2" or 3" concentric

[njbillt]

I'm looking at the installation manual of a Rheem condensing tank water heater, and I see a contradiction. The intake and vent are 2", but they show a 2" to 3" reducer within a few inches of the unit, and they say to use only a 3" concentric. But in a table they say less than 30 feet can use 2" piping. Can anyone tell me why they would have 2" intake and outlet on the heater and then go up to 3" pipe and concentric? Is there an advantage? Can I keep everything 2" concentric and all? Thanks for any input, but if you're going to tell me to use a tankless, please don't respond..

 

[Rich B]

I have 2 Rheem condensing gas furnaces. Installed one in 2006 and used 2" PVC for both the exhaust and inlet air pipes....both go into a concentric vent. It is a short run from the furnace to the exit....about 20 feet total. The furnace works fine and has since day one....

I installed a second furnace a couple years ago. It is a 2 family home and has 2 heating units. When I read the install manual for the second unit it called for 3" piping and just like you say the furnace has 2" pipe connections. I did run 3" for the exhaust and the inlet and again they terminate in a comcentric vent....and the pipe run is the same distance.

Both furnaces work perfect....No noticeable difference in the pressure of the exhaust exiting from the 2" versus the 3"

I must have misread the pipe size requirement on the first one or they changed it....I don't know for sure...

I thought at some point I would redo the first one and up it to 3" but never got around to it....and there really has been no issues with either one....

The 3" pipe is a lot bigger and tougher to make the run, I have the 3" concentric vent but never installed it yet.....

 

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[njbillt]

Rich. Thank you for the reply. I have 2 Goodman condensing furnaces both vented with 2" and they too work fine. Rheem has since updated their manual on my water heater and say it can use 2" or 3", but they still say in the chart to use ONLY a 3" concentric, which makes no sense to me at all. I really think it's an oversight. Their tech support told me I'd be fine with 2" all the way. I haven't installed it yet but will probably use 2" a concentric. I know it's easier to work with AND I already have one. The 3" I'd have to buy. Thanks again for your reply.