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Boiler Sizing

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Brent H.
Brent H. Member Posts: 161
Greetings,



I'm planning on having a Buderus G115 Boiler installed in my house. I've been running some heat loss numbers in order to have something to compare to the contractors numbers. I've run the numbers a few different ways and get between 70K to 81K BTU. The house when built 15 years ago was a pretty standard 24 x 36 cape with 2x4 construction. I've since built 2 additions(2x4 construction also) so the house is now approx. 3000 square feet. I have a couple of questions I'm hoping you guys can provide answers/opinions to:

1. Is 70K - 81K BTU heat loss seem reasonable for this size house located in central MA?

2. My current boiler is a 87K Gross/76K Net BTU New Yorker and seems to have heated the house fine over the last year. The Buderus comes in a 3 section 85K/74K or a 4 section 109K/95K version. Is there any reason not to go with the 3 section?

3. I'm planning on using a 46 gallon indirect. Will the 3 section Buderus be sufficient for heating this?

4. I believe the 3 section boiler uses the Riello F3 and the 4 section uses the F5. Is there an advantage of one burner over the other.

Most contractors seem to want to push the 4 sections but I'm doing this upgrade to save money on oil and don't care to oversize. I'm hoping to get a heat loss back from one of the contractors tomorrow which will be interesting to compare to my own.

Thanks for the help.

Comments

  • Ironman
    Ironman Member Posts: 7,379
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    Your Numbers Look Good

    With  the age and type of construction of your house, you should be in the 25 btu's per sq. ft. range (assuming nothing unusual). I'd go with the 3 section. Keep in mind that you only need that 75k btu's when it's zero outside: at 35*, you would need half that. You can up fire the Buderus 10%, but you cannot down fire it. I would also go with the 2107 Logamatic control - you'll see at least a 30% reduction in fuel consumption with greater comfort.



    That boiler is plenty for an average household as far as firing the indirect, but more info is needed to determine the size of the indirect. How may baths? How many people in the house? How many teens? Any large demand, like a hot tub or multiple head shower?
    Bob Boan
    You can choose to do what you want, but you cannot choose the consequences.
  • Brent H.
    Brent H. Member Posts: 161
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    Hot Water and Gross/Net Output

    The house has 3.5 baths but is on a well that wouldn't supply all of them even if desired. No hot tubs or any big loads like that. My family is 2 adults and 2 young girls that will eventually be 2 young teen girls :) My preference would be to supply 2 showers at once at a minimum.

    One other question... should I be using the Net or Gross boiler output numbers? The boiler is in our basement which is unheated but does keep my workshop warm.



    Thanks.
  • Ironman
    Ironman Member Posts: 7,379
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    Indirect and Boiler output

    A 40 or 50 gal indirect should be sufficient. Go with more storage, not more boiler, if you think you'll need more hot water. Keep the boiler sized to the space heating load. The 3 section G115 will keep up with about 2 gpm domestic draw if it's got priority. Then you have the storage capacity of the indirect in addition to that.



    The net output rating of the boiler includes a 15% piping heat loss. If your pipes are in a conditioned space or are insulated, then add the 15% back in or use the gross output.
    Bob Boan
    You can choose to do what you want, but you cannot choose the consequences.
  • Brent H.
    Brent H. Member Posts: 161
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    Still struggling with sizing

    Greetings,

    I'm still struggling to decide with confidence whether to go with a 3 or 4 section boiler. I ran another heat loss tonight and came up with 80K with no safety factor. The supply house ran a heat loss and came up with 70K. I have a bit more confidence in my numbers since it was all accurately measured wheras the supply house did a lot of estimating. If my heat loss was 20K higher or lower the decision would be easy. My installer thinks I should go with 4 sections but will install 3 if that is what I want. Any thoughts on how to settle this in my mind?



    Also, any estimates what the additional cost or loss of efficiency would be going 4 sections instead of 3?

    Will 4 sections along with a Superstor 45 provide any noticable amount of additional hot water over a 3 section?

    Many Thanks.
  • kcopp
    kcopp Member Posts: 4,443
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    Don't over think....

    this. The 3 will be fine. If there is ever an issue you can always up-fire the 115. But you can't down-fire a 4. I have a 115-3/21 and have a ssu-60 gallon. No issues and I have a bigger house... and much older.... 1800 ad . I have a tekmar 260... controlling the whole shebang.
  • Ironman
    Ironman Member Posts: 7,379
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    Stop Struggling

    When it comes to boiler sizing, bigger is not better. You would only need the full capacity when it's single digits outside. That will happen less than 3% of the time and only at night usually. When daytime comes, the load will be less.



    Also, all heat loss programs have a "fudge factor" to cover their rear ends. Usually, about 15%.



