I was advised to also replace the vacuum relief valve when I recently
had to replace our home's (leaking) water heater. Since the part was
cheap enough and I'd never thought to replace it in the 23 years (and
two previous water heater replacements by me.) since its original
installation, I went ahead and bought one.

Then I started wondering just what conditions would cause enough
negative pressure to collapse a water heater tank.

*******
The description of Watts' vacuum relief valve reads:

Series N36 Watts Water Service Vacuum Relief Valves are used in water
heater/tank applications to automatically allow air to enter into the
piping system to prevent vacuum conditions that could siphon the water
from the system and damage water heater/tank equipment.
*******

Since our home is at the crest of hill, I suppose that if the city
pumping system went bad, or a water main below us burst there could be a
long enough column of water "hanging down" to pull up to one atmosphere
of vacuum in our home's supply, which is probably more than enough to
collapse a water heater tank.

A plumber I asked said that a fire engine pumping out of a nearby
hydrant could also create a negative pressure in the main.

Are those the expected kind of things a vacuum relief valve protects
against?

Thanks guys, and Happy New Year,

Jeff
--
Jeffry Wisnia
(W1BSV + Brass Rat '57 EE)
The speed of light is 1.8*10^12 furlongs per fortnight.