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Do all hot water tanks need an expansion tank?

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cwilliams2000
cwilliams2000 Member Posts: 140
in THE MAIN WALL
I haven't ever had an expansion tank on my DHW tank. Only on my boiler. But I see there are tanks made for potable water. Can someone tell me why people need these or don't need them? Thank you in advance.

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  • Danny Scully
    Danny Scully Member Posts: 1,424
    edited October 2017
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    We install them on all our water heaters now (and mixing valves). The expansion tank does as its name suggests. It allows for expansion in the domestic water piping. Without an expansion tank, any domestic water expansion will cause your relief valve to drip. While you may have never had one, it’s just good practice.
  • leonz
    leonz Member Posts: 1,127
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    I guess I better have one in mine then. I always thought that they were used with municipal water supplies.
  • JUGHNE
    JUGHNE Member Posts: 11,061
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    Has your WH tank relief valve ever dribbled or opened?
    Typically in muni water systems you might have no backflow or check valve on your cold water supply. As your water heats and expands it pushes back into the cold supply. (you can notice this with the cold inlet becoming hot if not much hot water is used.) It doesn't expand tremendously but water will not compress and any minor expansion will raise the pressure a fair amount in any system.

    But if you are on your own well water you have some form of expansion tank to supply water when the pump is not running.
    That gives you the air cushion to allow a little water to expand and back up into your cold supply.

    If some form of check valve/back flow device is installed in the cold water supply on your system then you need some form of expansion tank.
    Just another item in the system that could be troublesome. I would not have one if not needed.
  • Jamie Hall
    Jamie Hall Member Posts: 23,286
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    JUGHNE said:

    ...
    If some form of check valve/back flow device is installed in the cold water supply on your system then you need some form of expansion tank....

    That is the key to it. Trouble is, it can sometimes be a bit difficult for a person not experienced in plumbing to tell if there is -- and there is no absolutely iron clad rule. Many municipal systems do not have a check valve or backflow preventer or pressure reducing valve on the supply, but increasingly they do, and if yours does you have to have an expansion tank. Almost all private water systems have pumps from a well; they will have a check valve and must have an expansion tank in the system -- which mostly controls the pump. There are a few private water systems, though, which are gravity feed and don't need one at all.

    An expansion tank isn't that much of a hassle -- if I were in doubt, I'd install one.

    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • EBEBRATT-Ed
    EBEBRATT-Ed Member Posts: 15,524
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    Put a tin can under your hot water tank relief valve. Check it occasionally. If there's water in it you need to check your water pressure and determine if you have a problem and you may need an expansion tank as part of the fix.

    If your relief valve never has had to relieve water pressure you don't need one. Just another thing to go wrong.

    I have had a gas hot water heater for 31 years...no expansion tank.

    If your water supply has a check valve or a PRV reducing valve...then you need an expansion tank
  • HVACNUT
    HVACNUT Member Posts: 5,835
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    In ground sprinklers also require a check valve, so a domestic tank should be installed.
    The tank should be pressurized to match the incoming street pressure.
    They also help control water hammer.
  • L Thiesen
    L Thiesen Member Posts: 54
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    Why would a sprinkler system need an expansion tank? I have never seen that done but then Ive only been a plumber for 45 years and I still haven’t seen everything.
  • Danny Scully
    Danny Scully Member Posts: 1,424
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    @EBEBRATT-Ed, you’ve had you direct water heater for 31 years?! Guess you’re a gambling man :wink:
  • HVACNUT
    HVACNUT Member Posts: 5,835
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    > @L Thiesen said:
    > Why would a sprinkler system need an expansion tank? I have never seen that done but then Ive only been a plumber for 45 years and I still haven’t seen everything.

    Sorry, the domestic side IF you have a sprinkler system.
  • Jamie Hall
    Jamie Hall Member Posts: 23,286
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    HVACNUT said:

    In ground sprinklers also require a check valve, so a domestic tank should be installed.

    The tank should be pressurized to match the incoming street pressure.

    They also help control water hammer.

    Some jurisdictions a reduced pressure zone BFP.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • EBEBRATT-Ed
    EBEBRATT-Ed Member Posts: 15,524
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    @Danny Scully well the hot water tank has been changed 2 or 3 times but still no expansion tank.
    In fact the tank in there now is is a 2003 8 year tank. So my fingers are crossed. I shut the water off if I am not around. It's going to pop any day now. Sump pump works too LOL
  • Larry Weingarten
    Larry Weingarten Member Posts: 3,293
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    Hello, Why not have a look at the anode? If there is much of anything left of it, you likely can keep the heater going. I've gotten over fifty years from conventional glass-lined tanks. ;)

    Yours, Larry
  • SlamDunk
    SlamDunk Member Posts: 1,580
    edited October 2017
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    I installed one with my new water heater last week. Meter has a check valve. Expansion of a liter or two wouldnt compromise my plumbing but it may if you have plastic plumbing, any plumbing with a compromised joint, old kitchen appliances or old faucets. I never had a problem .
    I just did it because my interpretation of the code said to.

    I pressurized mine toa little less than regulator psi, not city pressure. did i do it wrong?
  • BenRestin
    BenRestin Member Posts: 3
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    In Arkansas the code in 2003 required an expansion device, expansion tank, a ballcock that would discharge higher pressure, Watts made one, or a bleed off valve that piped off with 1/4 OD tube. Several water heater manufactures deep down in the instructions require something or dealers won't warranty a tank if it fails, a flue collapse for sure.

    A good sign of a problem is 3 or 4 TP valves
    on top the water heater and a 175# installed.
    This system needs an expansion tank and a125# TP valve, and in some case a pressure reducing valve just in the cold supply to the tank.
    Ben Restin
  • unclejohn
    unclejohn Member Posts: 1,833
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    The instructions say to install the expansion tank between the check or reducing valve and as close to the water heater as possible on the inlet of the tank. If there is no check or reducing valve you can not install it as per the manufacture's instructions.
  • hot_rod
    hot_rod Member Posts: 22,142
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    Even with a private well system, a DHW thermal expansion tank is a wise choice. More and more variable speed well pumps going in with very small, if any tank attached, so you may not be able to count on the well tank for thermal expansion in the future.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
    JUGHNE

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