Grey and black tanks using the same vent pipe

[Bowmans]

We are going to be putting black and grey tanks in a 1950 liner. I am wondering if both tanks could use the same vent through the ceiling. Both tanks will be close together so tying them wouldn't be an issue. This would eliminate a second hole in the roof. Only issue I can think of is if both tanks are draining at the same time, it might not allow enough air into both tanks for air displacement. We usually drain black, close black, then drain grey so may not be an issue there but wanted to hear if anyone has done this or if a plumbing expert on here might know of another reason this might not be a good idea.

Thanks,
Tadd

[Nuvite-F]

That's the way it's usually done.

[nmbosa]

That's what we did. Works fine.

[Bowmans]

Thanks for the quick replies. When we rebuilt the inside of our 1979, there were 2 vents from floor to ceiling. I assumed one was grey, other black. Now that you confirmed one vent for both was the way it is done, i started thinking and the second vent must be fresh water tank. We have never used it and I forgot it was even there... The confirmation will make vent placement easier in the 22' liner.

[ROBERTSUNRUS]

Hi, my trailer has a black tank vent through the roof and two gray tank vents under the counters for the bathroom sink and the kitchen sink.

[TG Twinkie]

One vent will be fine. You should always dump the black tank first. When required. Then use the grey water to rinse the system.


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[nmbosa]

I've seen several trailers that have separate vents upstream of the gray tanks for the waste plumbing. For example, many of the '60s California built trailers have a vent pipe at the rear sink that looks like a pole going from the sink base up to the roof. They also will typically have another vent pipe going from the kitchen sink area up through the roof. These vents allow the system to draw air in (as opposed to the gray and black tank vents that are pushing air out) and maintain neutral air pressure within the drains so the drain plumbing work correctly and doesnt pull the water out of the traps. My trailer was built in Ohio and did not have vents as originally built and I simply rebuilt it like original. But I now wish I had put some vents in as the drains could use the help.
Norm