Note 6 of table 703.2 of the California Plumbing Code states that the horizontal length of a vent is "not to exceed one third of the total permitted length". Similar terminology seems to exist in most plumbing codes. My question is WHY?? I've googled for the technical answer but have come up empty so I'm turning to the experts here. I assume there must be a "good reason" since most codes have engineering and/or experience behind them. Is it something in fluid dynamics related to gravity? The formula's I've seen in this realm do not contain a gravity variable. Very curious. I'm asking because I need to run a 1.5" branch vent from the combined 1.5" vents of 2 kitchen sinks in an attic to an existing branch vent through the roof as the owner doesn't want another hole in the roof. All of the vent run will be able to drain by gravity, as I can "peak" the vent line on its horizontal path. Vertical would be about 8', and the horizontal run would be about 18'.

Thanks all

I have never worried about it.

me too...

Google IDPH (IL.Dept. of Public Health)
Sect.890 Appex. A Table K under there plumbing code.
Pay attn.to agency note at bottom of page!

over long horizontal runs the "effective" pipe diameter can decrease causing the vent to be choked and thus ineffective. this is just my guess...great question. This is what makes plumbing interesting to me. The reasons behind the code.

Air is a "gas" and unless it is under a lot of pressure the "effective diameter" of the pipe is inconsequential, especially for the distances possible in building.

next they are going to tell us the vent termination is too close to the moon...

It depends on which phase of the moon it is.