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How much does size matter?

scatgo
scatgo Member Posts: 26
edited September 3 in THE MAIN WALL

I plan on replacing my 97 year old furnace with a Weil-McLain AquaBalance Heat-Only Wall Mount Gas Boiler. My option is 65,000 97000 or a125000 BTU unit. After a quick calculation I should need a 93000 BTU unit. Just how close do I have to be to the 93000 for thing to work right?

383-100-045 - Weil-McLain 383-100-045 - AB-120H AquaBalance Heat-Only Wall Mount Gas Boiler, 97,000 BTU (NG)

Comments

  • mattmia2
    mattmia2 Member Posts: 13,942

    Is 93,000 btu/hr the heat loss of the house that you calculated? Since it is a modulating boiler it can fire at a lower rate much of the time. I would go with the 97,000 btu/hr output model though the actual output may be a little less depending on what type of emitters you have and how well they deal with low temp water. Are you heating DHW with it or just space heating?

  • scatgo
    scatgo Member Posts: 26

    Just space heating with cast iron radiators.

  • scatgo
    scatgo Member Posts: 26

    Boiler BTU Calculator - SupplyHouse.com

  • hot_rod
    hot_rod Member Posts: 25,541

    A 97,000 input will be around an 84,000 output, derate for efficiency.

    If you trust the load calc, go with the 125, it will turndown about the same.

    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • mattmia2
    mattmia2 Member Posts: 13,942

    it is 97,000 output, but that is in a lab test. how close it comes to that will depend on how cool you can run the ci radiators and get the design output. if the house is 2 story that calculator is way off because it will be guessing about 2x the roof area you actually have.

    EdTheHeaterMan
  • hot_rod
    hot_rod Member Posts: 25,541

    That is a pretty simplistic load calculator. I would run another load calc to tighten up that number.

    What size is the existing boiler?

    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
    mattmia2
  • GGross
    GGross Member Posts: 1,598

    that calculator gives me a load of 103,554 BTU/hr. My boiler is an 85k mod/con and when I checked up on it this past winter when it was -15f outside it satisfied a call for heat in less than 10 minutes. That calculator is pretty far off, that being said a 125k with a 10:1 turndown is going to be fine, I just don't think your actual load is anywhere near what SH suggests.

    mattmia2ethicalpaul
  • EdTheHeaterMan
    EdTheHeaterMan Member Posts: 10,909
    edited September 3

    How much does size matter?

    This question is better asked in private with your significant other.

    As far as the boiler size is concerned…

    If you do the load calculation based on I=B=R form 1504WH or ACCA Manual J, you will find a much lower requirement. I have a feeling that you will be able to use the WM AB-80H with a 65,000 NET output, and have the most efficient setup. If you don't want to go to the trouble of an actual Load Calculation for your home where you measure the actual amount of doors, walls, windows, ceilings and floors, then you will be guessing that you have something less that 97,000 NET and I would choose the WM AB-120 with the 97,000
    NET output. Either way, the boiler will only use the amount of BTUs required to heat the home and rarely fire more than actual load requirements. Where you will waste fuel is at the low end when you only need 6000, BTU on a spring evening and the turndown ratio only goes to 9700 BTU so the boiler will cycle on and off to provide the lower BTU requirement.

    Edward Young Retired

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

  • EBEBRATT-Ed
    EBEBRATT-Ed Member Posts: 18,419

    @EdTheHeaterMan

    When I read the post title, I had visions of what your comment would be😊

    mattmia2Larry WeingartenEdTheHeaterMan
  • DCContrarian
    DCContrarian Member Posts: 1,219

    I recommend the process detailed in this article for sizing based on actual fuel usage:

    https://www.greenbuildingadvisor.com/article/replacing-a-furnace-or-boiler

  • EdTheHeaterMan
    EdTheHeaterMan Member Posts: 10,909
    https://forum.heatinghelp.com/discussion/comment/1863964#Comment_1863964

    I need to tread lightly here. Some of my comments have been removed in the past and I don't want to get banned. I think I did this one properly.

    Edward Young Retired

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

  • GGross
    GGross Member Posts: 1,598
    https://forum.heatinghelp.com/discussion/comment/1863971#Comment_1863971

    Need to start a mailing list for your spicy jokes. 'Ed Young: Too Hot For The Wall"

    EdTheHeaterManmattmia2
  • ZackR
    ZackR Member Posts: 30
    https://forum.heatinghelp.com/discussion/comment/1863968#Comment_1863968

    That is what I did and was able to size down my boiler and save approx. 8% on my gas usage. 2 heating seasons so far and no problems keeping things warm in the winter.

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