Black Tank Flush

[CloseToRetirement]

My CRS was in full swing until a recent post about the infamous KantLeak valve surfaced recently. Cate&Rob commented to the thread and noted about a unresolved divergent conversation in yet another thread about KantLeak replacement.

Following Rob's recommendation to start a dedicated thread, here it is. I am planning on relocating my AVB (Atmospheric Vacuum Breaker) that is used in the black tank flush system, hidden behind the shower faucet in the linen closet of my Solitude. My research took me to a plumbing engineer for clarification due to the ambiguities I was reading in some of the materials I could find.

***From an earlier post********************************************** *********************************

[Uniform Plumbing Code, Section 603.4 (2018)] Upright position, no valve downstream. Minimum of 6 inches or listed distance above all downstream piping and flood level of receptor.

He stated the water is discharged at the spray port (receptor) on the tank and consequently at the lower valve to drain the tank. He stated if the interpretation is the flood rim of the toilet that would be incorrect because the toilet has a seal and the top of the vent pipe would be the highest flood rim. The physics of the atmospheric vacuum breaker is to ensure any water backing up the valve outlet port closes the valve to stop the backflow on the non-pressurized source (city water) because the tank vent prevents any back pressure. So if the spray port fails thus creating back pressure, the valve closes preventing any back siphoning of waste water. If the city water source continues to run, the water would eventually go up the vent pipe onto the roof and no back pressure would be present due to the incoming water.

************************************************** ************************************************** *****

So during another conversation with said plumbing friend recently we continued the discussion of the AVB. In the days following he provided me an article from the International Association of Plumbing and Mechanical Officials (IAPMO) providing additional details about the use of the AVB. http://forms.iapmo.org/email_marketi...2017/Aug10.htm He also noted that brass valves are preferred but some plumbers will use plastic valves.

This helps reinforce my decision to relocate my AVB (I will be replacing with brass) behind the shower valve to the storage area, making it more accessible. In the event the AVB fails I won't have water running down under the shower pan and into the basement making for a costly repair.

I would very much like to hear from others on the topic.

Thanks,
Lyle

[Cate&Rob]

CloseToRetirement

Hi Lyle,

Agreed that CRS kind of lost this earlier discussion from all sides . . . .

The location of the the Black Tank Flush vacuum break seems to depend on what is considered to be "the flood level rim of the receptacle served". See section 1.2 of https://law.resource.org/pub/us/cfr/....1001.1990.pdf Grand Design and their supplier, B&B Molders are interpreting this requirement to be the rim of the toilet, per GDRV Compliance Manager Jeff Christner.

If a black tank flush line were left filling beyond the capacity of the black tank, the next point of overflow would not be the vent stack on the roof, it would probably be the kitchen sink. This is because on most GDRV products, all tank vent pipes tie into a common vent stack to the roof. Water rising in the black tank vent pipe would flow into the grey tanks and eventually up the sink and shower drains, overflowing at the lowest of these points. By this interpretation, the vacuum break would have to be 6" above the kitchen sink rim and/or the shower base rim.

This could go the other way, depending on whether the vent pipes connect to the vent stack above or below the kitchen sink rim or the shower pan. For example, if a faucet at the the bathroom sink were left running, would the overflowing bathroom grey tank back up the vent stack and into the kitchen grey and/or black tank, before or after it flooded the shower pan? In any of these scenarios, behind the shower wall is still above the overflow location, regardless of where the water comes from. At the water panel in the pass through would not be above these overflow points.

The issue that I have with the location chosen by Grand Design (behind the shower wall) is not only that it is difficult to get at, but in the case of the Reflection 303, it is above the furnace. From the B&B Molders installation instructions . . . "Vac/Check cannot be installed in an inaccessible location where venting of water from device during normal operation causes damage." What I encountered was that a small amount of water from normal operation of the vacuum break would dribble down on to the control board section of the furnace.

Rob

[CloseToRetirement]

Hi Rob,

A couple of comments to the standard that B&B is using. That standard was updated in 2017 making the one they are using 27 years old. Also note this is the performance standard of the AVB not the installation standard to the Uniform Plumbing Code for the AVB. I believe both standards apply to the AVB, performance which B&B is held to for manufacturing and installation (Uniform Plumbing Code) which GDRV should be in compliance.

I agree with your comments on the section outlining location. Mine is concealed in the wall between the linen closet and the shower stall. It is accessible but the standard uses an "or" not an "and" in the verbiage.

Table 603.2 of the Uniform Plumbing Code states an AVB cannot have a downstream valve. The black tank has a gate valve downstream of the AVB. IMHO this is a violation of the UPC especially if the consumer elects to close the gate valve and fill the tank with the black flush sprayer. This potentially can create back pressure to the AVB which is not permitted according to both the ASSE Performance Standard and the Uniform Plumbing Code.

Physics tells me the if the AVB is installed 6 inches above the top of the black tank, non-pressurized water will only rise in the outlet pipe of the AVB six inches below the outlet port. Once again back pressure is not permitted especially with the installation of the tank drain gate valve.

