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Re-routing the furnace vent pipe

0 votes

I'm remodeling a bathroom and among other things I need to re-route a furnace vent pipe.

The furnace is on the first floor and the bathroom is on the second. On the second floor the vent pipe comes vertically through the floor and then has a 45 degree elbow close to the ceiling, then straight up into the attic (obviously, through another elbow). The pipe used to be enclosed in a soffit, which I removed.

What I want to accomplish? Ideally, I'd like to re-route the vent pipe as close to the ceiling (or conceal it completely) since due to the 45 degree elbow now it's in the way.

The vent pipe is a double-walled one as required for gas burning appliances.

I did some research on this site and I found that I have to comply w/ two main requirements:

1) keep at least 1" clearance between the vent pipe and combustibles (e.g. studs in the wall/ceiling)

2) If there is a horizontal part of the pipe, its length should not exceed the vertical one

My question is on requirement #2: sounds like I can replace the 45 degrees elbow with a 90 degrees one (and then another one for the vertical into the attic) IF the length of that horizontal section is less than the vertical through the attic (in my case, about 3ft horizontal followed by 4-5ft vertical). I do not account for the vertical part of the vent pipe between the first/second floors. Is this correct? Any problems with having the horizontal section concealed in the ceiling (provided that I keep the 1" clearance)?

Thank you very much in advance for any answers/advice you might have.


asked in Heating and AC by ivo (130 points)
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1 Answer

+1 vote


Hi Ivo, 
There are actually several more requirements that need to be followed and since I don’t know all the details of your installation I can’t give you precise answer. 
Some of those important things are the diameter of the vent pipe, number of appliances venting / using it (furnace only or maybe furnace – water heater?), total number of elbows – vent pipe capacity depends on that (bigger / more elbows limits capacity), natural draft or fan induced draft, BTU’s, is there a sufficient space inside the ceiling for a minimum of ¼” per linear foot of slope on the vent pipe… and a few more.
What I would suggest to follow this link - http://0323c7c.netsolhost.com/docs/gama%20venting.pdf - to Gama tables, read pages 2, 3, 4, 5, and use one of the examples on page #19 and Table#2 (single appliance) to get all the numbers for your installation. 
Please let me know if you have more questions after going over the pdf file.
answered by darekrudy (21,730 points)

Thank you so much for the prompt answer, much appreciated!

I read through the pdf doc and I think that I can now answer my original question.

My case is represented in pic. 3 (so, table #1) - single appliance, double-walled vent pipe. (The vent pipe out of the furnace is single-walled, but then connected to a double-walled within 1 ft, so I guess, that qualifies). The vent pipe is 4" in diameter. From table 1, knowing L/H values (which I'll measure), I can determine the FAN min/max and NAT max permissible values. I will inspect the rest of the venting path to figure out the total number of 90 degree elbows and then reduce by 10% the FAN/NAT numbers for each additional (after the first 2 accounted already for in table 1) elbow. At that point depending on the BTU capacity of my furnace, I'll know whether my plan is feasible or not.

Dumb question: I'll find the BTU capacity on the unit itself, but what about the FAN/NAT type? I can probably look the model up, but is there an easy way to tell from the device itself?

Terrific site, supper helpful!



Thanks Ivo, just trying to be helpful smiley...

Fan column in those tables is for the appliances equipped with an induced draft fan. 

NAT column is for the natural draft appliances, they have no fan to push exhaust gasses from the combustion process. 

If you have an induced draft fan equipped furnace you can usually hear it before the burners ignite. It should start running ahead of the ignition and turn off some time after the burners go off.

The fan assembly is mounted above / below the burners... depending on your configuration.

This all makes sense now. I'll report back once I'm done with the job.

Thanks again,