Black Tank Vacuum Break -- How it Works

[Cate&Rob]

Moderator note--the post below was copied (and slightly edited) from the Winterizing our Rigs thread, post 47. For a full discussion of winterization or comments about that process, read and/or comment in that thread: https://gdrvowners.com/forum/operati...izing-our-rigs -Howard

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A vacuum break functions such that when pressurized on the inlet side (with water or air) the valve "opens" allowing this pressurized source to flow through to the output side (the black tank flush nozzle). When the inlet side pressure is removed, the valve "closes" but opens a vent which allows air into the output side which causes water in that side to drop into the tank, leaving the line (theoretically) empty. See attached diagram.

For the line between the vacuum break and the tank to be truly empty, it must have a constant downward slope all the way to the tank. This is not always the case. Blowing out the black tank flush line with air might not remove all the water from a low section of this hose, which is why antifreeze is recommended. The reason that many owners get away with not using antifreeze is that this flush line is usually soft hose which can tolerate freezing expansion of the water inside, as long as the water is not trapped in a fitting than can break.

[JCR GD]

Keep in mind that the OEM valves GD uses also have a plunger type check valve built into the discharge side. (all one unit)

[Scott'n'Wendy]

Cate&Rob
I'm no plumber so if the questions seems stupid...well..forgive me.
In the drawing above I'm thinking the bottom connection is coming from the nautilus panel and the connection on the right (O/P) goes to the black tank. Yes?
If so, when there is no pressurized water or air flow from the nautilus panel the vacuum break is venting the line to the black tank. Wouldn't the 'aroma' of the black tank permeate the trailer if this is how it is plumbed? I know it doesn't so I'm sure I misunderstand something...but what?
Also - two part question..lol - my previous 5er had a black tank flush with no vacuum breaker. And the black tank is vented to the roof anyway, so what does that vacuum breaker in the GD trailers accomplish? I'm willing to bet the line to the tank would drain without it. The unvented line back to the nautilus sure does! I'm thinking a line straight to the black flush nozzles with a cap...like it already has. Simple and effective.
So school me on vacuum breakers and why the added complexity over previous generations of trailers. What is the benefit here?
I'll have a popcorn and soda while wait for a reply.

[Country Campers]

Scott'n'Wendy

Please see post 6 in this thread Safe use of black tank flush - Grand Design Owners Forums (gdrvowners.com). There is a check valve on the tank side that prohibits water from coming back up the line from the black tank, it also prohibits water from draining from the valve to the black tank. The main reason this valved is used is to prevent contamination exiting the black tank and getting to the water source. In a previous TT I removed this valve because it always provided a spurt of water in the cabinet where it was located when it was used, never had any issues with it being removed. I do not like these valves for the way they operate. A small spurt of water at each usage will surely cause water damage at some point.

Hope you enjoyed the popcorn and soda, or pop as it is know here.

Brian

[TedS]

The check valve in the outlet of the B&B Molder vacuum break valve is supposed to prevent stink from coming from the tank. The vacuum breaker valve is required by a plumbing code to be at least 6" above the flood rim. The flood rim in the rv to which the valve is connected is the toilet, which is valved itself. Makes no sense. And the inlet side would drain out the hose inlet when you disconnect the hose. Is there not a check valve there? Messy, if the black tank contents were drawn back due to the siphon action. Having a vented tank does not prevent a siphon from working. Admitting air at the top of the potential siphon does break the siphon flow.

[Scott'n'Wendy]

I do not like these valves for the way they operate. A small spurt of water at each usage will surely cause water damage at some point.

Hope you enjoyed the popcorn and soda, or pop as it is know here.

Brian

Thanks for that link..enlightening..another "How it's Made" episode. lol
Yes that water drainage is annoying. On the previous 5er I was mentioning the port was located on the outside wall...GD should think of doing something like that.

I'm still not clear on what benefit the vac brkr is over just a straight piping shot to the nozzles though? One other thing, is this the vac brkr I have heard is behind the shower stall tap?

[trkester]

Thanks for that link..enlightening..another "How it's Made" episode. lol
Yes that water drainage is annoying. On the previous 5er I was mentioning the port was located on the outside wall...GD should think of doing something like that.

