Quote:
Originally Posted by RavensFan24
My trailer has a warning to not close the tank valve while running the black tank flush. I know some of you said you close it up to let the water build up, which I've considered doing. I guess I figured if you closed it then water could potentially come out of the toilet and thats why they warn against it.
It seems a few of you have had no issues while running the tank flush hose while the valve is closed. I'll have to give it a shot. I love when everything is fully empty and as clean as possible.
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Let's "analyze" the black tank flush operation and dissect it down to its individual parts/functions.
First, most "typical garden hoses" will supply about 3 gallons per minute at full flow. The black tank flush has a "restrictor" on it (the spray head) that reduces that flow to create a spray pattern. Typically, you can expect to get around 2 gallons per minute flow rate. Most RV holding tanks are around 30 gallons, (give or take) so you're looking at about 15 minutes to fill the tank using the black tank flush. Let's say that you have "full flow" through the flush system, you're still talking about 10 minutes to fill an "empty" tank. So, there's really no need for "paranoia" about overfilling after 2 or 3 minutes of having the dump valve closed.
Second, the black tank is vented to the roof of the trailer and "supposedly" the toilet inlet to the tank is a sealed, water-tight connection. The "ball valve" (flush valve) at the bottom of the toilet bowl is sealed with a rubber gasket and won't allow water to "backflow" up from the tank (if it is operating properly). So, if you should happen to "forget that you're adding water to the black tank" via the black tank flush system, you shouldn't get anything flowing up into the toilet. It is possible that you might have a leak at the toilet flange (on the floor) but it also is "designed to be leak proof" and if there's anything flowing out around the base of the toilet, you have a malfunction in the toilet installation. If you do "overfill" the black tank, the excess "should" flow up the vent stack and out onto the roof of the RV. Yup, that would be a "mess" (pun intended) but it would be outside your living area, not flowing through the bathroom and down the hallway. This is NOT the case with a black tank that also has the bathroom sink or an outside kitchen sink plumbed into it. Depending on whether that sink has a P trap or a "one way trap", it is possible to get the black tank contents to flow up into the sink, but most RV's don't have sinks plumbed into the black tank.
Many people "misunderstand" the black tank flush (which is prone to pressure damage and to freeze damage) and think that if they have a water leak during a flush operation that they will have sewer contents flowing into the trailer. That won't happen. The anti-siphon valve is mounted high above the black tank and is a vacuum breaker to prevent any tank contents from "siphoning" back up through the system. Is it possible to get tank contents in the trailer? Yes, if you remove the anti-siphon valve and have a plugged vent stack, but if you're pushing clean water "into" the tank, there isn't any possibility that the tank contents could "backflow" against that water pressure. So, the potential to get a "trailer full of poo" by forgetting that the black tank flush is filling the tank is extremely remote, you'll likely have it flowing over the sides from the roof before you damage the tank.
There are some people who "guess" at how much water they are adding to the black tank when they close the dump valve and let the black tank flush operate. Some say they allow 2 or 3 minutes for the "tank to fill" while they "stand there so they won't forget". Realistically, they are adding somewhere between 4 and 9 gallons of water to the tank, so there is really no chance of "bursting the black tank" with that sequence of operation.
I usually let my black tank flush "run freely" for about 5 minutes, then close the dump valve, let it run for about 7 or 8 minutes (yes I stand there and use my watch to determine how long it's been), then pull the dump valve open, allowing the black tank flush to continue to operate. During this time I'll "get busy with other chores" while the tank flushes and "cleans itself".
As for the "warning not to close the valve with the flush system operating". That's a "safe process for the manufacturer" type of statement. Consider it more of a "warning not to forget" rather than an "absolute, don't do it". It's sort of like a speed limit. Try going the speed limit and see how many cars pass you. Similarly, ask around about not closing the valve and see how many campers aren't following the same "warning".....