Draining system to eliminate water hammer.

Discussion in 'Plumbing Forum, Professional & DIY Advice' started by Terfer, Nov 27, 2008.

  1. Terfer

    Terfer New Member

    Messages:
    2
    I have received instructions on how to drain my domestic system and then re-charge in order to eliminate water hammer in the system.
    The procedure calls for shutting off the main supply and then turning on all faucets, water feeds etc. and then shutting off the water supply to the hot water tank. (gas-fired).
    My question is, will this not empty the tank, and if so, won't it be damaged by the gas heating system trying to heat the empty tank?
    Last edited by a moderator: Dec 1, 2008
  2. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

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    Way back when (and unfortuneately still today) people put a capped vertical stub of copper pipe near valves thinking that they would stop water hammer. Well, they work for a short while, but then fill up with water as the air is absorbed in the water. Basically, they are a short-term solution. The thing that does work is an engineered hammer arrestor. These either are a chamber with a bladder filled with air, or a piston with a seal and air on the other side. In either of those, the water never touches the trapped air, and those can work for years before they wear out. So, if you have water hammer rather than some other problem you've misdiagnosed what you are planning won't work for long.

    But, if you want to drain the system to introduce some air to be trapped in those short pipe stubs, first, turn the water heater off. then, if it is drained, you won't ruin it. Don't turn it back on until you've turned the water back on then opened each faucet (hot and cold) so you get a full flow of water without air...then you can relight the water heater without fear that it is going to burn up because there is no water in it.
  3. Redwood

    Redwood Master Plumber

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    Location:
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    Don't bother! The draining will not work for long. Install hammer arrestors where needed.

    Have you checked your water pressure with a gauge? You should!
  4. hj

    hj Moderator & Master Plumber Staff Member

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    Location:
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    air chambers

    Don't even bother trying to drain them. The air chambers are a capped riser. Therefore,
    1. You will NEVER drain all the water out of the system so that there will be air at the entry of every air chamber.
    2. Because it is a sealed riser, there is no way for air to rise to the top of it so the trapped water will drain down. This is a fact of life whether it is an air chamber, a water heater, or a pop bottle.
  5. Gary Slusser

    Gary Slusser That's all folks!

    And unless you drain the water heater with its drain valve, there is no way to get the water out of it once you shut off your water to it, or the house, and open a faucet to relieve the water pressure, even if the heater is in an attic; it won't siphon out.
  6. Terry

    Terry Administrator Staff Member

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    Location:
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    Well, actually then can.
    In the city of Bellevue new electric water heaters are required to be plumbed with vacuum relief valves so that they won't siphon.

    Once the top element hits air, it burns out in seconds.

    [​IMG]

    http://www.watts.com/pro/_productsFull_tree.asp?catId=64&parCat=331&pid=815&ref=2

    If you have water chambers previously installed, draining down the home will replenish the air in them for a little amount of time.
    So little that now mechanical hammer arrestors have become code.
    Those won't waterlog.
    http://www.siouxchief.com/B_WaterHammer_Fall.cfm

    Attached Files:

    Last edited: Nov 28, 2008
  7. hj

    hj Moderator & Master Plumber Staff Member

    Messages:
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    Location:
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    heater

    The heater can "siphon" out under the right/wrong conditions, but it will not drain out unless the drain valve is opened, AND air is admitted into the top of the heater.
  8. nhmaster

    nhmaster Master Plumber

    Messages:
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    S. Maine
    Most fairly new water heaters have the vaccuum relief built into the dip tube in the form of a small hole drilled about 2" down.
  9. Gary Slusser

    Gary Slusser That's all folks!

    Hey Terry, thanks, I didn't know about the new water heaters having a vacuum breaker/relief valve. But you have a typo, it should be so they will siphon, not so they won't siphon.
  10. Terry

    Terry Administrator Staff Member

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    15,121
    Location:
    Bothell, Washington
    hj and Gary,
    The water heaters are the same, we just install a tee on the incoming cold side and put the vacuum relief on it to prevent waster from the dip tube from going backwards up into the cold supply line when the main is shut off.

    If the water heater doesn't have the vacuum relief, I sometimes loosen the incoming cold flex connector at the top of the tank to achieve the same purpose. That will also prevent the water heater from draining through the cold supply.

    They do, good.
    Now I need to check with Bellevue and see if that works for the inspectors.
    That would be a lot quicker for me.
    It also now makes sense why some heaters siphon and some don't.
    Last edited: Nov 28, 2008
  11. Terfer

    Terfer New Member

    Messages:
    2
    Still confused

    Thanks for all the replies, but I'm still confused.

    Simply put, is it safe to drain the house system without shutting off the gas to the hot water tank?

    Terfer.
  12. Terry

    Terry Administrator Staff Member

    Messages:
    15,121
    Location:
    Bothell, Washington
    Gas?
    It should be fine.
    The only reason I worry a bit about the electric ones is the elements.
    And most of the time, that isn't an issue either.
    If you are just shutting down, and bringing back up, the tank never really drains.
  13. hj

    hj Moderator & Master Plumber Staff Member

    Messages:
    26,815
    Location:
    Cave Creek, Arizona
    heater

    Water heaters had the opening in the dip tubes back in 1955, and have had them since then also. It minimizes the possibility of siphonage, but is not a perfect cure since it can scale over and become ineffective. I do not see a typo, since the vacuum relief valve is to PREVENT it from siphoning, not LET it siphon.
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