Changing hot water heater today: ... do I need expansion tank on domestic side?

Discussion in 'Water Heater Forum, Tanks' started by staceyneil, Feb 6, 2011.

  1. staceyneil

    staceyneil New Member

    Mar 2, 2007
    cumberland foreside, maine
    We currently have an electric hot water heater installed in our domestic HW system. We are changing to an Amtrol CH41BZ. The instructions for the Amtrol call for an expansion tank to be put into the cold water supply line. Our current electric HW heater does not have one installed and we have no problems. Do I need the expansion tank?
  2. Redwood

    Redwood Master Plumber

    Dec 15, 2007
    Service Plumber
    If the domestic water supply in your home is a closed system you will need an expansion tank...
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  4. ballvalve

    ballvalve General Engineering Contractor

    Dec 28, 2009
    "retired" and still building and troubleshooting
    northfork, california
    Here we go again! Someone give him a link to the previous cluster f&**^


    At 60 pounds pressure, water boils at 307.4 degrees
    At 0 pounds pressure, water boils at 212 degrees
    At 50 pounds pressure, it releases the same energy as two pounds of dynamite.
    Last edited by a moderator: Apr 3, 2011
  5. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Sep 2, 2004
    Retired Systems engineer for defense industry.
    New England
    As said, you need one IF your home's water system is closed. A closed system means water can come in but is blocked from going back out. This can be caused by a check valve or a PRV (pressure reduction valve, even if it has a bypass). If you do not have a closed system, you do not need one unless your local codes require it (some do). The reason is that when the dense, cold water goes into the tank, then gets heated, it expands. Since few of the pipes are elastic (say the washing machine hoses, and some supply lines to the toilet or faucets), but most of it is pretty solid, the pressure rises almost instantly. Some stuff can swell, and some may leak, which relieves that increased volume and therefore the pressure. The definitive way to tell if you need one is to buy a $10 water pressure gauge, put it on say a hose bib or the drain of the WH, and leave it there for 24-hours. If you get one with a tattle-tale (peak reading hand), and it rises above your normal pressure, then yes, you need one. If it doesn't rise, you probably don't unless codes require it. Note, many water utilities are going around and putting new meters and check valves in. This could change your system from open to closed and you may not know about it. If you had a functioning expansion tank, you'd never know. If you don't, you'll probably get discharge from the safety T&P valve on the WH. Depending on where that is, you could get water and mess up the floor or walls. It isn't much, but it happens after any significant hot water useage.

    Technically, code requires items to be installed per the manufacturer's instructions. SO, even if you wouldn't normally need one, if an inspector read the installation instructions, he would require one to be installed, because the manufacturer requires it (if I understand what you said from the instructions). Now, they rarely read the installation instructions....
    Last edited: Feb 6, 2011
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