Why Vacuum Relief Valves on Water Heaters

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
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Jeffry Wisnia
(W1BSV + Brass Rat \'57 EE)
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I would guess that the main concern is not siphoning action collapsing the tank, but it taking enough of the water out so that either the electric elements then burn out or steam is generated with a gas unit, exploding the tank. Of course the TPR valve is there to hopefully prevent the latter.
Here in NJ, I've never seen one used, even in new construction. So, apparently here they are not required by code. The first time I ever even saw one on a water heater was on a recent episode of This Old House, when they were removing a water heater. All in all, unless I'm missing something, I don't see the need.
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snipped-for-privacy@optonline.net wrote:

I hadn't thought of that "overheating" angle, thanks.
I suppose it would require someone to open a hot water faucet and leave it open long enough for the tank to have enough of its contents sucked out through the dip tube to cause that kind of problem. Since there's no telling what sorts of things folks might decide to do, your point sounds valid to me.
It was also mentioned to me by my plumber aquaintance that present codes here in Red Sox Nation require power disconnect switches adjacent to new electric water heater installations for the same reason they've been used for a long time near outside AC units. They can prevent repair folks from getting zapped or whacked when someone else decides to flip a panel breaker back on while they're messing around inside the equipment.
I wonder what's next?
Jeff
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Jeffry Wisnia
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All new vehicles will come with no tamper GPS receivers. if tye GPS breaks the vehicle doesnt run.
it checks the speed limit on the road you are on and acting like a govenor prevents speeding.
in a chase situation where police are chasing a vehicle they send out a signal, all close by vehicles drop to 5 MPH limit
national tracking of all people begins, technically it already has with cell phones.
every citizen gets a new national ID card, in exchange for DNA sample, picture and fingerprints, plus retinal scan.
great for searching out bad guys. making credit card purachase? just one card will work for any person, with fingerprint scan and id number charge gets applied to your account. cash is becoming obsolete for better tracking.
dont laugh its all for our safety
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Ha ha ha.
Too bad it will take two people for every one of us to effectively monitor our activities to the extent that the "big brother is watching" conspiracy theorists often spout off about.
Soooo, who is paying the salaries of the 600 million "big brothers" watching the 300 million of us? Where are they hiding 600 million government employees?
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On Mon, 29 Dec 2008 10:10:21 -0800 (PST), snipped-for-privacy@optonline.net wrote:

If siphoning would suck enough water out to uncover an electric heating element, having a vacuum relief valve wouldn't help. It doesn't just make the vacuum disappear. It gets rid of it by allowing air to enter.

I don't know about gas, except the kind we can't discuss politely.

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mm wrote:

I'll have to take issue with that statement, mm.
If something going wrong with the home's water main supply causes a negative pressure at the heater's cold water inlet and a hot water faucet is opened, water could get sucked out of the tank (and the hot water pipes coming from the open faucet) through the heater's dip tube, lowering the level in the tank down to the bottom of the dip tube, which will certainly expose the element(s).
A vacuum relief valve teed onto the heater's inlet pipe "breaks" the vacuum right there so water won't get sucked out of the tank.
Capice, mm?
Jeff
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Jeffry Wisnia
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On Tue, 30 Dec 2008 00:16:49 -0500, Jeff Wisnia

I get it. Thanks.
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mm wrote:

I had someone tell me today that some heater's dip tubes have a small hole near their top to prevent sucking down the water level in the tank through the inlet fitting. That could prevent the burnout problem that trader4 brought up, though it wouldn't stop a collapse of the tank under the "wrong" conditions.
Jeff
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Jeffry Wisnia
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