I had a Rheem direct vent gas furnace today with the vent terminations located directly under a deck. I looked up the manual and on one page it says not to do it, that the termination point must be above the deck. A few pages later it gives a graphic of allowed termination clearances, showing 12" clearance needed under a deck. Lol, it’s hard enough to remember everything, why can’t the manufacturer stick to one criteria?
Ryan the installation instructions state “avoid installation” under the deck but does give you clearances if that is the only place you can locate the water heater. Often the building dictates where equipment can be installed. It’s not a conflict but clear direction.
I did note the “avoid” but they go on to say “as long as the terminations are not under the deck,” which implies they cannot be under the deck, lol. They could fix it by stating “as long as the termination under the deck meet the required clearance shown in the table.”
I do realize these are long and technical documents that probably have other errors, but the statement as is causes much confusion.
The manufacturer’s install instructions that I have read have all referenced the 12" clearance, Ryan.
I agree Ryan the wording in the manual is poor but does allow installation under the deck with specific clearances. Kudos for reading the manual. I do that all the time as it’s my go to for my warranty and new home inspections. This is an excellent example of referencing more than one section of a manual or in some cases a code.
I’ve seen multiple furnace direct vents terminate under an open deck. I point it out as “information only” in my reports, and warn the clients not to sit on that area deck for long periods of time during the winter when the furnace is running. /s. 
Yep. And technically there is probably some effect on the structure from the acidic exhaust.
I went with the 12" in my report instead of the total ban. I’ve seen it a few times before but never right up against the bottom like this one.
Yep, and one I inspected had moistures stains after 10 years underneath the decking that was no more than discoloration. Noted that too. Depends of where you are, the climate and vent use. Varies considerably.
It’s a pretty minor concern for sure. I imagine the biggest effect is on the fasteners rather than the wood. I couldn’t access the underside of this deck today due to lattice wrapping around the entire deck. If it collapses, they will fall about 20", lol.
Depending on how tight that space is under there. Just make sure the exhaust can not re circulate into the fresh air. That would shut the furnace down.
Where is the ledger board or joists? looks like there is nothing under the deck boards.
Ryan what the manufacturer means is that the vent piping must be extended to the exterior band of the deck and not vented directly under it. So the vents must go the full length of the deck joists and vent at the far end of the deck, not at the ledger.
It’s also so occupants aren’t sitting on the deck breathing in combustion gases.
It was a free standing deck only 20" or so off the ground. It was wrapped with lattice and had snow drifts around it. I was only able to get a small view of the underside through an area that had a hole in the lattice. There were a few problems I could see from that vantage point and called them out but otherwise disclaimed the support structure of the deck.
I agree that is one way to handle it. But the diagram I posted from the manual does not seem to imply that, in my opinion.
I always call it out as a defect whenever I see it.
Not the best instructions. The diagram doesn’t have a deck on it, just that little balcony, so it’s hard to decider from the drawing. The key word in the language is “terminations”. The piping can be under the deck but the terminations must be past the deck so the exhaust is extended away from people sitting or the deck or from potential snow blockage. The exhaust piping should be insulated as well.
