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

I have seen plenty of them. As to "useless", I can't personally say.

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.

Yeah, right.

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.

Don't take "vacuum" too literally. They would more accurately be called
"negative pressure preventers". They let air in as soon as the water
pressure is less than atmospheric. So there is still absolute positive
pressure in the pipe and on the water.

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

Water heaters are designed to hold pressure. It is very easy for a thin
wall tank to hold pressure (e.g., soda bottle). A thin wall does not
hold negative pressure ("vacuum") at all. I don't have figures, but I
would guess that a couple of psi would crush a tank. For example, a
tank 50" high and 30" in circumference has an area of 1500 sq in. 1 psi
would exert 1500 pounds force on the tank.

Awaiting an experienced reply,
Bob