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Thanks. I was not looking for code to quote, but when I started digging into the code with some free time I had, I got a little confused. I knew negative slope and a bunch of 90s was obviously wrong and enough to write up. But when the other inspector mentioned this stuff to me, I wondered to myself, what is the minimum/maximum elbows or vent size for that situation. Just trying to keep as sharp as possible. What if there was one 90 and one 45? Is that too much, etc. That is why I started looking and am still unsure.

I am a little confused about the following code areas: G2428 vs 2427, specifically 2428.2.3 which allows 90 degree elbows in flue pipe vs 2427.6.4 which only allows 60 degrees. This is IRC 2003 and the following is the example that I am trying to understand: I was not present, but another inspector saw something that did not look right to him and wanted a code reference. Two GAS water heaters mounted within a few feet of each other, joining at a common flue. I believe there was a wye, or something at the connection. He said that it appeared that all flue was 3" Bvent. The vent went on to do a couple of 90s and even sloped downward at a point. Here are my questions. Is this under 2428? The main flue would need to be larger than the 3", but do I use the charts or the 2427.6.8.1 where you take not less than one plus 50% of the other to calculate size? Slope of 1/4in per foot is in effect in 2427.10.8 as far as I can tell. I guess my main question is the difference between 2427 and 2428 relating to gas appliances and venting as they appear to be contradictory. What section, or process would you use in venting the setup in my example?