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How to determine boiler size.

Greg_23 Member Posts: 22
Go with the 115-21 and the S120. Plenty of hot water! On priority it should only take 10-15 minutes. Besides if the family expands or they take longer showers, you can crank up the aquastat and install a tempering valve and get another 1/3 capacity out of the same tank.
Good luck!


  • Hi Guys,

    Great forum. I have a question on picking the right size oil boiler for my home. I'm going with the Buderus G115 series with Logamatic 2107. My heat loss is 41,500.

    I will be also getting an in-direct water storage tank either 32Gal or 40Gal. I have a ranch with 2 bathrooms. One with a tub/shower and the other one is in the basement with shower only. Would the S-120 32Gal be enough hot water or would the St-150 40Gal be a better choice. Also, is there much of a difference (workmanship, efficiancy) between the S and ST series or can anyone recommend a better option for an in-direct storage tank.

    Thanks in advance.
  • Given your heat loss

    the G115/21 would be more than enough. That, I believe, is the smallest Buderus oil-fired boiler.

    The best thing I can tell you about indirects is that I have a Triangle Tube 40-gallon unit out there that didn't run out even with two teenagers in the house. The 40-gallon Buderus indirect has a slightly lower first-hour rating than the TT unit but should still be fine.

    Unless you have three or more teenagers that is ;-)

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  • Ron Schroeder
    Ron Schroeder Member Posts: 998

    Hi Steve,

    With your heat loss, the G115-21 would be the one to go with.

    The S-120 has a 3/4" diameter coil and the ST series have 1" diameter coils. The head loss thru the ST series is a lot less. The ST series are a little better insulated than the S-120 but the S-120 is insulated well enough.

    Personally, I would go with the ST-150 or ST-200. The greater storage volume will reduce the number of boiler cycles which will improve the average seasonal efficiency, especially in the non heating season.

  • Thanks for the replies.

    When adding an in-direct water storage tank. How many btu's is needed for it and does the size of the indirect matter. Hope that makes sense.

    Also, I had one gentleman come to my home and he suggested going with the G115/28. He explained, the in-direct water storage will have priority and in the winter when your taking a long shower. The larger boiler will be needed to heat the water up faster so if there is a call for heat it will be ready. Is this correct?

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