Grey and Black Tank Vent(s)

[Kevin Means]

Okay... we're building an RV out of an Army M1083 truck, and all the designing has been done in a CAD system. I'm working on the plumbing and I've come across an issue that seems to have inconsistent answers, so I'm trying to get some opinions here.

I need to vent the grey and black tanks. The tanks are right next to each other between the truck's frame rails. Do I need to run two separate vents - one for each tank, or can the vents be combined, enabling me to only have to run one vent pipe to the roof? If you plug that question into the internet, you get completely different answers, but the consensus seems to be that, as long as the tanks are close to each other (like ours) the vent pipes can be combined.

Kev

 

[SpencerPJ]

My two tanks on my travel trailer combine to one vent. ps: sounds like a cool project, keep us informed, pics

 

[Rene T]

I would conbine the two. Chances are you won’t be dumping both tanks at the same time

 

[NY_Dutch]

As said, you can combine them. About the only time separate vents are used is when the tanks are separated too much to make combining practical.

Also, make sure you install air admittance valves (also called "inline vents") at each sink to vent those drains without needed additional roof vents.

 

[Kevin Means]

Pretty much what I thought I'd hear. A combined vent makes sense to me too, so that's what I'm going with. If it starts stinking in the RV, at least I can blame you guys. Thanks

 

[Rene T]

As said, you can combine them. About the only time separate vents are used is when the tanks are separated too much to make combining practical.

Also, make sure you install air admittance valves (also called "inline vents") at each sink to vent those drains without needed additional roof vents.

I was just thinking. If an air admittance valve would work for a sink why wouldn’t it work for a gray tank air admittance device instead of a vent going up through the roof. I don’t see why it wouldn’t work as long as the piping would be the same size as the roof vent

 

[NY_Dutch]

I was just thinking. If an air admittance valve would work for a sink why wouldn’t it work for a gray tank air admittance device instead of a vent going up through the roof. I don’t see why it wouldn’t work as long as the piping would be the same size as the roof vent

The air admittance valves work as a vacuum breaker to keep the draining water from siphoning water out of the odor blocking traps. The tank vent serves to relieve air pressure in the tanks as the water enters from the sink drains, as well as allowing air to enter as the tank drains. Basically, the air admittance valve is a one way air check valve, while the roof vent serves to allow air flow in both directions. Believe me, if there was simpler way to vent the plumbing than running a pipe through the roof, home, commercial, and RV builders would have found it years ago. Image a builder being able to just use low cost valves and no roof penetration instead of running vent pipes up through the roof of a multi-story building. An RV is no different, just on a smaller scale.

Oh, and I didn't mention the smells that are released as the air is pushed out of the tanks! We've seen here from time to time how bad the smell is when an air admittance valve fails open.

 

[Rene T]

Thanks Dutch