Black Flush Anti-siphon Valve

[MNRon]

We’ve chased black tank odors on and off for quite a few years, for the most part they’ve been resolved BUT once in a while we get a slight odor in the fridge/pantry corner of our VL320. Recently I started investigating the black flush anti siphon valve that hangs on the back of our shower faucet (coincidentally just behind our pantry). When I purchased a new brass on to replace the plastic OEM valve thinking that might help I discovered that the way the anti siphon works is that when not forcing water into the black flush input, it provides an air path down to the black tank (and visa versa) - basically a black tank vent into my coach.

Am I missing something? Is this how all coaches deal with backflow prevention on the black flush?

The black flush already has a check valve at the input, should I just rely on that an eliminate the anti siphon ‘vent’?

Appreciate any insights or wisdom.
Thanks,
Ron

[JBarca]

I have had other issues with this tank flusher anti-siphon valve (3-way quick-release valve). All the valves may not be the same, but the style I dealt with worked like what BB_TX posted.

I found on two campers that the vent poppet would not seat correctly when the water pressure was applied and blew water into the wall cavity, which was not good...

It appears different brands of tank sprayers address the issue differently. The Camco spinning tank sprayer has a check valve in the head of the sprayer to deal with the black tank backflow. They also supply a water pressure hookup backflow preventer. If you route the tank flusher supply flush hose downhill toward the water connection, it will drain out the unused, clean flush water when the incoming water pressure drops off to prevent freezing in the winter. This method has no valve that vents buried in the wall of the camper and accomplishes tank backflow, draining the excess clean flush water and protecting the incoming water source.

The tank sprayers with a nozzle and no check valve need extras to prevent tank backflow and trapped water in the hose from freezing. For the camper OEM, it may be easier and cheaper to use the tank nozzle with no check valve, install the cheap in wall-mounted backflow prevention to meet a code requirement, and not have to deal with an external backflow prevention valve. I agree this is not a good long-term setup. It works, but when the backflow valve in the wall stops working right, it can create an issue that is not easy to deal with.

[BB_TX]

MNRon wrote:
BB_TX - thanks for your analysis and thoughts. We don’t get this issue often, nor is it strong. But it’s my current theory that seems to fit. Just dumped tanks yesterday so it will be a while before I get the chance to do the sniff test around the anti siphon, but plan on doing that over the next few weeks.

Assuming the sniff test confirms my hypothesis,do you have any recommendations for a solution? Is an internal anti siphon valve like this standard across the industry on black flush lines?

Many, including myself, have removed the anti siphon valve, bought parts and connected the two pipe ends together, and use an eternal vacuum breaker. That was done primarily due to the frequent leaking problems that occurred. But would also stop any potential odor issue also.

https://www.lowes.com/pd/Arrowhead-Brass-Female-Vacuum-Breaker/3695370?cm_mmc=shp-_-c-_-prd-_-plb-_-...

[wildtoad]

From experience, I was getting oder from my black tank flush valve. It has the cheap plastic valve that many come with. I removed it and took it apart. Some debris had gotten into the valve and kept a little ball from seating properly. I replaced it and made sure the input line had a screen filter washer. Since then all good from it.

Part Two: I’ve notice an oder in the bath. The ceiling vent is less than two feet from a tank chimney and when the wind blows in the right direction, or events draw air into the vent from outside, the place stinks. So we keep the vent closed unless the exhaust fan is in use.

[wa8yxm]

MNRon wrote:



Am I missing something? Is this how all coaches deal with backflow prevention on the black flush?

Ron

For the most part yes.
You can add a spring check after the vacuum breaker but I'd not recommend it as that would leave water in the line that can freeze and do damage if you store cold.

A better method (lot harder) might to be to put the breaker in sort of a cup. and then seal the cup to the outer skin and install a vent there so it vents outside the RV.

[onecb]

I found this sistuation years ago. I put a hose on the far side of the antisiphon and ran the hose to the black tank vent pipe going out the roof. From that time on no more oder in the coach. This is what worked for us. Alvin

[MNRon]

BB_TX - thanks for your analysis and thoughts. We don’t get this issue often, nor is it strong. But it’s my current theory that seems to fit. Just dumped tanks yesterday so it will be a while before I get the chance to do the sniff test around the anti siphon, but plan on doing that over the next few weeks.

Assuming the sniff test confirms my hypothesis,do you have any recommendations for a solution? Is an internal anti siphon valve like this standard across the industry on black flush lines?

[BB_TX]

The anti siphon flush valve is not intended as a vent. But you are correct that it does have a direct vent to atmosphere under that cap and any positive pressure in the black tank could push air back up thru it. And that atmosphere is inside the RV.
If the vent opened, and all the water drained out of the pipe into the black tank, and the tank water level was below the spray head, then any positive pressure on the black tank could push odors back up that pipe to that vent. The tank roof vent should prevent positive pressure. But positive pressure could result from flushing with a blocked or partially blocked roof vent, or strong wind blowing just right to push air down the roof vent, or …..
Have heard people mention unexplained sewer odors but never heard of anyone actually tracing it to that anti siphon valve. Certainly possible.

[MNRon]

Douggrainer - that’s what I thought before looking at the new brass anti siphon I purchased. Definitely an anti-siphon and not a check valve. The ‘in’ side has a plastic valve to prevent water from flowing backwards. The ‘out’ side, which flows directly to the black tank, is exposed to air except when water is flowing in. Makes sense as “anti-siphon” that when water not going in it let’s air in to the black side so it drains back into the tank and can’t be siphoned, unfortunately after the water has drained into black tank the black line is now exposed to air and becomes a vent. Seems like a poor design element on a black flush to me. Am I missing something??

[dougrainer]

It is NOT a vent. It is a Check valve. While it may look like it vents, it does NOT. It is installed above the tank to prevent a siphon effect if you lost water pressure to the black flush system. Thus contaminating the fresh water supply system to the RV. They CAN leak water when broken, but they will not vent odors inside the RV. Doug

PS, the input check valve is redundancy. It is nice to have to prevent what little water is in the pipe TO the anti siphon valve, from draining back into the water compartment.

[CA_Traveler]

Black or grey tanks should not every vent inside in my opinion. My tanks have a roof vent. Does your black tank have a roof vent? How can there be a back flow problem when the flush line has a check valve? Make sure the check valve is working and eliminate the anti siphon valve.