Black tank flush vacuum breaker.

[Cate&Rob]

Good discussion! (What this forum is all about)

TedS I think the "above the toilet bowl rim" is an RVIA carryover from residential requirements where this would be the backflow overflow point . . . but, this does not apply to an RV toilet with a sealed ball valve. An RV toilet is not going to overflow the rim from an over-full tank, unless someone steps on the pedal. As an aside . . . I did try to pose this question to the Regulations Compliance guy at Grand Design (because I couldn't find anyone at RVIA to talk to) but didn't get very far. They don't seem inclined to question the RVIA regs.

Jlawles2 The brass valve with the built in check valve is an interesting find . . . but, the check valve seems to be on the wrong side, unless the vent seal is sufficient to prevent overflow from the tank ?

JCR GD I think you have found the best combination of quality parts at a reasonable price!

Rob

[Cate&Rob]

One thing I remember is always use full water pressure. These valves do not like low pressure (leak).

Somewhere in the literature for the OE valve is the note that it takes 8 psi (if I recall correctly) to close the vent. Pressure less than this will push water out the vent instead of closing the vent and flowing to the tank. The most common cause of this is leaving a hose connected to the black tank flush (probably one of the most frequent campground "misunderstandings") and when the sun heats the hose and expands the water inside, it flows out the vacuum breaker vent.

Rob

[Jlawles2]

Cate&Rob Rob, the way I see the valve I posted is the bottom connection would be the fresh water inlet. This valve opens and water exits the right connection. When water flow is stopped, the non threaded side can then open to allow air to enter and let the right side drain. In the event water gets pushed from the right side into the valve, since both checks open only in the direction of the right fitting, the water actually forces the checks to seal harder.

Unless I am seeing the water flow the wrong way. Most of the other ones I saw, the air check side was gravity closed thus low flow or back flow could exit the vent side.

[Cate&Rob]

Jlawles2
Hi Joseph,

In the drawing you posted (post 11) . . . both connections are male . . . (which confuses your description )

Rob

[Jlawles2]

Cate&Rob Sorry been a long day. Using the drawing as a reference, the bottom (AKA female with my eyes not communicating with the brain today) would be the inlet. Sorry for the confusion.

Going up to fix it now.

[Beachcamper]

You guys are confusing me now on where the check valve should be positioned. Check out pic of what is now the vacuum breaker position and where the check valve will be added to the brass valve.

Would this be the correct way to assemble brass valve and check valve for installation?

[Jlawles2]

Vivian, note the arrow on both the black valve and the brass valve. These should be the flow direction.

As it appears the brass valve would be a direct replacement. As the picture of the brass valve is shown, the outlet is on the right instead of the left (just turn the valve over and it will match).

[Cate&Rob]

Beachcamper

The way the brass valve and check valve are assembled would be correct to replace the OE valve with water coming in at the bottom and going out to the black tank from the side. However . . . the brass valve appears to have pipe thread fittings with Teflon tape. This “might” seal to the OE lines but is not what the OE fittings are designed for and thread seal tape is definitely not required. The valve should have straight threads and a smooth end for the cone seal in the female fitting to seal against. If you are replacing the valve, you should also replace the soft cone seals in the hose fittings.

Rob

[Beachcamper]

Cate&Rob @jlawless2 thank you for responding and warning about the cone seals. Will have some handy for the install.

[TedS]

Cate&Rob I agree that the requirement to be above the bowl rim doesn't make sense for the valved bowl outlet. Unless someone flushes into an already overfull tank and the vacuum break valve is below the level of the bowl. My belief is that the flood level in the rv black tank system should be the high outlet of the tank vent. Unattended tank flushing with the tank outlet valve closed has resulted in water and stuff overflowing out the vent while the flushing system is running. When the flusher is turned off, waste water will backflow out the vacuum break valve. That valve should therefore be above the tank vent outlet outside. That would also put the normally leaky valve outside not causing water damage.

[Cate&Rob]

TedS
Hi Ted,

We have all likely heard of (or maybe seen?) a black tank flush left on and blowing out the roof vent. This would only happen if the roof vent was for the black tank only. With a roof vent stack connected to both black and grey tanks, overfilling the black tank with the flush connection would rise up the vent until it connected with a grey tank vent and then overfill that grey tank. The likely result being that the black tank contents would appear in the shower pan or kitchen sink.

Rob

[TedS]

A reasonable conclusion would be the installation a check valve to the outlet of the vacuum break valve. Which some folks have done. That would prevent any backflow while allowing the inlet plumbing to drain.

[JCR GD]

Another thought... the vacuum break could also become a black tank "air" vent (depending on tubing routing) without a resting closed check valve on the discharge side of the vacuum break, allowing odors to escape.

Edit: NOT something you want.

[TedS]

JCR GD venting the black tank out the vacuum break valve would release tank odors inside the trailer.

[JCR GD]

JCR GD venting the black tank out the vacuum break valve would release tank odors inside the trailer.

Yes, a bad thing... I should have been more clear