I'm still not clear on what benefit the vac brkr is over just a straight piping shot to the nozzles though? One other thing, is this the vac brkr I have heard is behind the shower stall tap?

Solved my issues with the vac brkr. I don't use the black tank flush. It's inferior to putting my Valterra wand down the toilet to clean things and I can avoid another minor flood due to my failed brkr.

Using the wand allows me to get things nice and clean and my tank level indicator actually works! Working the wand up and down and all around cleans the sensor.

If course if your toilet isnt a straight shot into the tank like mine is, then that's a different story.

[JBill9694]

IMHO, Scott'n'Wendy, the vacuum breaker on the flush line is there as an anti-doomsday device. While we call it it a vacuum breaker, it could also be called a siphon breaker. So, what is the doomsday that the device is protecting us from? Let's say that after flushing the black tank is filled with water to some point above the flush line outlet, let's also say that there is not a check valve on the flush line inlet (meaning water can drain back out of the line) and lets say that the flush line inlet and the spigot it is attached to are actually lower than the flush line outlet. With all three of these improbable conditions met, the flush line could start to siphon nasty water out of the black tank when flushing is stopped (similar to siphoning gas from a car) and the hose is disconnected from the intake or the spigot. The vacuum breaker opens on the negative pressure (suction) of the siphon, breaking the siphon effect.

I'll also opine here that the AVB should be normally closed, that is not letting air into the system, and so there should not be any water leakage at the start of the flushing. If there is, I would replace the valve.

[Cate&Rob]

Scott'n'Wendy Hi Scott,

Not clearly explained in this thread, but discussed elsewhere (that I can't find at the moment LOL) . . .

The vacuum break vents both the fill line and the line to the tank. The intent being that a line full of air can't start a siphon from the tank back through the fill port.

The one way check valve on the tank side of the vacuum break allows water or air to move towards the tank, but prevents flow of water or air (smell) back through the vacuum break vent.

Buried in the vacuum break manufacturer data is the statement that it takes about 8 psi water pressure to close the vacuum break vent. Thus, as flush water pressure is building, there will usually be a "spurt" of water out of the vacuum break before the vent closes. Why Grand Design puts these valves in hidden places (in my case, behind the shower and over the furnace ) . . . I cannot answer.

For those who insist on leaving a hose connected to the black tank flush . . . water expanding in that hose laying in the sun will never reach 8 psi, but will continue to drip out of the vacuum break vent.

Rob

[TedS]

I don't use the black tank flusher. I use the FlushKing on the dump outlet with satisfactory success.

Read the fine print for the vacuum breaker valve. It advises to not install the valve were leaking water presents a problem. So, GD installs it in an inside cabinet or inside the wall !!! WTH !


On edit: Where is the flood plane on the rv? Just a thought. The toilet connected to the black tank is sealed with a ball valve. It has happened that the black tank was inadvertently overfilled with a flush valve and the excess water flowed out the rooftop vent.

[Cate&Rob]

On edit: Where is the flood plane on the rv? Just a thought. The toilet connected to the black tank is sealed with a ball valve. It has happened that the black tank was inadvertently overfilled with a flush valve and the excess water flowed out the rooftop vent.

The flood plane is the shower pan or perhaps the galley sink . . . whichever is connected to the same vent stack as the black tank. If the black tank is over filled with the flush, the contents will rise up the vent stack until it reaches the Y connection to the grey tank vent. From there, down that vent stack until the grey tank fills up and then out the lowest drain connection. If the shower drain should happen to have a Hepvo valve instead of a P trap, it would be on to the bathroom vanity sink.

This was a "theory" until it was proven by an attendee at the National Rally . . . one of the "two hose gang" whose connected flush line was inadvertently turned on.

Rob

[Scott'n'Wendy]

This was a "theory" until it was proven by an attendee at the National Rally . . . one of the "two hose gang" whose connected flush line was inadvertently turned on.

Rob

Must have been a 'lovely' day for that couple.

Thanks for the extra explanation John, Rob, Brian, and Ted.