Digging further into back-flow and back siphoning prevention, a hose connection vacuum breaker at the inlet port (which I already have in place) in the utility center seems like a safer practice due to the tank drain valve and the consumer closing the drain valve, thus creating back pressure to the inlet port.

Lyle

[colan]

My two cents worth, with the RVDA/RVIA training standard for techs saying a PDI includes filling the black tank with water until it can be seen with from the toilet. I definitely want the syphon break and check valve above floor level. With many/most toilets not having a straight drop into the blank tank this means completely filling the tank to a level above the black tank flush connection.

So who cares, why can’t I retrofit to a lower and more convenient place? This recently happened, the perpetrator will remain nameless. Pulled into a new site, both the black tank flush and fresh water connection have quick connects. The black tank flush is about six inches above the fresh water connection. Some fool who again will remain anonymous connected the fresh water supply to the black tank flush. It wasn’t until the perpetrator’s wife asked why she saw water when she flushed that the mistake was discovered.


EDIT: Don’t trust your memory, pulled the manual out - fill until water is visible in the lowest fixture of the each system.

Forgetting about fools who unknowingly connect to the flush RVs holding tanks especially for dry campers tend to get rather full. While the check valve in a proper siphon break should stop bad stuff from happening why take this chance?

[Cate&Rob]

CloseToRetirement

Hi Lyle,

The 27 year old code reference that I used is something that I dug up on the internet . . . not something that I got from B&B or GDRV. My original question to GDRV was whether this was still the code requirement that they work to. They confirmed that it was, without referencing a later version of the code.

I am not sure that the tank drain gate valve enters into this as a "downstream valve". The tank is vented to atmosphere before the tank drain valve, so this should meet the requirement for no back pressure downstream of the vacuum break.

Having the vacuum break at the hose connection to the panel (as you suggest) makes a lot of sense. The problem is that there is no way to assure that all owners would do this. This idea should be coupled with a check valve at this location to make sure that the contents of an overfull black tank can't exit through this hose connection.

In "fiddling around" with these devices, I have learned that the plastic OE vacuum break appears to have a check valve built into the downstream (tank) side, whereas the "better quality" brass alternative does not have a built in check valve. Installation instructions for both note that some leakage from this device is normal and this has to be considered in choosing the installation location.

colan I agree with Colan's example that a black tank full to just below the ball valve in the toilet is going to backfill the flush line to this same level. If the vacuum break is below this level, it will be backfilled with the contents of the black tank if it does not have a check valve protecting it. This is why the spec is for the vacuum break to be higher than the toilet rim.

Rob

[Canyonlight]

Excellent learning thread ! Thanks, guys !

Dan

[CloseToRetirement]

CloseToRetirement



colan I agree with Colan's example that a black tank full to just below the ball valve in the toilet is going to backfill the flush line to this same level. If the vacuum break is below this level, it will be backfilled with the contents of the black tank if it does not have a check valve protecting it. This is why the spec is for the vacuum break to be higher than the toilet rim.

Rob

I agree that fluid dynamics come into play when the vacuum breaker is above the toilet rim. The common vent stack of the shower drain and toilet would be the next overflow.

ASSE 1001-2017 states the following.

• This standard applies to atmospheric type vacuum breakers (herein referred to as the "device") that are single pipe-applied, flushometer-applied, or integrally-applied (does not apply to water closet tank ball****s or similar devices that depend on float-operated valves to control flow). The purpose of these devices is to provide protection of the potable water supply against pollutants or contaminants that enter the system due to backsiphonage through the outlet. Under backsiphonage conditions, a small amount of water is permitted to exit through the air ports.
  The device shall:
  • Have its outlet open to atmosphere;
  • Not be subjected to backpressure;
  • Not be subjected to more than twelve (12) hours of continuous water pressure;
• Installation instructions shall include statements as follows:
  • The device shall be installed in accordance with the requirements of the local plumbing
    code. [Lyle Myers comment] This would be the Uniform Plumbing Code which states no downstream valve regardless of the vent stack.
  • The device shall not be installed where the venting of water from the device during its normal
    functioning causes damage.
  • The device shall be installed such that it is not subjected to backpressure.

My plumbing engineer friend looked at my Solitude and stated the design and standard of the AVB as it's installed in my coach does not meet all the standards, specifically the location of the device as outline in the installation bullets. Any backup in the tank would overflow into the shower and galley gray tanks, then the kitchen sink, shower drain and lastly the bathroom sink. Even locating the AVB in the bathroom more openly still risks water damage to the interior living space. He stated the intended use of the AVB is more commonly used in commercial sinks where the sink drains into the sewage plumbing, no downstream valve for water passthrough. The use of the AVB in my coach is pushing the envelope of the intended design and standards due to the use of the tank gate valve, waste tanks and vent stack system.

His recommended solution is the vacuum breaker hose connection with a straight pipe to the spray port valve. Any back siphonage on the hose connection vacuum breaker is vented at the hose connection vacuum breaker valve.

Lyle

[boyscout]

Cate&Rob amp;Rob, CloseToRetirement the vacuum break on my anxiety is being threatened with your thread. :-)

If I understand your conversation correctly, GD's design choice becomes clearer. (Where ELSE can the necessary elevation be reasonably achieved in many trailers?) However it doesn't reduce anxiety much.

