Quote:
Originally Posted by beeje
............ They are not needed as long as there's a high Loop in the drain line
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That is not true. The purpose of an anti siphon atmospheric vent is to break the vacuum to prevent the siphoning of water back thru a pipe. The siphoning would occur in the event of loss of water pressure in the public supply line. When pressure is lost in the main supply lines, the water begins to flow out the lowest places in the line as people open faucets or flush, pulling a vacuum on the upper parts of the line. That vacuum can pull water out of feeder lines going to buildings, outside watering systems, and yes, RVs. That is why you have air in your home water lines after the public water supply has been shut off. It would siphon water even from an elevated loop if there were no vent to relieve that negative pressure. Highly unlikely the specific conditions would be right for it to pull any black water contaminates from an RV flush system, but possible. And that is why it is mandated. If it is possible, someone will find a way to do it.
Think of siphoning gas from a car gas tank. You run a hose down the filler tube. It goes up in a higher loop and then down into some container (or the ground). And the outside end of the hose is lower than the hose end in the tank. You suck on the hose to create a vacuum to pull gas up thru the hose and down to the lower outside part of the hose. And then the siphoning will pull gas up thru the loop and down and continue to run out the outside end.