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

I have never seen a vacuum break on a water supply
line to a hot water tank, and it sounds completely
useless.

First if your fire truck scenario even worked, it
would be the responsibility of the water system to
install backflow preventers at the turnouts to the
houses. Second, collapse of a tank is virtually
impossible because I don't know how you would be
able to suck the water out through the supply
line. Water has a relatively low adhesion rate,
so a strong enough pull (vacuum) would break the
water column.

Second, if hot water tank lines did have vacuum
breaks and there were no back flow preventers and
the firetruck made the water flow "backwards,"
then the truck would soon be pumping water and air.

What would happen to the water if one end of a
tangled system of pipes filled with water is
closed and a strong vacuum is put on the other
end? Nothing significant. Yes if you got a low
enough pressure the water would boil and vapor
would be slowly drawn off.

You are not going to crush a water tank (or any
part of your water system) by putting a vacuum on
the system.