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Thread: Necessity of Vacuum Relief Valve - Not Pressure Relief Valve

  1. #1
    DIY Junior Member WillieK's Avatar
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    Default Necessity of Vacuum Relief Valve - Not Pressure Relief Valve

    Hi everyone,

    My new water heater states: "Install vacuum relief in cold water inlet line as required by local codes". My inspector says I need one. I read on one of the old posts that you only need one if your water heater is on an upper floor or in the attic. Mine is in the attic. What is the purpose of a vacuum relief valve? What could possibly happen if I didn't install one - other than the inspector killing me?

    Thanks in advance for your help.

  2. #2
    Forum Admin, Expert Plumber Terry's Avatar
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    It prevents siphoning of the water heater.
    If the water main is shut off to the home, and then a faucet is opened, it will siphon the water heater. When that happens, the top element burns out in seconds. With a non-functioning top element, you now have no hot water.

  3. #3
    Master Plumber master plumber mark's Avatar
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    Default not needed

    I have not heard that one in a long time....
    Inside most heaters today there is a port hole on the
    dip tube inlet at the top of the tank that prevents the
    heater from siphoning....... a heater cannot siphon down

    the valve they want you to install is not necessary


  4. #4
    Moderator & Master Plumber hj's Avatar
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    quote; Inside most heaters today there is a port hole on the
    dip tube inlet at the top of the tank that prevents the
    heater from siphoning....... a heater cannot siphon down

    the valve they want you to install is not necessary


    It is NOT to prevent siphoning. If the water supply to the building is turned off and the water is drained out, the tank will be subjected to "severe" negative pressure, i.e., a vacuum, and the amount of the negative pressure will be in direct corellation to how high the heater is above the drain point. The heater will withstand 300 psi of pressure but relatively LITTLE negative pressure so it will "collapse". If you have you ever seen a tanker truck that the driver opened the drain hose before he opened the lids, you have seen a collapsed tank and that was just the suction of a few feet between the truck and the ground. In Texas there was a city water storage tank that "imploded" and it was VERY impressive to see it look like an accordian. If you heater is in the attic, you DO need the vacuum relief valve, just as a safety measure.
    Licensed residential and commercial plumber

  5. #5
    DIYer, not in the trades LLigetfa's Avatar
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    Default

    Ja, hj has the right answer. A vessel designed only for pressure should not be exposed to a vacuum. There have been documented cases of various tanks collapsing from the improper implementation of a booster pump as well. Water softeners have also collapsed from an improper drain line installation.

  6. #6
    Retired Defense Industry Engineer jadnashua's Avatar
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    THey're required where I live, regardless of where it is installed, as is a tempering valve. Really pisses off uninformed people when it comes time to replace their WH (assuming they get a permit), as it can add some to the installed price.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  7. #7
    Moderator & Master Plumber hj's Avatar
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    quote; THey're required where I live

    That is the problem with beaurocrats, a special interest tells them how to solve a certain "occassional problem" and they decide to make it a universal solution. We have the same thing here with pressure reducing valves. Most areas do not have the pressure that would require them, but since they do not want to address just the areas where a PRV is needed, they just passed an ordinance that EVERY house must have one.
    Licensed residential and commercial plumber

  8. #8
    Master Plumber master plumber mark's Avatar
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    Cool what????

    HJ......

    I dont have a clue as to what you guys are referring to
    so please inform me as to what this is all about ...are you speaking about
    residential or commercial systems actully imploding??

    I have yet to see a water heater on the higher floors of any building
    "implode" because they have drained down all the water....
    Inside all heaters there is a dip tube with a anti siphon port hole
    to keep this from happenning...... and I doubt it could

    I would like to have an imploded heater for my store front window
    if one actually exists..... that would be an extremely rare find.....


    back a long time ago , this was something that was required on
    drain down solar panels. I had to tangle with some "solar enjineers"
    on this subject becasue they were so un-reliable that they would
    leak and had to be changed out all the time due to any pressure fluctuations
    in the system.......


    this vaccuum relief valve will only cause troubles and probably leak all
    over someones attic yearsdown the road when the system is turned off when
    the poor plumber attempts to change out a ballcock or hose bib.......

    this is just my humble opinion

  9. #9
    Master Plumber master plumber mark's Avatar
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    If anyone has a water heater that has imploded or even a picture of one
    I would really like to have a picture of it for my web site....