Why is the vacuum break and the risks of a cheap valve inside a wall required over a backflow preventer nearer the inlet? Or maybe it isn't... Rob are you suggesting an alternative would be OK when you say, "Having the vacuum break at the hose connection to the panel (as you suggest) makes a lot of sense. The problem is that there is no way to assure that all owners would do this"?

We have been required by county regulation in some Florida parks to have a simple and inexpensive brass backflow preventer on our hose connection, so presumably it's considered effective by them at preventing something in the trailer from flowing back out to the water supply. Why aren't these devices a simple solution to replace the cheap valve hidden high in the shower wall?

Here's the device we've been required to have:

https://www.amazon.com/Litorange-Con...s%2C153&sr=8-5

If this isn't the answer, after you guys have worked out an alternative I'm hoping you'll describe it. I'd like to remove the anxiety of the cheap valve hidden in the shower wall by replacing it with a more-comforting and easily-checked replacement system.

Thanks for all the thoughtful detail in your conversation... I'll enjoy it even more after I take a Xanax..

EDIT: Or how about this thing?

https://www.homedepot.com/p/Watts-1-...ine%20repairs.

[Second Chance]

Cate&Rob amp;Rob, CloseToRetirement the vacuum break on my anxiety is being threatened with your thread. :-)

If I understand your conversation correctly, GD's design choice becomes clearer. (Where ELSE can the necessary elevation be reasonably achieved in many trailers?) However it doesn't reduce anxiety much.

Why is the vacuum break and the risks of a cheap valve inside a wall required over a backflow preventer nearer the inlet? Or maybe it isn't... Rob are you suggesting an alternative would be OK when you say, "Having the vacuum break at the hose connection to the panel (as you suggest) makes a lot of sense. The problem is that there is no way to assure that all owners would do this"?

We have been required by county regulation in some Florida parks to have a simple and inexpensive brass backflow preventer on our hose connection, so presumably it's considered effective by them at preventing something in the trailer from flowing back out to the water supply. Why aren't these devices a simple solution to replace the cheap valve hidden high in the shower wall?

Here's the device we've been required to have:

https://www.amazon.com/Litorange-Con...s%2C153&sr=8-5

If this isn't the answer, after you guys have worked out an alternative I'm hoping you'll describe it. I'd like to remove the anxiety of the cheap valve hidden in the shower wall by replacing it with a more-comforting and easily-checked replacement system.

Thanks for all the thoughtful detail in your conversation... I'll enjoy it even more after I take a Xanax..

EDIT: Or how about this thing?

https://www.homedepot.com/p/Watts-1-...ine%20repairs.

Mark,

The problem with the backflow preventer you linked on Amazon is that it expels sprays water everywhere when the pressure is released... fine for outside, but not suitable for inside the rig. We have several because, like you've found in Florida, they are required in many Escapees parks. I keep one on the side of the Y-connector I use for the black tank flush (using a separate hose).

Rob

[boyscout]

The problem with the backflow preventer you linked on Amazon is that it expels sprays water everywhere when the pressure is released... fine for outside, but not suitable for inside the rig. We have several because, like you've found in Florida, they are required in many Escapees parks. I keep one on the side of the Y-connector I use for the black tank flush (using a separate hose).
Rob

Thanks Rob. That's an old post you quoted, raised from the dead recently in discussion in another thread about how high the black flush breaker must be mounted.

I've since purchased this valve:

https://www.amazon.com/Watts-Technol...s%2C162&sr=8-7

and will install it behind the shower wall. Ours (and yours) can be accessed through a large panel in the wall at the back of our pantry whereas some are more difficult to reach.

However if the valve in normal operation releases water or if it begins leaking water, it may not be detected until it has done some damage. I was hoping to move it somewhere else in the trailer but am dissuaded by advice that it should be located 6" higher than the kitchen sink, so it's already in one of the few places that AFAIK that can be achieved.

Have you figured out anything better?

[Second Chance]

Thanks Rob. That's an old post you quoted, raised from the dead recently in discussion in another thread about how high the black flush breaker must be mounted.

I've since purchased this valve:

https://www.amazon.com/Watts-Technol...s%2C162&sr=8-7

and will install it behind the shower wall. Ours (and yours) can be accessed through a large panel in the wall at the back of our pantry whereas some are more difficult to reach.

However if the valve in normal operation releases water or if it begins leaking water, it may not be detected until it has done some damage. I was hoping to move it somewhere else in the trailer but am dissuaded by advice that it should be located 6" higher than the kitchen sink, so it's already in one of the few places that AFAIK that can be achieved.

Have you figured out anything better?

No, I haven't figured out anything better for that vacuum breaker (I had to replace the shower faucet due to a leaking vacuum breaker on it, so I saw that one when I was in there). I've just installed a brass Y and short flex hose inside the utility bay to flush the black tank this winter, though. It will require mopping up in the bay when I disconnect the flex hose to let the flush line drain back - good thing that's a sealed metal box.

Rob