Venting gray water and black water tanks

[Mex-Busnut]

Hopefully, I won't get laughed at for asking so many questions, demonstrating my extreme ignorance in these matters.

1. What diameter of PVC pipe do you use to vent your gray water and black water tanks?

2. I have already acquired the small round vents that go on the roof for this purpose. Can I use a single pipe for both tanks with a T at the bottom, or do they need to be separate?

3. Does the fresh water tank also need to be vented?

4. Do these vent pipes simply need to be attached to the top of the tanks (with proper hole and sealing, of course!), or is there some other trick there?

Thanks in advance for educating me!

[thomasinnv]

I used 1 1/2" for the vent pipes, and I vented both grey and black to a single pipe through the roof. Works for me.

[Iceni John]

1.   Two 2" vents per tank.   The poo tank's rear vent goes up through a stainless steel roof pipe, and its front vent connects to the grey tank's front vent;  the grey tank's rear vent goes down toward the road.   This way I have a flow of air through both tanks (good for the aerobic bacteria in the poo tank), and any methane from the poo tank can rise harmlessly out through the roof vent.   Two 2" vents also allow sufficient air in when dumping through each tank's 3" outlets.   (Typically one would use ABS pipe for DWV, not PVC, not that it really matters.)

2.   see 1 above.

3.   Yes.   A 1/2" vent is sufficient, as it only has to equalize pressure inside the tank when filling or drawing water from it.

4.   If you're having fittings spin-welded in, have proper FPT fittings also put in for the vents, then you can use MPT elbows for the vent pipes.

There's been lots of talk about tank venting here and elsewhere, including some heated discussion about poo tanks' vent locations!

John

[chev49]

i have 3/4 inch vent lines for all tanks and have had no problems. and have each of them separate.  for freshwate i have 3/8 for each tank. have 2 vents on the black tank.

[Len Silva]

Chev, I'm surprised that you don't have some problems with only 3/4" vents.  The problem comes with emptying sinks traps when dumping without sufficient air coming in.

[chev49]

i had 1 1/2 in my first three busses, and 1 inch in the next. 
i have 3 grey water tanks in the current one, and each tank is vented.
they dont dump as fast as if i had a 3" vent line, but they work fine.

[thomasinnv]

when I posted that I used 1 1/2" i meant 1.5", not a single 1/2". Guess that could be confusing.

[jjrbus]

Hopefully, I won't get laughed at for asking so many questions, demonstrating my extreme ignorance in these matters.


You would get laughed at for doing things without asking and then asking how to fix it 

Now you are viewed as a wise person.    JIm

[PCC]

Not wanting to cut a hole in my roof, I ran my 2" vent line up to the window level and then back down to under the coach.

Like an island sink drain vent, it must rise a minimum number of inches above the highest point, to prevent liquids from getting into the vent, but that has saved me having to worry about any rain or wash water getting into the coach from around the vent pipe opening.

[Mex-Busnut]

Thanks to everybody for your replies!

Now I only have to figure out a way to keep the perfume companies from stealing the fragrances from my black water tank!

[Len Silva]

It just happened to work out for me that I could run the vent up the refrigerator vent.  I also recall that swimming pool vacuum hose would glue right into PVC fittings and made a cheap, flexible ventline.

[dougyes]

I've used garden hose: male thread x barbed brass fittings clamped to a garden hose. Drilled a hole in the roof and siliconed the fitting to the roof. No problems.