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