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DHW recirc - tankless + buffer - Qs

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rbphhc
rbphhc Member Posts: 126
edited March 2023 in THE MAIN WALL
Guys, can anyone share a schematic for DHW buffer tank *with* recirc loop?

Looking for periodic refreshing of *both* the 20 gal buffer and hot in the lines.

Return line is pretty short from last fixture to cold supply.

Assuming we want to tee into cold *after* the CK valve (B, not A).

Is there an ideal position for SS circ pump? In the return? Pre- or post- buffer? (1, 2, or 3?)

Thanks for any feedback.

Comments

  • hot_rod
    hot_rod Member Posts: 22,476
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    what is it you are trying to accomplish? Less cycling of the tankless? More dump capacity for high flow loads?
    How is the recirc controlled? timer, 24/7, thermostat?
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
    rbphhc
  • rbphhc
    rbphhc Member Posts: 126
    edited March 2023
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    hot_rod said:

    what is it you are trying to accomplish? Less cycling of the tankless? More dump capacity for high flow loads?
    How is the recirc controlled? timer, 24/7, thermostat?

    Trying to accomplish:

    1/ Instant HW at all taps (K and upstairs B are far from the tankless, and tankless takes a few seconds to come up to T, so it's a long wait);

    2/ DHW storage in the buffer tank for potential outages (northern VT sees intermittent power often);

    3/ Thermal battery for in-floor radiant, run off coil. 'Buffer' tank is actually a wall-mounted indirect (NTI S20W), but I didn't want to complicate the question.

    Controlling the recirc: I was thinking Tstat in the HW line farthest from the tankless, to Aquastat, to circ pump.

    (Would it be possible to have a second Tstat in the buffer/indirect, and wire so either Tstat hitting low Delta would trigger circ pump?)
  • hot_rod
    hot_rod Member Posts: 22,476
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    seems like an odd way of making a tankless into a boiler and back to a dhw source, but.

    You need to keep the tank hot always if it will supply radiant from the coil? So just run the tankless into the tank, pull the dhw and recirc as it were a standard HW tank. Radiant from coil.

    What temperature do you need for the radiant? If you are using a bottom coil you will need to run the pump from the tankless 24/ 7 to keep the coil in hot water.

    The circ for the tankless to tank needs to size for the gpm the tankless needs to trigger to high fire, maybe a high head circ depending on the pressure drop of the tankless, do you have that number?

    Then the DHW recirc becomes a small circ just sized to overcome the temperature drop in the DHW, probably the smallest DHW circ you can but.

    As for power outage, you need power to run all of this. Maybe some DHW from the tank, without recirc for whatever the capacity of the tank is, in a power outage.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
    rbphhc
  • rbphhc
    rbphhc Member Posts: 126
    edited March 2023
    Options
    hot_rod said:

    seems like an odd way of making a tankless into a boiler and back to a dhw source, but.

    You need to keep the tank hot always if it will supply radiant from the coil? So just run the tankless into the tank, pull the dhw and recirc as it were a standard HW tank. Radiant from coil.

    What temperature do you need for the radiant? If you are using a bottom coil you will need to run the pump from the tankless 24/ 7 to keep the coil in hot water.

    The circ for the tankless to tank needs to size for the gpm the tankless needs to trigger to high fire, maybe a high head circ depending on the pressure drop of the tankless, do you have that number?

    Then the DHW recirc becomes a small circ just sized to overcome the temperature drop in the DHW, probably the smallest DHW circ you can but.

    As for power outage, you need power to run all of this. Maybe some DHW from the tank, without recirc for whatever the capacity of the tank is, in a power outage.

    THANK YOU, Rod. Very much appreciate the response.

    Temp for radiant: @ 90-100f. Just one circuit, single room in-floor, 600sf.

    I was just coming to the same conclusion. Pipe it like standard HWT, tankless into tank. NTI S20W even has dedicated recirc port, so easy.

    Only downside: need to run 2 circ pumps. Both SS, which is expensive.

    QUESTION: *IF* I wanted to try to run only one SS circ pump, would it be *possible* to circulate both the indirect *and* the recirc line off the same pump? I spoke to Taco, and they spitballed that a 007-SF5 would likely do it, but I'd need to run the pressure loss calculations.

    We'd basically be pushing a slug of hot into the DHW lines periodically, off the top of the indirect, replenished from the tankless. Wouldn't that keep both pretty hot, even with the marginal radiant coil draw?

    Thanks again!
  • rbphhc
    rbphhc Member Posts: 126
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    Pressure loss curves for the Rinnai RU199iP, if that helps.


  • hot_rod
    hot_rod Member Posts: 22,476
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    rbphhc said:

    hot_rod said:

    seems like an odd way of making a tankless into a boiler and back to a dhw source, but.

    You need to keep the tank hot always if it will supply radiant from the coil? So just run the tankless into the tank, pull the dhw and recirc as it were a standard HW tank. Radiant from coil.

    What temperature do you need for the radiant? If you are using a bottom coil you will need to run the pump from the tankless 24/ 7 to keep the coil in hot water.

    The circ for the tankless to tank needs to size for the gpm the tankless needs to trigger to high fire, maybe a high head circ depending on the pressure drop of the tankless, do you have that number?

    Then the DHW recirc becomes a small circ just sized to overcome the temperature drop in the DHW, probably the smallest DHW circ you can but.

    As for power outage, you need power to run all of this. Maybe some DHW from the tank, without recirc for whatever the capacity of the tank is, in a power outage.

    THANK YOU, Rod. Very much appreciate the response.

    Temp for radiant: @ 90-100f. Just one circuit, single room in-floor, 600sf.

    I was just coming to the same conclusion. Pipe it like standard HWT, tankless into tank. NTI S20W even has dedicated recirc port, so easy.

    Only downside: need to run 2 circ pumps. Both SS, which is expensive.

    QUESTION: *IF* I wanted to try to run only one SS circ pump, would it be *possible* to circulate both the indirect *and* the recirc line off the same pump? I spoke to Taco, and they spitballed that a 007-SF5 would likely do it, but I'd need to run the pressure loss calculations.

    We'd basically be pushing a slug of hot into the DHW lines periodically, off the top of the indirect, replenished from the tankless. Wouldn't that keep both pretty hot, even with the marginal radiant coil draw?

    Thanks again!
    The single pump option would require a high head SS as I see it? The flow rate dictates the burner output, I think. At a 4 gpm flow, for example, you may not get the output for the tank and radiant.
    Which tankless do you have, what gpm are you looking for?

    Unless you go with a stainless ECM $$$, that high head circ may cost 120W, and running continuously. You would need a balance valve to get the recirc gpm split off from the total flow somehow.

    With a 2 pump, the bigger circ shut off when the tank is at temperature. Then a small recirc is all that is needed.

    Are composite circs any cheaper?

    Trade the tankless for a wall hung boiler or combi :)
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
    rbphhc
  • rbphhc
    rbphhc Member Posts: 126
    edited March 2023
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    Agree, 2 circ system is smarter. Especially after pricing pumps. I will stick to the original plan, and recirculate the tank and HW separately. Thanks!

    (Tankless is the Rinnai RU199iP. Brand new, so not looking to upgrade!)