This came up on another forum, as it does from time to time when someone suggests installing a backflow preventer on the black tank flush inlet or keeping a separate hose attached to the black tank flush inlet. Both are really bad ideas. In the interest of health and safety DO NOT put a backflow preventer on your black flush inlet, or leave a hose attached to it.
Some of the most plumbing-knowledgeable GD owners around including Cate&Rob have made a good case against doing it, and against leaving the black flush water supply connected to the inlet. Here it is.
The black flush water supply rises from the hose inlet through a plumbing line to high up inside your trailer, probably up behind your shower wall. It connects there, feet above the black tank, to a valve that creates a vacuum break. From there it continues back down from the valve and into your black tank.
When you send pressurized water into the black tank flush inlet it creates a feet-long column of fresh water up to the high-up valve before it continues flowing down into the black flush mechanism in the tank. That column of fresh water is a deliberately-designed buffer between the inlet and any bacteria-laden water in the tank and black flush mechanism.
Each time you finish flushing the tank you should remove the black flush supply hose and allow that column of fresh water to flow out. The feet of fresh water on the inlet side of the valve flows back out the inlet and the feet of mostly-fresh water on the other side of the valve flows down into the tank, carrying any bacteria on that side of the system with it.
Trapping that water in the system - by installing a backflow preventer or by leaving a hose full of water attached - defeats the purpose of the vacuum break safety system. It leaves a column of water - a pathway for pathogens - all the way from the tank to the inlet (and into your hose if left attached).
On the other hand, allowing the fresh water to flow back out of the system drains the pathway from both the up and down sides of the vacuum break system. You can then reattach the black flush hose if you want to.
Understanding this leads to another bit of advice that may surprise some people. You can use the SAME HOSE for black flush and the trailer's city water supply! As long as you disconnect from black flush immediately after you turn off the water to it, you can simply move the hose to city water and turn it back on again. Nothing but fresh water has passed through the black flush inlet. However if you trap water in there, that's when it can get risky.
The system works like this for a reason: health and safety. Grand Design doesn't generously load up our trailers with unnecessary things, especially things hidden inside walls like the black flush vacuum break system that most people don't understand. It's there for a reason.
Hope this helps.
Some of the most plumbing-knowledgeable GD owners around including Cate&Rob have made a good case against doing it, and against leaving the black flush water supply connected to the inlet. Here it is.
The black flush water supply rises from the hose inlet through a plumbing line to high up inside your trailer, probably up behind your shower wall. It connects there, feet above the black tank, to a valve that creates a vacuum break. From there it continues back down from the valve and into your black tank.
When you send pressurized water into the black tank flush inlet it creates a feet-long column of fresh water up to the high-up valve before it continues flowing down into the black flush mechanism in the tank. That column of fresh water is a deliberately-designed buffer between the inlet and any bacteria-laden water in the tank and black flush mechanism.
Each time you finish flushing the tank you should remove the black flush supply hose and allow that column of fresh water to flow out. The feet of fresh water on the inlet side of the valve flows back out the inlet and the feet of mostly-fresh water on the other side of the valve flows down into the tank, carrying any bacteria on that side of the system with it.
Trapping that water in the system - by installing a backflow preventer or by leaving a hose full of water attached - defeats the purpose of the vacuum break safety system. It leaves a column of water - a pathway for pathogens - all the way from the tank to the inlet (and into your hose if left attached).
On the other hand, allowing the fresh water to flow back out of the system drains the pathway from both the up and down sides of the vacuum break system. You can then reattach the black flush hose if you want to.
Understanding this leads to another bit of advice that may surprise some people. You can use the SAME HOSE for black flush and the trailer's city water supply! As long as you disconnect from black flush immediately after you turn off the water to it, you can simply move the hose to city water and turn it back on again. Nothing but fresh water has passed through the black flush inlet. However if you trap water in there, that's when it can get risky.
The system works like this for a reason: health and safety. Grand Design doesn't generously load up our trailers with unnecessary things, especially things hidden inside walls like the black flush vacuum break system that most people don't understand. It's there for a reason.
Hope this helps.
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