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Thread: Piping Diagram for Two Tanks in Series w/Isolation

  1. #1
    DIY Junior Member stinkpaw's Avatar
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    Question Piping Diagram for Two Tanks in Series w/Isolation

    Anyone know of a link to a piping diagram showing two tanks in series and allowing you to isolate each tank?

    I believe you would pipe like so, but I'd like an 'official' diagram to show my plumber.

    X is a valve that's shut , O one that is open.


    I'd Imagine its a big no-no to have both valves shut to a tank unless its energy source is off and its drained or BOOM.

    Question: If you were taking Tank #1 off-line would you just close the cold inlet valve and leave the hot open for additional safety and to drain the tank as hot water was used?
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    Last edited by stinkpaw; 06-03-2010 at 12:54 PM. Reason: Fix fonts for diagrams

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    In the trades Dana's Avatar
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    What's your goal here? (More capacity for endless showers, or monster-tub fills?)

    Isolating a HW heater isn't a huge problem since it's also protected by a temperature & pressure relief valve. An expansion tank on the isolated system would also limit stresses from normal heating without spitting a bit of water out the T&P valve.

    Closing just the cold inlet to the tank is sufficient to stop flow into & out of that tank, but it can't "drain the tank as hot water was used" with the cold water feed closed.

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    Journeyman & Gas Fitter Doherty Plumbing's Avatar
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    You mean something like this:



    Green dots are shut offs.

    This setup should let you run the tanks in series or individually.
    Last edited by Doherty Plumbing; 06-03-2010 at 03:01 PM.

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    DIY Junior Member stinkpaw's Avatar
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    Quote Originally Posted by Dana View Post
    What's your goal here? (More capacity for endless showers, or monster-tub fills?)

    Isolating a HW heater isn't a huge problem since it's also protected by a temperature & pressure relief valve. An expansion tank on the isolated system would also limit stresses from normal heating without spitting a bit of water out the T&P valve.

    Closing just the cold inlet to the tank is sufficient to stop flow into & out of that tank, but it can't "drain the tank as hot water was used" with the cold water feed closed.
    Both: Monster tub fills (~170 gallons) and high output shower (~7.5 GPMs). There is another thread on here I started detailing my situation and asking for best ways to supply water. 2 big tanks in series seems to be the way to go.

    Out of curiousity, why do we put shutoffs on the hot outlets of the tank if nothing will come out when the cold inlet is closed?

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    DIY Junior Member stinkpaw's Avatar
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    Quote Originally Posted by Doherty Plumbing View Post
    You mean something like this:



    Green dots are shut offs.

    This setup should let you run the tanks in series or individually.
    Thanks. Is the extra shut off you have between the the hot of Tank 1 and cold of Tank 2 needed (since you already have one on the hot of Tank 1)?

    Do you know of a manufacturer's diagram showing the same thing? I've talked to half a dozen plumbers in my area and they all are saying parallel is the way to go and have never done a series hookup. I want to show them something 'official.' AO Smith has one for parallel but I couldn't find one for series.

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    Moderator & Master Plumber hj's Avatar
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    Use three way valves instead of tees in the supply lines. In one position the water will enter the tank and the other will divert the water to the second heater or into the hot water piping depending on whether it is the first or second heater. If you want to be able to isolate the heaters, ball valves in the hot water outlet pipes will do that.

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    Journeyman & Gas Fitter Doherty Plumbing's Avatar
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    Quote Originally Posted by stinkpaw View Post
    Thanks. Is the extra shut off you have between the the hot of Tank 1 and cold of Tank 2 needed (since you already have one on the hot of Tank 1)?

    Do you know of a manufacturer's diagram showing the same thing? I've talked to half a dozen plumbers in my area and they all are saying parallel is the way to go and have never done a series hookup. I want to show them something 'official.' AO Smith has one for parallel but I couldn't find one for series.
    You could probably get away with eliminating 1 or 2 valves.

    Umm why do you need something "official". My drawing will work just as well as some drawing you get from a manufacturer. Who cares who drew it as long as it works ??

