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Sizing for Hot Water Boiler

razzor Member Posts: 20

I am looking to replace an 17 year old Burnham V75 boiler with a Biasi B10/4. Trying to figure out the proper BTU needed and have done a bunch of heat loss calculations. I used CoolCalc, Slant Fin, and even the Weil McClain manual version. My house is about 2800 sq feet and was built in 1997 so well insulated with decent windows in Southern Maine. My old Burnham says Net IBR our 166,000. Seems way too big for my house. Plus I heat the 952 sq ft first floor living space with a pellet stove in the colder months. All my heat loss calculations at worst say around 77,000 btu if I use real liberal numbers. Any thoughts?



  • razzor
    razzor Member Posts: 20
    Also roughly 190 feet of baseboard fin heating.
  • GroundUp
    GroundUp Member Posts: 1,742
    77k seems about right for 2800 sq ft in that climate given your newish construction. 27.5 BTU/sq ft isn't out of the ordinary
  • STEVEusaPA
    STEVEusaPA Member Posts: 6,375
    edited June 2019
    Which city? How much oil did your burn in the last year/season?
    Do you have a coil for domestic hot water productions?
    My guess is the smallest one is probably fine or slightly oversized.
    But a detailed heat loss will let you know for sure.
    HVACNUT Member Posts: 5,469
    The B-4 will give a net of 96K BTU so that's still overkill for what you described. Is your mind set on Biasi only? The contractor is paramount. Get some estimates and see what's offered comparable to what your looking for. Buderus, Trio. Weil McLain and others offer 3 pass high efficiency boilers. Also an Energy Kinetics System 2000, EK-1 Frontier is an excellent system to look into.
    Also, chances are any upgrade from the V7 will require a stainless steel chimney liner if not already there.
  • razzor
    razzor Member Posts: 20
    Follow up. My plumber seems to think I need enough btu to power the 190 feet of hot water baseboard but the heat loss calculations say otherwise. Multiplied by 580 = 110,000 btu. I get a Heatloss of 80,000 btu using Slant Fin or Cool Calc using conservative numbers. Kind of confused! Thanks.
  • Zman
    Zman Member Posts: 7,493
    edited August 2019
    Using the existing baseboard lengths tell you the max amount of energy your baseboards can emit, not what is needed to heat the house. The heat loss is the best way to size the boiler.
    It would be very surprising if your heat loss is greater than 77k

    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
    HVACNUT Member Posts: 5,469
    Sounds like you need a new plumber. Or heating contractor. The Buderus G115/3 has a net IBR of 74K.
    The Trio P3, 79K net.
    The Biasi B-4, 96K net.
    It seems you're more experienced than the plumber.
  • hot_rod
    hot_rod Member Posts: 20,740
    How has the home been heating, does it maintain temperature on the coldest days? Does the boiler cycle off on design days? Does the pellet stove need to run to stay comfortable?

    If the system was sized exactly correct and installed and controlled properly it would run non stop on design day.

    Good news that you have that much baseboard, if your load calc is accurate you should be able to run at temperatures below the typical 180F design, and increase efficiency of the new boiler.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream