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Thread: Expansion tank size problem

  1. #1
    DIY Junior Member Austin83's Avatar
    Join Date
    May 2011

    Default Expansion tank size problem

    I am getting a PRV installed due to high pressure coming into my house. I have two 50 gallon hot water heaters that are right next to each other and connected. The plumbing supply store has a 4 gallon, or a 20 gallon expansion tank, and they told me the 4 gallon expansion tank is for an 80 gallon hot water heater, but they think that it will be just fine for my situation. The difference in price is astronomical between those two tanks, and the local Lowes and Home Depot carry a 4.4 gallon tank. My choices are limited, and I'm sure any hired plumber in this area would slap a 4 gallon tank on it and be done with it, but I'd like to know if this is going to be alright for my home. Any information or help is greatly appreciated.

  2. #2
    Moderator & Master Plumber hj's Avatar
    Join Date
    Aug 2004
    Cave Creek, Arizona


    as a practical matter, almost any size expansion tank will be all right, as long as the air precharge is correct. Air is a gas, so it is impossible to "squeeze" it into "nothingness". Therefore there will ALWAYS be an air bubble in the tank to absorb the expansion.
    Licensed residential and commercial plumber

  3. #3
    Retired Defense Industry Engineer jadnashua's Avatar
    Join Date
    Sep 2004
    New England


    I think they're way off...go here: http://www.watts.com/pages/support/sizing_DET.asp and calculate what you need for yourself. The smaller one should be fine.
    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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