Shared venting configuration

It is common knowledge among inspectors that the smaller BTU gas appliance needs to be connected above, or downstream, of the larger BTU gas appliance when sharing an exhaust vent.

Generally we say that not doing so can cause backdraft issues and result in poor venting of the smaller BTU appliance.

But what is the technical reasoning behind this? What specifically happens when the smaller is installed lower, or upstream, of the larger? And, does the issue happen only when both appliances are operating at the same time?

I haven’t been able to find any real explanation or description of what actually happens “exhaust flow wise” with the improper configuration.

Here is a picture for reference from an inspection some time back…

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Are they actually shared? Not sure I can tell from the pic.

Yes, they join together. That is an old picture, not really wanting to scrutinize that particular install I guess.

Just curious as to the specific dynamics at play when a smaller BTU gas appliance is connected below or upstream of a larger one. There is a reason it is not allowed by code and I have came across it several times. But I’ve never really heard a great explanation as to why it causes problems.

Although I could be wrong, the only good reason I think of is to prevent backdrafting of the higher BTU appliance through the WH draft hood, along with causing the WH to backdraft when both appliances are lit.

Which IMO would be a good reason to have the proper configuration.

I’ve done some searching as well and couldn’t find any concrete technical information as to the reason why.

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I found one old post somewhere that explained if the higher BTU appliance is running, and then the lower BTU appliance starts, the lower BTU appliance will have trouble exhausting. They somewhat explained that this is due to the larger appliance downstream having a warmer exhaust temp.

I guess I can see this somewhat, in that the lower BTU appliance exhaust may be cooler and could have a hard time “intermingling” with the hotter exhaust above, or downstream?

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For me, I think it is to get as much rise as possible out of the smaller appliance. Code also allows for them to be at the same level ( less/equal 45 degree angle). Manifolds are also allowed. For me, this blows out the whole back-draft or intermingling theories.

2018 IRC


Do you really want to know? It is about fluid dynamics. I don’t want arguments about common sense perspectives…

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Do you have a cliff notes version perhaps? Or a summary, lol? Explain it like you would explain it to a client that is asking why it is in the report.