What is this under my bathroom sink?

[Chris_-FRF]

2016 Cedar Creek Silverback 5th wheel 33IK

I diagramed my water system and there were two blue people pipes routed under my bathroom sink that are connected together there by a small black device. I have no idea what it is or why it is there. Anyone know?

The picture of it uploaded sideways for some reason.

 

[5picker]

Welcome to the forum, I see you have but a few posts.

This item is an often discussed subject on R/V forums. A search of something like "what's this under my bathroom sink?" would have brought up lots of reading.

It is a check/anti-siphon valve for the black tank flush plumbing/piping.

By code, it has to be mounted higher than the toilet so it often gets mounted under the bathroom sink or inside a shower wall.

Its job is to keep black tank fluid from being sucked back into the supply line in the case of a catastrophic loss of supply water pressure.

I mentioned they are often discussed... that's not only because folks don't know what they are... but also because many of them leak, causing flooding in the bathroom when flushing the tank or because they often stick (internally) and don't allow proper flushing of the tank. They have also been reported to be connected backwards as well.

Good on you for mapping out your water system!

Here's a generic plumbing schematic that's been circulating on this forum for several years that shows it.

 

[NJKris]

Is that the same as an 'air admittance valve' I see some people have issues with?

 

[Capacitor]

No, an air admittance valve (AAV) is a vent the same way the plumbing that goes through the roof.

 

[5picker]

Is that the same as an 'air admittance valve' I see some people have issues with?

No, an air admittance valve (AAV) is a vent the same way the plumbing that goes through the roof.

Kris, as Capacitor points out, the AAV is in the larger, black colored ABS DRAIN plumbing to help vent the grey tank/drain pipe.

The black tank check/anti-siphon valve is in the SUPPLY (water) PEX plumbing.

Both are typically located under a sink although there "may" be one AAV at every sink/shower location.
There will only be one check/anti-siphon valve. (unless you have two black tanks and two black tank flush ports - some R/Vs do!)

 

[NJKris]

ok, so only on sewage/drain lines. Makes sense. What maniac would introduce air to a pressurized water line.

 

[5picker]

ok, so only on sewage/drain lines. Makes sense. What maniac would introduce air to a pressurized water line.

Well... the check/anti-siphon does introduce air to the pressurized water line, once the pressure is removed. I know, I know, not quite the same but almost.

Again, it's function is to create a vacuum break to keep the black tank contents from being siphoned back into the supply water in the case of a catastrophic loss of water pressure. Certainly, several things have to be in place for that to happen. One, the black tank flush has to be submerged in black tank liquid and two, the flush process has to be unmanaged so if the water supply happens to be compromised, no one simply shuts off the supply or unhooks the hose.

On a related/unrelated note... air admittance into pressurized lines happens frequently in the housing real world. Many iron removal filters introduce air into the water line (some even use a small air compressor) for the iron molecules to cling to, for rust removal.