    Go with the 3 section; it can be up-fired as much as 10%. But I don't think you'll need to.
    Bob Boan
    You can choose to do what you want, but you cannot choose the consequences.
  • lchmb
    lchmb Member Posts: 2,997
    edited October 2011
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    and

    Make sure to add a mixing valve to your indirect (I would go with a 40 gallon) and run the tank at 150 degree's. I do, and have never had an issue and water usage is huge in my house...

    btw, like other's have said. Dont over size the boiler, you'll be sorry in the long run for the loss in savings...
  • Brent H.
    Brent H. Member Posts: 161
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    Thanks....and some followup questions.

    Thanks for the feedback, it helps to put my mind at ease.

    A couple other question came to mind as I was looking at my old boiler tonight. My current New Yorker rated at 76K/BTU Net has a Becket burner with a .85 nozzle and the last combustion test shows 82%. The Buderus is rated at 74K/BTU Net with a .70 nozzle and 86% efficiency. How does the Buderus get basically the same output with a nozzle .15GPH smaller? It makes no sense to me when the efficiency number claim only a 4% difference.



    Also, like most houses, my house has more baseboard than required for the heat loss. On a cold day, wouldn't the return water coming back to the boiler be too cool due to the house having more baseboard than the boiler can provide hot water for?



    Thanks again.
  • Charlie from wmass
    Charlie from wmass Member Posts: 4,322
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    oil pressure

    The pressure effects the actual fire rate and this is why the same nozzle can be used for different sizes of boilers.
    Cost is what you spend , value is what you get.

    cell # 413-841-6726
    https://heatinghelp.com/find-a-contractor/detail/charles-garrity-plumbing-and-heating
  • Brent H.
    Brent H. Member Posts: 161
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    Hot Water

    Thanks for all the feedback, it's been quite helpful. I talked with my contractor tonight and got him onboard for a 3 section install. He also mentioned it could be upfired if necessary. His only concern has whether the house would have trouble catching up on heat after working to recover/heat the hot water tank(on priority so no heat for the house). My feeling is that the time it will take to do this is so small in the big scheme of things that I don't believe it is an issue.



    From all of my research and discussion with contractors, I've come to the conclusion that contractors oversize for fear of getting calls from cold customers..... I have had this fear a bit myself but you folks have helped me feel comfortable about it. One of the reasons for this install is to save money on oil.... if I oversize and waste oil then I've accomplished nothing.



    One final question, is there any advantage to going with a 60 gallon hot water tank since when my kids grow up I'll have 2 teenagers showering plus my wife? I doubt that more than one will shower at a time since I'm on a well and there just isn't the supply for 2 good showers. If I upgraded the well pump/tank, the most showers at any one time would be 2. I'm not sure how much more it will cost to keep 60 gallons hot as opposed to 45. I do like the suggestion of going with 45 and a mixing valve.



    Thanks again for all the help, it's greatly appreciated.

     
  • lchmb
    lchmb Member Posts: 2,997
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    either way

    go with a mixing valve and run the tank hotter. It keeps germs at bay.
  • Brent H.
    Brent H. Member Posts: 161
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    Up-Firing

    I called Buderus with a couple questions yesterday and while I was talking to them I asked about up-firing. The tech told me that they do not recommend up-firing the 115WS since it is already up-fired. Basically, the 115WS is and up-fired version of the older 115. Any thoughts?



    This makes me a bit nervous since I liked having the option to up-fire if needed and got me thinking about the difference between the 3 and 4 section boiler. Where exactly is the efficiency loss with using a bigger boiler? I understand that it may cycle more but don't modern controllers dump the extra heat left in the boiler into a zone so it's not wasted.



    Thanks for the education.
  • gennady
    gennady Member Posts: 839
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    boiler sizing

    Never size boiler by btu/sqf rule of thumb. these are rules of dumb. Use heat loss calculation by manual J only, do not forget include infiltration.
  • Ironman
    Ironman Member Posts: 7,379
    edited October 2011
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    Buderus G115/3WS

    Brent,



    Yes, the 115/3WS is an upfired version of the 115/3. You had stated that you were getting a 115/3 in your original post and our respones were based on that. The 115/3 has an input of 84k, but the 115/3WS has a 105k input. It's already upsized to almost the capacity of a 115/4. It's more than what you'll need.
    Bob Boan
    You can choose to do what you want, but you cannot choose the consequences.
  • Ironman
    Ironman Member Posts: 7,379
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    Rules of Thumb

    No one ever said to size a boiler by a "rule of thumb". The O.P. stated that he had done a heat loss calc. (and also the supply house) and wanted to know if the numbers looked reasonable based on the construction of his house and its location. My response was based on that and over 30 years of doing load calcs. Putting my response in context makes this obvious. :)
    Bob Boan
    You can choose to do what you want, but you cannot choose the consequences.
  • Brent H.
    Brent H. Member Posts: 161
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    Input BTU vs. Gross/NET BTU

    Bob,



    Thanks for the information.



    If the 115/3WS has a 105,000 BTU Input, how do the get 85,000 GTU Gross? I assume that Input BTU x Efficiency = Gross but that doesn't work since 105,000 x 86.7% = 91,035 not the 85,000 Gross specified. Any thoughts?



    Thanks.



    -Brent
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