Bob Engelhardt wrote:
Martin H. Eastburn wrote:
...
Many places have anti-back flow devices at the street ...

I don't know how many "many" is but I would say that many do not. I say
this because heater installation instructions call for a vacuum breaker
on the supply line. This prevent tank collapse if water is sucked out
the supply line, as when a fire truck is pulling water from a hydrant.
In this case water is flowing "backwards" to the street.

My $.02,
Bob

Bob
Unless we are using hard suction hose we cannot draw the pressure down
below about five pounds before the hose will collapse and stop the flow.
Even with "Hard Sleeve" the maximum pressure reduction a modern fire
pumper in good condition can achieve is roughly two thirds of
atmospheric. Would negative ten pounds actually collapse a water heater
tank?
--
Tom Horne

Well we aren't no thin blue heroes and yet we aren't no blackguards to.
We're just working men and woman most remarkable like you.