Vaccum Relief valve

Discussion in 'Plumbing Forum, Professional & DIY Advice, Tips & ' started by Randyj, Jan 8, 2007.

  1. Randyj

    Randyj Master Plumber

    Messages:
    1,047
    Location:
    Alabama
    I've only seen one water heater with the vaccum relief valve and am just ignorant about it's use or why it is there.... would someone just explain it to me....or direct me to a good reference. I've seen some in strange places on institutional plumbing and never understood why they were in the plumbing. I assume they are to stop the water from siphoning out of the water heater if the incoming cold water line were opened. I'm kind of trying to understand why it comes after rather than before the expansion tank.

    Thanks Steve for posting the picture!
    [​IMG]
    Last edited: Jan 8, 2007
  2. hj

    hj Moderator & Master Plumber Staff Member

    Messages:
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    Location:
    Cave Creek, Arizona
    vacuum vent

    It has limited effectiveness when the heater is in a basement or at ground level. When the heater is in an attic, however, if the cold water is shut off and a faucet is opened to relieve the pressure, the water could be siphoned out of the tank, if a hot water faucet is also opened, but most heater dip tubes have an opening in them to prevent more than a gallon or so of water from being pulled out. The main reason for it, is that the weight of the water in the cold pipe can create a vacuum, (and the strength of the vacuum is in direct proportion to the distance the heater is above the faucet that is opened), which, in theory, could collapse the heater's tank.
  3. Randyj

    Randyj Master Plumber

    Messages:
    1,047
    Location:
    Alabama
    Collapse a water heater tank???? ROFL... man that's goofy. That would be some more bodacious vaccum.
  4. hj

    hj Moderator & Master Plumber Staff Member

    Messages:
    26,679
    Location:
    Cave Creek, Arizona
    tank

    Like I said, it is in theory, but if you ever saw a tanker, (gas or milk), truck which had a collapsed tank because the driver did not open the lids or vents, you would understand that it could happen under the right circumstances.
  5. Randyj

    Randyj Master Plumber

    Messages:
    1,047
    Location:
    Alabama
    Well yah.... 3,000 gal tank with wall thickness the same as a 40 gal water heater??? Duh... ya reckon? I bet you'd need spars every foot or so to keep a tanker from collapsing after riding in hot sun with vent open the close it up in the evening with about a 1/2 tank... I see that with a 5 gal gas can if I tighten the lid.
  6. kordts

    kordts In the Trades

    Messages:
    551
    Location:
    exurban Chicago
    There is a water tower near Rockford, Illinois that partially collapsed on itself. What started it was a water main break, some other stuff happened and it pulled a vacuum.
  7. GoTanklessToday

    GoTanklessToday In the Trades

    Messages:
    89
    Location:
    Renton, WA
    Some may remember the old Marathon water heaters. They didnt have steel glass lined tanks like standard tank heaters. Sears used to sell these as their lifetime warranted tanks. They came with a vacuum breaker in the box with the heater for that exact reason. To prevent the "synthetic" tank from collapsing. Washington's adaption of the UPC calls for a vacuum breaker on a tank style heater any time there is plumbing fixtures below the water heater. "Below" is interpreted widely, it can mean a floor below, or in some cases like when a water heater may be installed on a platform above a sink, it would be required. Some crafty commissioned plumbers have decided that the drain valve on a water heater is a "fixture" and is "below" the heater, and they use that theory to extract more of their customer's money.
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