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Ratio Of Required BTU Between Direct and Indirect HWH?

D107 Member Posts: 1,791
edited July 2018 in Domestic Hot Water
I know the answer will depend on boiler and indirect specs, etc. but generally if your soon-to-be-replaced direct 50 gallon 30kbtu fired hwh has been plenty adequate for years (nominally fires at 40k but actually measures at 30k) would a 51k DOE/44kbtu ibr net boiler generally give equivalent results for a 50 gal indirect or a TurboMax 23? Household two people, back to back showers is highest draw, 2.2gpm flow. Or does this simply have to be calculated?


  • Robert O'Brien
    Robert O'Brien Member Posts: 3,441
    The 50K boiler, even an 82% has a much higher combustion efficiency than any atmospheric gas HWH. As long as pump/piping can move that energy, the recovery will be greater.
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  • D107
    D107 Member Posts: 1,791
    Looking at old direct ratings: first hour 80gph, 40gph 90º rise, which I assume is the continuous rate.

    Note that we'll have to add onto table rate that set temp is 140ºF, mixed down to 120º for direct, standard indirect or Turbomax 23, which should increase output considerably.

    TurboMax 23 ratings are 88gph first hour and 75gph continuous based on 80º rise and boiler water swt of 160º and return of 140º and boiler net output of 50Kbtu. So only issue with Turbomax is how much do DHW requirements interfere with outdoor reset curve for the buffer tank function?
  • Zman
    Zman Member Posts: 7,303
    As Robert mentioned, the indirect will outperform your existing setup.
    The reverse indirect will not do as well if you get aggressive with your outdoor reset curve.
    Do you have a drawing of the system you are proposing?
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
    EBEBRATT-Ed Member Posts: 11,907
    If the boiler fires 50k you don't get 50k out of it
  • D107
    D107 Member Posts: 1,791
    edited July 2018
    @Zman no drawing yet, thanks. @EBEBRATT-Ed I miswrote boiler spec. Fires at 60, DOE 51, IBR 44. (Peerless M185-03). I think the DOE is used for boiler net output for HWH. Given a heating microload of about 3Kbtu in Zone 2 on a 50º day, a buffer would seem to be a wise choice. (2 zones of cast iron, one under-radiated basement baseboard.) Not going mod-con, want atmospheric.