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Old 08-23-2015, 09:53 PM   #1
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Boxite's Avatar
2008 22' Sport
Spicewood (W of Austin) , Texas
Join Date: Mar 2015
Posts: 1,720
Black Tank Flush Anti-Siphon valve replacement

Having read thru a couple of the threads on the Black Tank Flush anti-siphon/vacuum-break valve I came to the conclusion mine was fouled up.

I dreaded trying to locate it between the walls but it turned out to be easy. In my 2008 22' Sport, it's found by looking inside the wardrobe. On the aft wall of the wardrobe was an 8" X 8" wood panel held by two screws (what's that..?? an access panel..??) and removing the two screws revealed the anti-siphon valve attached to the PEX plumbing via two simple, 1/2" threaded screw-knobs, similar to what is often found beneath bathroom sinks and toilets which use flexible hoses at the shut-off valves.

One of the other posters expressed considerable frustration in attempting to dispense with the anti-siphon valve and using brass angle fittings instead. He had trouble with leaks. I suspect that was due to the fact the screw knobs do not seal against the /2" pipe threads when fitted up against the a-s valve..but use the rubber gaskets/O-rings inside the fitting and ordinary 1/2" plumbing fixtures do not provide for that. Teflon tape and thread sealants also will not work.

I found a simple solution to replacing the a-s valve. (Yes... I know... the a-s valve is a plumbing code thing... but it's really overkill when you think about it. The system is not continuously hooked up to public water so a back-siphon is not going to occur unnoticed.) My valve was filled with what looked like tobacco leaves.... an insect nest apparently. I cleaned it out successfully but upon examining the valve I discovered the vertical plunger/seal can spit a little water out in between the walls of the Airstream and I don't care for that a lot more than I worry about a little siphon action on a system that is never allowed to remain hooked up without pressure applied to itl

Anyway, here's the SIMPLE SOLUTION I used: go to Lowes or Home Depot. Obtain two "Shark Bite" or "Blue Hawk" 90-degree-angle slip fittings for PEX pipe. They cost about $5 ea. Using an ordinary PVC cutter, cut the existing PEX just below the two hand-knobs which screw to the valve. Be certain to cut the PEX to the same height. If you cut the risers about 3" shorter you can use one piece in the next step.
Using a short piece of PEX, connect two slip-fit 90-degree angles to each other, then to the PEX risers. (Connect the two slip fittings to each other with a short 3" piece of PEX you just removed.)

FINISHED! The flush line is fully connected and will work perfectly.

In other words, the supply side of the flush line rises up to a 90-degree slip fitting, which is connected to another 90-degree slip fitting which sits atop the PEX going to the black tank flush sprayer. The two slip fittings are well above the black tank which should prevent any sewage from ever making it over the "hill" to escape the system, especially if not left connected before or after the flush-operation.
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Old 07-08-2018, 07:51 AM   #2
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2014 19' International
Stafford , Virginia
Join Date: Apr 2014
Posts: 12
Great post - I had the same problem this year. Never thought to winterize the black water tank flush inlet. This year, the anti-siphon valve blew apart under the sink and water leaked everywhere.

I took your advice and replaced the A-S valve with a basic connection of pex. I suspect I am not up to some code, but the A-S appeared to be a PoS and I could not find an exact replacement.

See the pics - I basically created a large elbow out of two 1/2" male sharkbite adaptors, one 1/2" PEX connector/elbow, and some 1/2" PEX pipe cut very short. It was less than $10 as I already had a bag of 1/2" PEX compression rings and the PEX tool. Home Depot.

Good luck all!

First pic is the original a-s valve (top gone!);
second is the new "problem" to solve";
third is my final result:
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2014 19' AS Intl'
2013 Toyota Sequoia
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