    I feel like I am asking for a real picture of BIG FOOT

    send me those pics..... I

    am gonna Google imploded heaters and see what I can find


    http://www.homeownershub.com/plumbin...ater-2077-.htm


    send me what you got HJ

  10. #10
    Moderator & Master Plumber hj's Avatar
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    Default tank

    Not a water heater but the same thing happens on a smaller scale with a water heater, since it also does not have internal baffles, (which pressure vessels such as a sumbarine do have to resist collapse from exterior pressure), to strengthen it.
    http://www.ebaumsworld.com/video/wat...?autoplay=true

    MPM, the "hole" is to prevent siphonage, NOT suction. Siphonage will only occur if a faucet is opened to let air into the system. Vacuum depends on the entire system being closed/sealed. In fact, if a faucet were opened so the hole stopped siphonage, it would ALSO break the suction and eliminate the need for a vacuum relief valve.
    Last edited by hj; 11-14-2012 at 07:40 AM.
    Licensed residential and commercial plumber

  11. #11
    Forum Admin, Expert Plumber Terry's Avatar
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    Mark,
    Often when I'm draining down the house to do a pipe repair or to replace shutoffs, I loosen the flex connectors on the water heater to prevent the tank from siphoning. If it's electric, I just turn off the breakers at the panel so I don't let the water drop too low and burn out the top element.

    Siphoning happens. We just take the right steps, knowing that.

    In the city of Bellevue, if the electric heater is on the first floor, the vacuum breaker is not required, only if it's on a second or higher story.


    Water heaters will siphon!
    Last edited by Terry; 11-14-2012 at 10:15 AM.

  12. #12
    Master Plumber master plumber mark's Avatar
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    Default Terry ...you have a firm grip on the obvious

    Quote Originally Posted by Terry View Post
    Mark,
    Often when I'm draining down the house to do a pipe repair or to replace shutoffs, I loosen the flex connectors on the water heater to prevent the tank from siphoning. If it's electric, I just turn off the breakers at the panel so I don't let the water drop too low and burn out the top element.

    Siphoning happens. We just take the right steps, knowing that.

    In the city of Bellevue, if the electric heater is on the first floor, the vacuum breaker is not required, only if it's on a second or higher story.


    Water heaters will siphon!

    Terry ...you have a firm grip on the obvious

    I completely agree with you terry..... yes they will siphon
    till they hit the siphon hole on the dip tube in the heater..
    and sometimes its not a bad idea to open the t+p valve
    or something like you stated


    YES a water heater will siphon,

    but unless you put the heater to a test in an experiment
    to see how much negative pressure you need to use to make it
    implode..... the odds are it will never , ever implode. in a real world
    situation.....,,,


    I am gonna go get my power ball tickets now...
    its up to 169 million and my odds are better to win
    the power ball than see a water heater implode in my lifetime...



    on another note..... how have you been doing lately???
    feeling ok and all???.

  13. #13
    Forum Admin, Expert Plumber Terry's Avatar
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    I'm not worried about one imploding.
    Hey, I like this big text.


    Feeling good. I spent a few days doing a repipe in a crawl. That gave my stomach muscles a workout. Just waiting for snow to fall so I can start using my skis.


    Last edited by Terry; 11-14-2012 at 04:51 PM.

  14. #14
    DIY Member tom12's Avatar
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    Not sure if i grasp hj's argument, but for what it's worth:

    1. the 1/8th hole near the top of the dip tube is called an anti-siphon hole ( for occasions of negative pressure ) and described as such by some mfr's of dip tubes and water heaters.

    2. siphonage, as i understand it, is suction and gravity at work.

    3. It's recommended ( can't remember by whom ) that bottom supplied water heaters ( which, obviously, have no installed dip tube ) have a vacuum-relief valve installed,no matter the location of the water heater.

    4. A number of local codes in the Cal. Bay Area require vacuum-relief valves in attic installations - but i dont recall even ever seeing a vac. relief valve except in pics.

  15. #15
    Retired Defense Industry Engineer jadnashua's Avatar
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    WHere I live, if you want to pass code, you need a vacuum relief valve and a tempering valve on any new or replacement WH installation.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

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