# Terminating a furnace vent under a deck



## NH09 (Sep 24, 2010)

I have a contractor who is replacing a rusted furnace vent on a 15 yr old + furnace. The vent was run through a ledger board of a deck, in the joist bay, and out the rim joist. He is replacing the vent and wants to know if it's OK to terminate below the deck (approx 1' lower than the old vent) and the deck is 4' off the ground open on 3 sides. There are no other options for sidewall venting since this is surrounded by condos on both sides and the deck goes across the whole back of the unit. There is nothing in NFPA 54 to cover this -has anyone else come across this situation?


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## klarenbeek (Sep 24, 2010)

I'm assuming it's not a category I appliance since it's sidewall vented?  If it's not, the only thing (other than common sense) preventing terminating under a deck or overhang would be the something in the furnace manufacturer's instructions, which would be required to do the installation and inspection anyway.


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## Dr. J (Sep 24, 2010)

How "open" is the deck?  IOW, do the products of combustion waft up between the boards, or is it tight enough that they mostly have to run under the deck and escape out the 3 open sides?

I don't think I am concerned about the proper operation of the furnace - the sidewall under the deck is probably the same atmospheric zone as above, so it should vent properly.  However, the concern would be products of combustion getting re-entrained into the building, and/or fumigating anyone on the deck.  I would expect that this would emit a foggy cloud on very cold days.    Since this is outside it is probably only a nuisance, and if it gets smelly, the occupants would likely move before anthing bad happened.  No worse, probably better than a direct vent fireplace that pokes out of the wall right in a deck/patio area. Good idea?  No.  Fatal, probably not.

Since this sounds like a replacement of an existing system, and assuming no one had issues before, I would be inclined to accept this situation, although I am a design professional, not an AHJ.

One other concern, this is going lower that the original, and the deck is only 4' above grade - any snow drift concerns?


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## NH09 (Sep 24, 2010)

Since the furnace is so old, the HVAC contractor cannot track down the installation instructions. I don't like installing the vent under a deck, but I can't find anything preventing it.


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## NH09 (Sep 24, 2010)

No snow drift concerns, and lowering the vent was to move it out of the joist bay. I'm inclined to agree w/Dr. J, it's probably going to be more of a nuisance than anything else.


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## mtlogcabin (Sep 24, 2010)

How about extending it out to the edge of the deck?


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## Paul Sweet (Sep 24, 2010)

I'd be concerned about condensation on the underside of the deck if it terminates below the deck.


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## NH09 (Sep 24, 2010)

We did talk about condensation and that seems to be one of the reasons the existing vent rotted out, the original vent went out through the rim joist via the joist bay.


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## north star (Sep 24, 2010)

** * **

NH09,



I'm with mtlogcabin on this one!    Could the exhaust pipe be extended

out to the end of the deck, ...maybe even install a blower fan to assist

in moving the exhausts out to that termination point?

Also, if the model no.  &  manufacturer of that particular furnace

could be provided, maybe some one on this forum could locate the

manufacturer' installation instructions / limitations.   Just a thought!



** * **


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## That Inspector Guy (Sep 24, 2010)

I had a situation recently where a H/O was finishing his basement with a powder room, the ejector pump required that it be vented.....He would have had to vented it under the deck, adjacent to the slider. Plumbing code specifically says no within (IIRC) 4 feet of a window or door......Doesn't the Mech Code say the same thing for combustion exhaust???


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## peach (Sep 25, 2010)

Bad practice.. yes.. prohibited.. I don't think so.  Get the products of combustion as far away from openings as they can get.


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## RJJ (Sep 26, 2010)

Agree wwith MT: Get it out from under the deck.


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