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    Moderator & Master Plumber hj's Avatar
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    COmbine the three valves on the blue line at the right into one three way valve where the tee is. Connect the red hot into the blue line just to the left of the 3 way valve. Eliminate the red line between the two tanks. Combine the other two valves in the blue line, along with the one on the red line by the tee, into another three way in place of that tee. By doing this you CANNOT leave any valves in the wrong position, which would be a real possibility when you have 7 or 8 valves to manage. Those who have NEVER installed a series system have absolutely no concept of how it works, or its benefits.

  9. #9
    Plumber jimbo's Avatar
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    The actual diagram of the connections is not nearly so important as the contstruction. tanks in parallel must have inlet and outlet manifolds MATCHED to very close tolerances, otherwise one tank will do all the work, and you will not realize the full amount of heated water from two tanks

  10. #10
    Moderator & Master Plumber hj's Avatar
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    And they will ONLY be matched at the time of installation. After that, corrosion, and other factors, will alter the flow characteristics, so you have absolutely no control over HOW the water is flowing through the tanks.

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    DIY Junior Member stinkpaw's Avatar
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    Quote Originally Posted by hj View Post
    And they will ONLY be matched at the time of installation. After that, corrosion, and other factors, will alter the flow characteristics, so you have absolutely no control over HOW the water is flowing through the tanks.
    Quote Originally Posted by hj View Post
    And they will ONLY be matched at the time of installation. After that, corrosion, and other factors, will alter the flow characteristics, so you have absolutely no control over HOW the water is flowing through the tanks.
    Thanks everyone for their input.

    Is this how the piping would look using 3-way valves?

    Another question: with tanks connected in series, do you use expansion tanks or just the T&P valve on each heater? If you need the exp tanks, how would that fit into the picture?
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    DIY Junior Member stinkpaw's Avatar
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    Quote Originally Posted by Doherty Plumbing View Post
    You could probably get away with eliminating 1 or 2 valves.

    Umm why do you need something "official". My drawing will work just as well as some drawing you get from a manufacturer. Who cares who drew it as long as it works ??
    I agree your diagram will work and I appreciate you taking the time to create/post it.

    But for whatever reason the 6+ local plumbers I have talked to look at me like I'm crazy when I want the tanks connected in series not parallel. Maybe its a regional thing... I think they think I'm making it up. One guy actually started chuckling when I mentioned it!

    I figured if I showed them a diagram from Rheem or someplace 'official' they might 'get it.' Or at least not laugh.

  13. #13
    Moderator & Master Plumber hj's Avatar
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    Funtionally, your diagram with the 3 way valves is correct. In actual practice, however you have to know WHICH port is the common one, usually the center one, but not always. It depends on the porting of the valve. In your diagram, if that were the porting of your valves, they would be rotated 90 degree CCW so the feed from the left entered the center of the valves. Your local plumbers may have been programmed to NOT think of any ways other than what they learned in school.

  14. #14
    DIY Junior Member stinkpaw's Avatar
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    Quote Originally Posted by hj View Post
    Funtionally, your diagram with the 3 way valves is correct. In actual practice, however you have to know WHICH port is the common one, usually the center one, but not always. It depends on the porting of the valve. In your diagram, if that were the porting of your valves, they would be rotated 90 degree CCW so the feed from the left entered the center of the valves. Your local plumbers may have been programmed to NOT think of any ways other than what they learned in school.
    Thanks. I think this should be more accurate.


    Where would we put the expansion tanks, or do we just rely on the T&P valves?
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    Moderator & Master Plumber hj's Avatar
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    Functionally, you can put an expansion tank anywhere in the piping. If you do NOT install an isolation valve on the first tank's hot water outlet, then you can install the tank between the right hand valve and the water heater. It will ALWAYS be at system pressure regardless of the valve settings, either through the cold feed to the first heater or by backfeed from its connection to the "feed" to the second heater.
    Last edited by hj; 06-08-2010 at 06:23 AM.

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