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using DHW recirc with on demand heater

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archibald tuttle
archibald tuttle Member Posts: 1,085
in Domestic Hot Water
i've got a manifolded building which i kind of wish were 3/8" home run because it takes literallly 5 minutes to get hot water to the extremities of the system. the original piping had a half inch gravity return from the old storage heater that worked marginally. have replaced with on demand heater. I can put a recirc pump on that 1/2" return and tee it in at the cold water feed for the heater. Will a standard recirc pump move enough volume to trigger a good sized (180,000 btu) takagi on demand heater?

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  • Alan (California Radiant) Forbes
    Alan (California Radiant) Forbes Member Posts: 4,002
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    You've got to check the pressure drop through the Takagi heat exchanger and piping. Check the Takagi literature; they will usually give you that information.
    And then choose a large enough pump to overcome that pressure drop.
    8.33 lbs./gal. x 60 min./hr. x 20°ΔT = 10,000 BTU's/hour

    Two btu per sq ft for degree difference for a slab
  • Leon82
    Leon82 Member Posts: 684
    edited June 2019
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    Depending on the model of heater it may void the warranty recirculating it.

    You can also use a small storage tank to recirculate and when the faucet is opened the tankless will feed the storage tank
  • archibald tuttle
    archibald tuttle Member Posts: 1,085
    edited June 2019
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    Alan (California Radiant) Forbes said:

    You've got to check the pressure drop . . . choose a large enough pump to overcome that pressure drop.

    Knew this was the basic science of the question. I was kind of wondering if anyone had done this. I am going to check with Takagi because you gonna need a reliable flow rate over the minimum to trigger it.

    And then there is the question of how the heater responds when you start bringing back warm water after 5 minutes kind of thing. I guess one way to deal with this is to shut it down when hot water reaches the high 1/2" return, put a sensor there. I was thinking of putting the pump in the utility room in the basement where the return comes back although I could potentially put it at the apex of the return and then the sensor would be adjacent to the pump.

    The control i think is the headache, both as regards warm return at relatively low flow rates because the units probably aren't designed to balance that well. Maybe i'm wrong because they run in series tandem occasionally for high volume applications so maybe they are used to looking at incoming water temp or responding quickly to high outgoing temp although i would think in recirc the problem is going to be they can only turn down so far.

    The other issue is that i'm looking for some kind of kitchen occupancy sensor, or maybe even just a pushbutton in the kitchen to run the system. It kind of defeats the purpose to keep warm all the time but use patterns are varied and i couldn't do it with a time of day control very well.

    I'm just off setting this all against home running half a dozen apartments with 1/2" pex on the hot side. I'm kind of on the bubble or thought maybe i could come up with a trial on the recirc and if not happy start opening chases for repiping.

    thanks,

    brian

  • archibald tuttle
    archibald tuttle Member Posts: 1,085
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    Leon82 said:

    Depending on the model of heater it may void the warranty recirculating it.



    You can also use a small storage tank to recirculate and when the faucet is opened the tankless will feed the storage tank

    i was put off by the idea of going back to a tank after getting rid of one, but a highly insulated 10 gallon might do the trick. i wonder about the temperature integrity if i start mixing cold right back in to a small tank though with no consumption to trigger the takagi.

    i could do an approximate calculation on volume in the pipes. how much room temp water can i return to a small tank and still have filled the pipes for useful temperature and then the tank it self is ironically a buffer compared to input from the takagi so you don't immediatley pop back up to full hot. the more i think about this the more i think pex home runs might be in order despite the beautiful manifolded cooper set up. If it weren't for the pressure drop I'd even go 3/8".

    I'm going to clock my current volume to hot from standing start and while i can figure it by theory with a single piece of 1/2" I easily run a piece of pex up the outside of the building and into a bucket near the furthest kitchens and emperically accomplish a head to head experiment.

    thanks,

    brian
  • Alan (California Radiant) Forbes
    Alan (California Radiant) Forbes Member Posts: 4,002
    edited June 2019
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    Most if not all on-demand heaters have an inlet sensor that will adjust or turn off the firing rate when the inlet water temperature rises.

    Install a timer to run the circulator during occupied hours. Since it's a multiunit building, I'd turn it on at 5am and off at midnight.

    What is the make and model of your heater? There are usually instructions and diagrams for a recirc. system.

    Add a tank, sensors, remote actuators and some muffler bearings if you want to make a simple system more complicated. Put in a nice, properly sized bronze or stainless pump with Webstone isolation flanges, a timer and a check valve. I like simple systems and that's what I would do. See disclaimer below.
    8.33 lbs./gal. x 60 min./hr. x 20°ΔT = 10,000 BTU's/hour

    Two btu per sq ft for degree difference for a slab
  • Leon82
    Leon82 Member Posts: 684
    edited June 2019
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    Yes, with the small tank it has to be maintained more as the annode is very small. I used a 4 gallon to absorb the cold water sandwich and a stainless pump to circulate.

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