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Please help with this code question...


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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?

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I guess my question to you would be why do you want to quote back to the code? If your read 2427 & 2428 it is confusing as heck, but the underlying message in them is that it goes back to what the manufacture of the equipment says.

I don't think that you need the code if you have a negative slope and a bunch of 90 degree elbows. You would be safe in reporting that it is not to manufactures standards and that for increased safety it must be corrected by a licensed and qualified plumber.

I think that you could use either 2427 or 2428, both would cover the problem. 2427 covers it the best, IMO.

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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.

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We go through this every few weeks here. The first thing that you have to understand that the rules given in the codes don't necessarily mean that the installation is going to perform correctly. The next thing is that ten people reading the same code can conceivably come up with ten different interpretations. Although that certainly isn't what's intended, it seems to often be the case.

The codes presume two bends of 90 degrees, but state that bends of more than 45 deg. are not allowed, with the exception of one at 60 deg.. Go figure.

There's a really good article about proper venting of multiple gas appliances on The Journal of Light Construction Online at the link below. Check it out. It's free. Just click here.



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2428 is listed in 2427 as an exception so it would conditionally apply. I would not be surprised to see the mistake caught in the next edition. The IRC fails to distinguish between vent conectors and vents in this case. 1/4" rise per foot always applies to gravity venting systems and the +50% of the smaller rule would need to be met in the case you describe m. The 2-3" water heaters need to go to a 4" common vent.

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