Is it normal to hear water flowing thru the plumbing system from the vent stack on your roof?

For example, from the roof vent, When a toilet is flushed I can clearly hear the flush and the pipe quickly filling and draining.

I was trying to clear debris from my roof vent and I was thinking if it was totally restricted, you would not hear water flowing thru the system. Is that a fair assessment?

yes and yes

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"Measure Twice & Cut Once" - Retired U.A. Local 1 & 638

You are correct, you would be able to hear the water flowing when any fixture is drained, but particularly a toilet because of the volumn. One must remember that the vent and drainage charts in the code books were developed long before low flush toilets were. These newer toilet tanks drain faster than the older less than 3" flappers allowed, so the toilet vents, many of which are 2" (when there are more than 2 toilets in a home and the toilets are a certain distance from each other) are burdened with taking in more air into the vent. When a liquid moves through a pipe there is friction loss. I believe that air moving through a vent also causes friction loss. It could be that the vent charts need to be revisited by the engineers that write the codes to allow for this and help the lower volumn flush toilets to do their job. American Standard makes a 4" flapper that even drains fast enought to flush a dozen golf balls down the drain. The engineers that write the code should be and probably are looking at this. If someone disagrees with my contribution, please let me know why you disagree.

Srloren-

Obviously I'm no plumber but your post has me thinking about a couple things-

#1 does a toilet even need a vent? I've heard different things on this, including claims that unvented toilet branches flush better...

#2 a low flow toilet drains faster, but isn't this a function of having less water, rather than the speed of flow thru the pipe? For example, a given 3" pipe can drain at a particular rate. Are you saying the water drain rate is variable?

#3 what is the effect of hydraulic effects on this theory? Assuming that liquid flowing through a given pipe cannot be compressed, regardless of the pressure it is under... Air flowing thru the same pipe can be compressed as the pressure on it changes. Therefore can't that pipe allow "more" air thru it than water thru it? Like a jet effect.