Plumbing Question

[mgramann]

We had our rough plumbing inspection yesterday, round 2 actually, and things looked good except for one thing.  Apparently, the inspector says if we flush our toilet upstairs, the downstairs will flush as well.  He says that we should tie in the shower drain between the downstairs toilet, and the wye from the upstairs toilet.  Now, while this is my first time plumbing, I just don't see it.  If the upstairs toilet is flushed, water will flow down the pipe, pressure being offset by the vent(first positive, then negative)  While it does create a wet vent situation, even through the wye, the vent is the path of least resistance, and when the vent is wet, gravity will offset the negative pressure.  The only possible issue I could see would be the positive pressure on it's way down, but the septic should offset that.  Am I missing something?  Is he missing something?

A picture is worth 1000 words.  Below is a diagram of the situation.  All grey water is separate until just before it exits the house, for the purpose of adding a grey water system later, so it really is this simple when it comes to the toilets.



Thanks in advance!

[MushCreek]

I'm no expert on this; my forehead is battered and bloody from banging my head against the wall trying to figure out how to do DWV. What I think I see is no vent between the downstairs toilet and the stack. My understanding is that you need a vent there because the rush of water down the stack will create a vacuum as it goes by the branch. The negative pressure can suck the water out of the toilet trap. This water in the trap is what keeps sewer gas from entering the house, so if the trap is sucked dry, you have a potentially stinky (and dangerous) situation. I don't see what good tying in a tub will do; the water would get sucked out of the tub trap instead of the toilet trap.

It took me a long time to figure out that the vents aren't just to help a fixture drain, but also to protect it from being sucked dry by waste traveling through the system. None of my books pointed out this rather simple theory. Of all the disciplines I've learned while building my new house, plumbing is by far the hardest, mostly because good, clear advice is hard to come by. Electrical, on the other hand, is much easier to find good info on.

[mgramann]

I understand what you are saying about the trap, but by the time the water going by would create negative pressure, the wye would be open to outside air.  I think his thought is that the tub vent would keep an open line of air preventing the exact scenario you describe.  The thing is, I don't know that the scenario is valid based on the plumbing diagrams I have seen.  For example, it is common for 2 toilets to share a soil stack with both of them draining in via sanitary tee.  Now, my scenario is slightly different with the wye at the bottom, but a wye is the proper way to transition from vertical to horizontal.  I'm just not sure if that wye creates the need for additional venting.  He seems to think so.  I'm going to likely make the change, but was more curious as to the theory behind it

I'm with you on electrical.  Passed first time.  This is the biggest plumbing job I have ever done, and it was anything but simple.

[Jimbo Ricketts]

it only going to make a differance if shower is vented too , otherwise the shower will ,might, girgle too . the codes dept in metro nashville and especally franklin city change something every 2-3 yrs . that shower vent should b 2in 1,1/2 vent is a no at least around here.     how far awaw is the toilet from the 3 in vent? if within 12 ft its just codes bs it would werk         

[JRR]

I think Jimbo has it... the inspector was expecting the shower to be vented.

[mgramann]

That's the thing, the tub IS vented.  1.5 out, and a 1.5 vent for about 3ft until it expands to 2", and the second toilet is well within the max distance to the waste stack. 

I don't honestly think it is needed, but it will add one more path of ventilation by tying the two together, so I won't fight it.

Thanks!