O.K., I’ve definitely hit a wall on this one and could use some input. The picture below is of a 4" flue for a gas fired heater. The heater is in a small, properly sealed mechanical closet in the middle of the home. The flue is being run directly above and out but through what is suppose to be the upper combustion air opening.
Yes, the opening should not be screened and yes the flue is improperly supported by the screen mesh. What I can not find is a restriction for routing the flue through the upper combustion air duct. Yes it is wrong but I need a reference for the client to take to the builder. Between the upper and lower CA openings there is more than adequate CA opening, even with the flue in the way.
I can think of possible consequences but can’t find a good reference.
A second question is that the lower CA opening is screened with 1/2" opening mesh. We all know why it should not be screened but the IRC only specifies not screening the end of the duct that terminates in the attic:
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That reference was intended for the space in front of the combustion area. But reading the IRC Commentary for the section you referenced they made reference to the following which I totally missed:
I also am not aware of any regulation that would prohibit this installation, but like yourself think it “looks wrong”.
My thought is that convection currents resulting from the heat of the flue in the vent would iinterfere with the flow within the vent.
One thing to bare in mind though is that the IRC is not the last word on this subject as the Fuel gas section of the IRC is just a condensed version of the full International Fuel Gas Code, which may have more to say on the subject.
