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expansion tanks on DHW systems

how important is an expansion tank on a residential or even light commercial system? i figure it could prevent some water hammer but can't think of other major reasons to add them to every installation. can anyone explain the physics of this issue for me, and help me become informed? thanks


  • icesailor
    icesailor Member Posts: 7,265
    Expansion Tanks:

    One word comes to mind, EXPANSION of water that is heated. The expansion has to have a place to go. If there are back-flow devices installed on the water service, you will have dripping DHW relief valves that cost many $$$$ to replace.

    Others more articulate than I will give you more reasons.
  • pardon_my_ignorance
    pardon_my_ignorance Member Posts: 18
    thanks for the reply.

    yes, i'm with you on that score, but wonder if the pressure diff between street (45-60 psi) will build enough to trip a 125psi setting on the t/p valve. i'm no ttrying to cut corners, just wanting to have a complete understanding of the physics. another quetion i have is given the temp diff from street to heater, how much pressure will build? thanks if you reply again. 
  • icesailor
    icesailor Member Posts: 7,265

    It depends on if your water system uses check valves on the meter sets. EPA considers municipal water systems to be pristine and protected water. Water in a house is considered of an unknown source.. Therefore, if there is any failure of the municipal water system, they don't water of an unknown quality to be introduced into the public systems. If the check valves are installed, you need a pressure tank.

    If you don't have checks, you probably don't need a DHW expansion tank.
  • Larry Weingarten
    Larry Weingarten Member Posts: 2,961
    edited April 2012
    water heaters...

    ... cause thermal expansion.  So you're dealing with the difference between say 50 and 120 degrees and the volume of the heater and hot side piping.  If the inlet is blocked with a back-flow preventer, pressure reducer or check valve, there is a real need for an expansion tank or a relief valve set only a little higher than line pressure.  If it's not there, all the plumbing in the house gets stressed, not just the hot side. We like to avoid stress!

    Yours,  Larry

    ps.  Icesailor is plenty articulate or he would not have used such a word  :~).
  • Kevin_in_Denver_2
    Kevin_in_Denver_2 Member Posts: 588
    Toilet Valve can also solve the problem

    In researching this issue, I discovered this toilet valve:


    When the water in the tank expands, this guy will just dump a little down the drain at 80 psi, say, instead of the 150psi setting of the T & P relief valve that is on the tank. In new construction, this saves a little money.

    When Denver suddenly required backflow preventers in multifamily buildings, all my water heaters started failing. Duh. The pressure swing from 60 to 150 psi a couple times a day did them in pretty fast. I think it causes a crack in the porcelain glass coating on the inside of the tank, and the unprotected steel tank rusts out in about 3 years.

    Just another one of the many unintended consequences of code requirements.
    Superinsulated Passive solar house, Buderus in floor backup heat by Mark Eatherton, 3KW grid-tied PV system, various solar thermal experiments
  • Mark Eatherton
    Mark Eatherton Member Posts: 5,840
    Sorry Kevin...

    But I think this idea is a terrible waste of our most precious resource, and I would never encourage anyone to waste water. Bad idea.

    To the original poster, there are pressure gages with a "memory" hand that will show the highest water pressure seen. It attaches to the drain valve of the heater, and if for example, the incoming pressure is around 60 PSI, but the hand show a peak of 120 PSI, then yes, you probably need a potable water expansion tank, and it is critical to air the tank diaphragm up to the static water pressure (60 PSI in this example)

    I had this very conversation with an AHJ this last week. The customers house didn't have a back flow preventer on the incoming service. He said that the requirement for potable water expansion tanks is required on ALL installations because there is the POSSIBILITY of a replacement water meter being installed, and all new meters have a check valve in their construction, hence the universal requirement on new installs.

    It's not so much a case of "You got what you paid for", as it is a matter of "You DIDN'T get what you DIDN'T pay for, and you're NOT going to get what you thought you were in the way of comfort". Borrowed from Heatboy.
  • Kevin_in_Denver_2
    Kevin_in_Denver_2 Member Posts: 588
    I agree

    The other issue with the toilet valve pressure relief is that when it fails, nobody will bother tracking down the correct replacement valve, and then the water heater will have the same problem.
    Superinsulated Passive solar house, Buderus in floor backup heat by Mark Eatherton, 3KW grid-tied PV system, various solar thermal experiments
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