Vents- what is really needed?

[luvoldstuff]

Okay I'm about to finalize all of my new drain plumbing in my '68 AS Globe trotter project.
I have done away with two of the three roof vents due to complete re-plumbing. I have the newer one way vents at each drain location(sinks, shower), I am definitely going to vent the new separate black tank through the roof, my question is:

Do I need to tie the grey water tank into that roof vent? It would be easy enough, but I'm a little afraid of smells etc(even though I have traps)
I think the one way vents will be fine, but my only reason for asking is my thoughts about when I drain the grey tank- it might "chug-a-lug" annoyingly without a bigger roof vent?

[luvoldstuff]

My new walls are only half inch thick, so I am only using the original oval vent pipe that is in the wall by the kitchen sink.

[65CV]

You absolutely need to vent the grey tank or it will empty VERY slowly as it "chug-a-lugs".

IMHO, if you have adequate traps, you can probably merge the vents, although your vent diameter does sound small.

[luvoldstuff]

The grey tank, at this point,(if I don't tie into roof vent) is vented by the 3 undercounter "one way" vents. I would think that would be plenty(for dumping)?.....but
I will probably go ahead and tie the grey vent to the stack since I'm just not sure, if I get smells I could re-route(cap tie in) it later and see how it drains??
I shouldn't get any smells with traps though- home sinks are tied into the sewer-although no holding tank?

[awryawry]

I believe that the air-admittance valves will adequately vent the GW tank for gravity drainage. As noted on another thread, I believe that the OEM valves are inadequate, and should be replaced with modern, code-approved AAV's. Talk to a plumber or a plumbing-parts supplier.

wry

[Wazbro]

I'm not a plumber but wouldn't only having "one way" vents cause a problem?

I mean air has to leave the tank to let water in and air has to enter the tank to let water out.

[luvoldstuff]

That really is an interesting thought!!!?........I never thought of it that way. You may be right! I ended up venting both tanks through that one roof vent (today) anyway, just hope no sewer smells somehow make there way through, but maybe I'm over-thinking it anyway, I'll post some pics, 'cause threads without pics kinda suck!

[InsideOut]

Originally Posted by Wazbro

I'm not a plumber but wouldn't only having "one way" vents cause a problem?

I mean air has to leave the tank to let water in and air has to enter the tank to let water out.

I'm not a plumber either - but that's how I thought it worked too...

Shari

[awryawry]

What happens is that the drainage water introduced into a waste tank very gradually compresses the air within the vessel, and that pressure builds and builds over time until it explodes with a biblical fury -- and that's why there are so few AirStreams around today ....

Beware the toilet cough!

wryawry

[65CV]

Try picturing it this way:

- you pour a gallon of water into one of your sinks
- in order to get that gallon of water to drain, 1 gallon of air has to leave the gray tank
- the other two sinks have traps that prevent stinky air from getting into your trailer. They won't let that gallon of air out of the tank.
- the sink with the gallon of water in it has a air admittance valve -- it will allow air to go into the tank, not out.

If you don't let that gallon of air out of the tank to the outside world, how would you expect to get that gallon of water into the tank? (If it does drain, that stinky air is getting by a defective trap into your trailer.)

You need a vent.

Hope that helps.

[luvoldstuff]

Thanks, that makes sense. Good way of explaining it. I went ahead and vented the grey tank into the roof vent and everything seems to be draining fine.