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Thread: Expansion tank location with a new tankless water heater

  1. #1

    Question Expansion tank location with a new tankless water heater

    I'm interested in the best place to put an expansion tank in my cold water line with a new tankless water heater install.

    Currently with my tank water heater (to be replaced), an expansion tank is located right next to the cold inlet to the unit. The way the house is set up... Cold water is fed to bathrooms, kitchen, showers, etc, then on that cold line the expansion tank is next and then the HW heater. So in the big scheme of things... the expansion tank is located between appliances/faucets/etc and the cold water inlet to the HW heater.

    With my new tankless HW heater unit, I will have the following configuration. Cold water will come into the house, go to a water softner. Out of the softner, cold line will go to the new tankless HW heater unit, then continue on through out the house to feed cold water to appliances/faucets/showers/etc.

    So here is my question. Should I install the expansion tank on the cold line between the water softner and the tankless HW heater? Or. Should I install it between the continuation of the cold line after the tankless HW heater and the rest of the cold line faucets/appliances/etc? In either case, I plan to install the expansion tank to be within 1-2 feet from the cold input to the tankless HW heater. Or. Does it really matter what side....as long as it is close to the tankless HW heater?

    Sorry if long winded, hopefully I explained my question enough.

    Thanks in advance for any help.


    "HokieAmongLions"

  2. #2
    DIYer, not in the trades LLigetfa's Avatar
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    It matters not where on the cold side the tank is as long as it is downstream of the regulator/checkvalve.

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    In the Trades mtcummins's Avatar
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    i think you can put it on the hot side as well if that makes installation easier... that's where my plumber put mine and it passed inspection. after HW tank, before fixtures, and before radiant floor heating loops. i'm not sure about the code on this, just where mine is and the inspector didn't say anything.

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    In the Trades Jerome2877's Avatar
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    The real question is do you even need an expansion tank with the on demand water heater? While it will not cause a problem, the reason for the tank is to protect against thermal expansion due to the heating of the water. Now with a regular HWT you have a large amount of cold water being heated while the system is closed so this causes thermal expansion and thus an expansion tank is a good idea. With on demand however the only time the unit is heating water is when there is an open hot water tap, then when it closes the heater shuts down. If anything the static pressure would actually decrease slighty as the water left in the pipes cools down.

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    In the Trades mtcummins's Avatar
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    hmm, good point. i haven't ever actually installed a tankless yet, so never interacted with it this way, but i think you are correct, there should be no need for one at all.

    i was going to use one in my house remodel, but with the radiant floor heating, it just doesn't make much sense to go tankless. huge inefficiencies are created with the tankless in this application. for potable hot water though, they definitely seem like the way to go. i'm hoping that we'll see them actually lasting the 20 years they're claiming, that would seal the deal that there's no other way to go for potable water.

  6. #6

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    Thanks.... My gut was that it wouldnt matter where... but after checkvalve... but wasnt sure.

    Thanks again everyone.



    BTW.....Jerome2877...
    Previous thread question that I posted months ago... the answer was yes to put one in.

    Link....
    http://www.terrylove.com/forums/show...on-tank-needed



    Quote Originally Posted by Jerome2877 View Post
    The real question is do you even need an expansion tank with the on demand water heater? While it will not cause a problem, the reason for the tank is to protect against thermal expansion due to the heating of the water. Now with a regular HWT you have a large amount of cold water being heated while the system is closed so this causes thermal expansion and thus an expansion tank is a good idea. With on demand however the only time the unit is heating water is when there is an open hot water tap, then when it closes the heater shuts down. If anything the static pressure would actually decrease slighty as the water left in the pipes cools down.

  7. #7
    In the Trades mtcummins's Avatar
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    I don't know one way or another about this, but Jerome's point seems logical to me. Maybe code does not differentiate and requires one regardless, I don't know. But can anyone explain why (in the real world, not code world) you would need one?

    Thanks

  8. #8
    Retired Defense Industry Engineer jadnashua's Avatar
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    Some tankless units actually have a small tank in them, and then, it would be a good idea.

    The potable water expansion tank is designed for cold water...installing it on the hot side will shorten its life a lot.
    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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    In the Trades mtcummins's Avatar
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    Ah, good to know. Thanks Jim.

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    In the Trades Jerome2877's Avatar
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    I think the reasoning for code requiring one on a tankless unit would be to protect against a home owner either changing to a tank or adding one in the future. I'm not saying you shouldn't install one(as code does require one), I was just thinking about it and was interested in other opinions.

  11. #11
    Moderator & Master Plumber hj's Avatar
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    The reason for a "code" requirement is because many agencies do not "THINK", and once they decide something is a "good idea" make it manditory for EVERYONE so they do not have to decide whether it is needed or not in any particular installation. That way the inspector can just look for it and if it is there approve it and if not turn the job down. Tankless heaters with small tanks use it for recirculation purposes and it creates a minimal, if any, thermal expansion. In any case thermal expansion does NOT occur unless you have a closed system in the first place.
    Licensed residential and commercial plumber

  12. #12
    In the trades Dana's Avatar
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    What HJ said about the volume of mini-tanks- a coupla quarts isn't going to need an expansion tank any more than 50' of half-inch plumbing does. They designed those in to avoid the "cold water sandwich" scenario, where a slug of cold water passes through the tankless unheated during the ignition cycle creating an annoyingly varying temp at the tap after a few short-draws.

    Tankless heaters can be used in radiant heating applications with reasonable efficiencies, but like anything else it has to be designed & implemented correctly to get the efficiency out of it. If you have a radiant floor heated by a condensing boiler, an indirect fired tank HW heater on the boiler would usually be a better/cheaper/more-efficient system than adding a tankless. Short-cycling losses on tankless HW heaters are high (the EF test over-rates them), and the standby losses on an indirect tank are low.

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