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Thread: Pressure regulator install for water hammer

  1. #1
    DIY Junior Member boaterbill's Avatar
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    Smile Pressure regulator install for water hammer

    Background info:
    Water dept says they deliver 80 psi. Turning off all cold water faucets quickly, fluidmaster 400 toilet valve and washing machine cause hammer. I
    have a Bosch on demand gas tank-less water heater. Water meter located in utility room, mount vertically 2' above concrete floor with 60 year
    old shutoff valve below it. Shutoff valve does not fully stop water flow, while closed fine stream of water coming out. Water department shutoff
    4 feet below ground somewhere in the yard.

    The Plan:
    Close in house shutoff valve ( fine stream of water still flows in closed position) best I can and install threaded 1" ball valve on out flow threaded
    nipple to become working shutoff valve. There is a floor drain 2 feet away. From this point continue on to install PRV. Since I have tank-less
    water heater would I still need an expansion tank? Would this stop the water hammer?

    Would lowering the water pressure affect the operation of the tank-less WH?

    Thanks in advance. All comments welcome.

    Boater Bill NE Ohio
    Last edited by Terry; 05-03-2013 at 05:09 PM.

  2. #2
    Jack of all trades DonL's Avatar
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    "would I still need an expansion tank?"

    I would say Yes, to do it properly.
    Last edited by DonL; 05-01-2013 at 11:55 AM.
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  3. #3
    In the Trades Gary Swart's Avatar
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    Yes for sure on the expansion tank. The PRV will create a "closed" system which means when the water heater does its thing, the water will expand and have no place to go. The result will mean the T/P on the water heater will open every time the water heater is on. 80 psi is on the high end of acceptable, and is quite a bit higher than necessary. I have no words of wisdom on the tankless water heater, but I would guess that 50 to 60 psi would be plenty.

  4. #4
    DIY Junior Member boaterbill's Avatar
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    I should add that I will be using the Watts 25AUB-Z3 which has a bypass valve to allow for expansion back into the line. Would I still need the expansion tank with the PRV having this feature?

  5. #5
    In the Trades Jerome2877's Avatar
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    No, you would NOT need an expansion tank!

    The on demand water heater only heats the water when it has water flowing through it and when the flow stops it shuts down. Then the heated water that is still in the pipes will cool so the pressure would never increase past whatever the PRV is set to.

  6. #6
    Jack of all trades DonL's Avatar
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    Quote Originally Posted by Jerome2877 View Post
    No, you would NOT need an expansion tank!

    The on demand water heater only heats the water when it has water flowing through it and when the flow stops it shuts down. Then the heated water that is still in the pipes will cool so the pressure would never increase past whatever the PRV is set to.

    Water Hammer can work on Hot and Cold water.

    What is your theory to back up your claim ?


    I guess I may not understand.
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    In the Trades Jerome2877's Avatar
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    Quote Originally Posted by DonL View Post
    Water Hammer can work on Hot and Cold water.

    What is your theory to back up your claim ?


    I guess I may not understand.
    I am talking about thermal expansion and the lack of it in an on demand water heater.

    Water hammer is caused by water stopping quickly in the pipe and is best controlled with a schock arrestor as close as possible to the end of the run. Sometimes a PRV will help stop or lesson this but an expansion tank is not designed for stopping water hammer.

  8. #8
    Moderator & Master Plumber hj's Avatar
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    quote; The PRV will create a "closed" system which means when the water heater does its thing, the water will expand and have no place to go.

    When water is flowing there is no "closed system", and since the water heater only operates when water is flowing, thermal expansion is not possible and thus an expansion tank is not needed. HOWEVER, an expansion tank, which is just a very large "shock arrestor", MAY absorb pressure spikes from turning off faucets too quickly, depending on where it is installed. A PRV may NOT stop the water hammer unless your pressure is set so low that it is unacceptable.
    Licensed residential and commercial plumber

  9. #9
    DIY Junior Member boaterbill's Avatar
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    Quote Originally Posted by hj View Post
    quote; The PRV will create a "closed" system which means when the water heater does its thing, the water will expand and have no place to go.

    When water is flowing there is no "closed system", and since the water heater only operates when water is flowing, thermal expansion is not possible and thus an expansion tank is not needed. HOWEVER, an expansion tank, which is just a very large "shock arrestor", MAY absorb pressure spikes from turning off faucets too quickly, depending on where it is installed. A PRV may NOT stop the water hammer unless your pressure is set so low that it is unacceptable.
    Thanks for the response and thanks to all that have posted.

    I'm dealing with a 65 year old copper supply system which has been very trouble free. This is a post WW2 little spec.veterans home, may be 650 sq. ft. on a slab; 1 bath and kitchen. All exposed plumbing is in utility room 5' x 12'; furnace, washer, dryer, utility sink, tank-less WH are all in here. It's between kitchen and bathroom. I cannot afford any major plumbing failures or it would destroy everything on the ground floor.

    I agree that if PRV is installed it may not stop the water hammer. The water hammer now will trip the tank-less WH safety valve. The Fluidmaster tank valve seems to trigger this when the toilet tank valve stops filling. I have the tank valve barely open to stop this.

    I think the expansion tank will arrest the hammer as you suggest but would it just create a link in the system that will fail in time and flood the house? Would small arrestors at each delivery point be more fail safe in the long term? Just looking for some long term peace of mind.

  10. #10
    Jack of all trades DonL's Avatar
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    You may just want to try it after you get the pressure lowered, and just see how it works.

    A expansion tank may be a bit of a overkill, but would work.

    http://www.pexsupply.com/Watts-02695...stor-8729000-p


    Good luck.
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  11. #11
    Moderator & Master Plumber hj's Avatar
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    Individual arrestors, when they work, will ALWAYS be better because you can install them at the "hammer causing" locations. One central "arrestor", or expansion tank, MIGHT do it, since most of your plumbing seems to be localized, but there is no guarantee.
    Licensed residential and commercial plumber

  12. #12
    DIYer, not in the trades LLigetfa's Avatar
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    Yes, at the end of the line is where they do the most good. Water cannot be compressed so consider the line of water as being a mass with inertia. The gas inside the arrestor compresses and works like an airbag, absorbing the impact.

  13. #13
    Jack of all trades DonL's Avatar
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    You can build a arrestor using a Piece of PVC pipe and a T and a end cap.

    But when it gets waterlogged you have to drain the water out of it.


    You may need a special permit to buy PVC pipe and end caps at the same time, now a days.
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  14. #14
    Moderator & Master Plumber hj's Avatar
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    quote; But when it gets waterlogged you have to drain the water out of it.

    Good luck "draining the water out of it", unless the entire system was set up with that in mind.
    Licensed residential and commercial plumber

  15. #15
    DIY Junior Member boaterbill's Avatar
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    I picked up a water pressure gage and the water pressure is 80 psi. When cold water is turned off at all locations it jumps to 140 psi. The hammer noise is coming from inside the tank-less water heater. If I shut off cold water valve to the tank-less water heater all is quiet.

    I'm thinking my only solution would be as Gary Swart and hj suggested, PRV and the expansion tank. What about PRV first and see what happens, then add expansion tank? Placing expansion tank may be a problem. Could I place it 10' away and still be beneficial for water hammer?

    Will the banging inside the tank-less destroy it?

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