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best overall efficiency for800mbtuh boiler

i'm in the process of reccommending two 800mbtu
natural gas boilers with fully modulating burners and outdoor reset. the units are driving reheat coils in vav boxes. i would like to get to the mid 80% figure in a cast iron boiler. any suggestions ?


  • kf_2
    kf_2 Member Posts: 118
    The water

    temp for reheat coils is usually pretty low, have you considered using condensing boilers and really taking advantage of outdoor reset.

    If you are hooked on cast iron, consider the Viessmann Rondomat with full mod. gas burner and boiler protection package. Efficiency 84%.

  • Steve Ebels
    Steve Ebels Member Posts: 904
    Check out

    A Vertomat if you want max efficiency. Full modulation available, unlimited low temps. Viessmann lists 90.2% eff. at supply temp of 158 and return of 122*. Drop the supply to 104* and the return to 86* and the eff pops up to 95.1%. Those numbers are for a VSB-22 firing at 846,000.
    Flue gas temp is a max of 27* above return water temp.

    If Cast Iron is what you want to use, I like the Rondomat from Viesmann or Buderus' 315 series. The 315-9 has an output rating of 768,000 with a combustion eff of 84.5% on gas and 87.1% on oil. The Buderus Thermostream design allows for some mighty low return temps as far as an iron boiler is concerned.
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