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Indirect heater temp?

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I've got a Bradford White SW2120L indirect hot water heater I'd like to put into my geothermal system that is running at 110F. I would like to use it to preheat water going to my DHW tankless heater. Can I safely run this unit at less than 120F or will bacteria be a problem?

Comments

  • Ironman
    Ironman Member Posts: 7,379
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    That’s the perfect temp for breeding Legionella.

    We set indirects at 140* and mix it down to 120* with a tempering valve.
    Bob Boan
    You can choose to do what you want, but you cannot choose the consequences.
    HomerJSmitholdgit
  • dko
    dko Member Posts: 611
    edited September 2023
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    It won't have the effect in increased gpm you are expecting unless you are also going to also increase output temperature. Also the minimum fire may bring your temp over your setting into scalding temps or not fire at all that close to set point letting the water bypass the tankless.



  • WMno57
    WMno57 Member Posts: 1,349
    edited September 2023
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    https://noritz.com/legionella/
    "According to the American Society of Plumbing Engineers (ASPE), the ideal growth range for Legionella is between 90°F and 108°F, but the bacteria can survive in a wide range of temperatures:

    68 ℉ and below — bacteria dormant
    77 to 113 ℉ — growth range
    90 to 108 ℉ — ideal growth range
    118 to 122 ℉ — can survive but not multiply

    Depending on the strain, Legionella bacteria will begin to die over time when temperatures rise above 122 ℉, with the bacteria dying instantly at 158 ℉. "
    Even if your tankless can output 140 F DHW, the problem is the upstream storage. Based on the above, I would say NOT SAFE because your indirect storage temp is in Legionella growth range and probably ideal growth range. Also, you cant assume 100 percent uptime for your geothermal system. What if the geothermal is down for some amount of time? How will that affect temperatures?
    Don't do it.
    I DIY.
  • Hot_water_fan
    Hot_water_fan Member Posts: 1,886
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    Does the geothermal have a desuperheater? Those usually have high temp outputs, which could then be routed to the preheat tank.
  • WMno57
    WMno57 Member Posts: 1,349
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    And if the geothermal system has downtime for any reason, he now has an indirect tank full of warmish brackish water. Possibly full of Legionella. Bad idea for a number of reasons.
    I DIY.
  • hot_rod
    hot_rod Member Posts: 22,271
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    So pre heat to a tankless running 140f. No problem with that, unless the tankless fails It is a common solar preheat application 

    Residential dhw systems turn enough water on a daily basis to present serious legionella concerns

    Its buildings with dead ends or hotels with empty floors  for extended periods that cause issues

    You are more at risk walking by a fountain in the mall, or downtown San Francisco

    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
    oldgit
  • EdTheHeaterMan
    EdTheHeaterMan Member Posts: 8,128
    edited September 2023
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    Residential dhw systems turn enough water on a daily basis to present serious legionella concerns

    @hot_rod Bob, did you mean to say "Prevent". One letter reverses the meaning of the statement

    Edward Young Retired

    After you make that expensive repair and you still have the same problem, What will you check next?

  • hot_rod
    hot_rod Member Posts: 22,271
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    Residential dhw systems turn enough water on a daily basis to present serious legionella concerns
    @hot_rod Bob, did you mean to say "Prevent". One letter reverses the meaning of the statement
    Prevent it is
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream