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Thread: water heaters in series

  1. #1
    Plumber canton's Avatar
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    Default water heaters in series

    Is it ok to put a indirect storage tank in line with a gas fired h.w heater. In this case, there is an existing 41 gallon gas fired h.w heater, he has a forced h.w water system and an 41 gallon Amtrol indirect storage tank that he wants to use as a backup or supplement. Is this ok to do?

  2. #2
    In the Trades Bob NH's Avatar
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    You/he need to think through how the system will be operated. Is the forced hot water system going to be kept on all summer to heat the indirect heater? That tends to waste energy.

    If the indirect heater is after the gas heater, then there is nothing to keep it hot during extended periods of no usage like a long weekend away.

    The indirect heater could be first and the forced heating system could be used when there is extra demand.

    You could install a small circulator for the hot water between the gas water heater and the indirect heater. That would provide storage while firing either unit. Firing only one unit will save some energy.

    Success starts with the planning. If the owner doesn't want to be bothered with operating details he is never going to like it.

  3. #3
    Plumber canton's Avatar
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    Default Good idea?

    The indirect is after the h.w heater, he was thinking of using the gas fired in the summer and the indirect in the winter when the boiler will be running.

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    Retired Defense Industry Engineer jadnashua's Avatar
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    Unless the one you turn off is before the other, it won't work...think about it...you've got maybe 40-50 gallons of potentially cold water sitting in the tank after the one that is heating. WHen you start to run water, hot goes in the bottom of the cold one, rises to the top warming the the tank. Not until the whole tank is filled will you have hot water, but by then, you've run the entire contents of the first tank out...no hot water at all.

    If you want to turn the indirect off in the summer, it needs to be the first one in the chain. You still need to keep the gas one on all year, though.
    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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    I&C Engineer (mostly WWTP) Lakee911's Avatar
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    Why not parallel them? Then it doesn't matter which is off or on.

    Jason

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    Moderator & Master Plumber hj's Avatar
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    Default ?

    [QUOTE=Lakee911]Why not parallel them? Then it doesn't matter which is off or on.

    You try to balance a parallel system where the heaters are completely different in the first place. And if you could get it to work, then you would ALWAYS be mixing the cold water from the "OFF" heater with that of the "ON" one which means you would ALWAYS get cool water, no matter how hot the active tank was.

  7. #7
    In the Trades Bob NH's Avatar
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    The way to get two heaters to work when the firing unit for the indirect (second heater) may be off for a while is to circulate water from the outlet of the second heater to the inlet of the first heater.

    A small bronze circulator pump can be connected to operate whenever the thermostat for the indirect heater is calling for heat. As it circulates not-so-hot water from the second indirect heater to the first gas heater, the gas heater will turn on.

    The thermostat of the gas heater should be set a little higher than the thermostat of the indirect heater. Otherwise, the indirect heater will not reach its set temperature and the circulator will never turn off.

    I would get a pump with a flow rate of 2 to 3 times the recovery rate of your gas heater. That is because the recovery rate is based on heating cold water and the water coming out of the indirect heater tank is nearly hot.

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