ANYTHING TO IT. At least not without contacting a proffesional HVAC guy. You run the risk of the exhaust fan blowing too much pressure into the water heater exhaust and then causing the exhaust to backdraft into the residence.

This = DEATH by CO.

Most of the time the exhaust for any heating device is carefully sized to allow a draft or uses a small powered fan to move the gases out. If for some reason the duct had a powered ventilator on top you might have more luck but you still need to analyze the total CFM's going out and make sure what you are putting in will not exceed that.


Senior HTF Member
Feb 1, 2001
It is not a sensible idea. I assure you there is not one code out there which allows a hood exhaust (of any kind) to be connected to a water heater flue (of any kind).

Dennis Nicholls

Senior HTF Member
Oct 5, 1998
Boise, ID
Real Name
Think of it this way. The water heater flue has very hot gasses in it. The range exhaust will be full of grease. Grease + heat = A FIRE.


Senior HTF Member
Jan 30, 1999
Real Name
This is a very bad idea. The water heater flu is intended to vent not only hot gases, but natural gas that may occasionally leak from and accumulate around the water heater. Range hood vents are pressurised (because of the fan) and require their own flu to prevent back draft. If you connect the range vent to the water heater flu, you will pressurize the flu every time you run the fan, potentially blowing explosive natural gas, along with the range exhaust gases, back into your house around the water heater instead of allowing it to vent to the outside.

Don't do it.

Grant B

Senior HTF Member
Mar 29, 2000
Home depot has Fine Homebuilders Code Check.
It's never been wrong and SF has some of the hardest codes in the nation (so Ive been told)

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