Anti siphon valve for black tank flush

Cummins12V98 wrote:

dougrainer wrote:
..........

They are installed to prevent holding tank ODOR from venting out the inlet fitting. Most BT flush inlets do NOT have a check valve like the City Water inlet because of the inside Check valve. If the OP's is NOT factory installed, then there is no upper Check Valve. The INLET and the inlet to the tank has a check valve, that comes with the aftermarket BT kit. Doug

They are check valves on the grey and black tank flush lines. They are there to keep contaminated water from siphoning into the public water supply.

Holding tanks vent thru the roof!

^^^^^^ This ^^^^^^
There are there for the same reason codes require anti siphon valves on outdoor water connections such as outdoor faucets and lawn sprinkler systems. There to prevent siphoning contaminated water back into the public water.

Thank you!

Thanks everyone. I'll follow line and keep looking. If it is in the wall I'll just quit using it and go back to pooh wand.

Cummins12V98 wrote:
Thank you!

Tell you what, If you have the valve removed and have a pipe bypass, SMELL the entry fitting. OR, Overfill the Black tank and see what comes out the entry hose fitting. Yes, it prevents backfeeding to the city water system. But it also stops the odor. Doug

dougrainer wrote:

Cummins12V98 wrote:
Thank you!

Tell you what, If you have the valve removed and have a pipe bypass, SMELL the entry fitting. OR, Overfill the Black tank and see what comes out the entry hose fitting. Yes, it prevents backfeeding to the city water system. But it also stops the odor. Doug

The anti siphon valve has an atmospheric vent that allows air to enter and water to drain out of the pipes when there is no water pressure. That is why they must me mounted high above the tank. Air is allowed to enter under the cap. That is why they are so prone to leak when they do not seal properly. If any odor was in the pipe, it would be released into the RV. More than one person with a leaky valve found there to be a bug had entered under the cap and the valve could not seal.

OP asked about anti-siphon for black water flush. This is also known as a backflow
preventer. It is in-line with the flush line.
It is not an air gap. The tank is vented through the roof.

enblethen wrote:
OP asked about anti-siphon for black water flush. This is also known as a backflow
preventer. It is in-line with the flush line.
It is not an air gap. The tank is vented through the roof.

Yes, the tank is vented thru the roof. IF YOU REMOVE THE ANTISIPHON VALVE AND PLUMB STRAIGHT THRU(REMOVING THE VALVE) IT WILL VENT BACK THRU TO THE INLET FITTING. You will get Black tank odor in the Water Bay in transit. You can stop that by installing a screw on plug on the BT inlet fitting. Or, just install a standard City Water inlet with the built in check valve. Doug

enblethen wrote:
OP asked about anti-siphon for black water flush. This is also known as a backflow
preventer. It is in-line with the flush line.
It is not an air gap. The tank is vented through the roof.

They used to be a vacuum breaker but it looks like the newer ones are just a check valve. More cost cutting.

RPBD or reduced pressure backflow device is what public buildings use to keep their water from returning into the city water. Also used in many medical/dental facilities on equipment to keep contaminated water at the source and not into the building.

I have/had as pictured atmospheric vacuum breakers that are designed as shown to keep water from returning from the tank to the public water supply. By design yes they will keep odors on the tank side. Not really designed for that but yes they will.

Mine started leaking so I removed BOTH and installed PVC fittings in their place. I have my waterline connected to the two flush lines at all times with a "Y" that has 1/4 turn valves. I open and close the valves as needed. I used the sprayer hose as the water supply, I never have to connect of disconnect anything when flushing. I have a timer on my phone set each time so I don't overfill the tank.

Technically this is not "LEGAL" but it sure works well. If I sell the RV the new owner will be informed to the condition so they can make an informed decision to return to stock or not.

enblethen wrote:
OP asked about anti-siphon for black water flush. This is also known as a backflow
preventer. It is in-line with the flush line.
It is not an air gap. The tank is vented through the roof.

The 90 degree black tank flush anti siphon valves with a cap on top are air gaps as the cut away view shows. The brass one shown a few posts below is the same type. That is how they allow the water to drain from the flush lines. If no air vent, water cannot drain out of the lines. Mine cracked year ago and I removed it. And it looked exactly like the cut away view, with vent to atmosphere.

And yes, the tanks are vented.