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[BB_TX]

I agree with you that it is highly unlikely black water can come back out the inlet. But it is not there to protect against an over full tank. Antisiphon valves are required in any application where a loss of supply water line pressure and the resulting siphoning occurs that can suck all the water back out of an open line. Think of when you lose water pressure at home and you end up with air in all your water lines because the water was siphoned out due to the negative pressure being pulled. But home inside water connections have no direct connection to contaminated water and an antisiphon valve is not required. But if you have a lawn sprinkler system it requires an antisiphon valve as it could suck dirty water back into the line.

The only way a black tank flush could send contaminated water back into the water supply lines is if you had no antisiphon valve, the spray head was or had been immersed in black water, you started doing a flush, and the water supply suddenly lost pressure and began pulling a vacuum on the line and sucking that water out of that line. The odds of all that coming together are very slim. Slim but possible.

[beeje]

I agree that is entirely possible (but un likely), when I flush mine out (like most others) I am standing right there listening and observing what is happening. I would more than likely notice the reduction of water pressure flowing in. I suppose in the politically correct world we live in a backflow preventer would be in order.