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Bosch Greenstar FS 131 (KBR 35-3) Oversized?

glimmer00 Member Posts: 2
Been considering switching out our oil boiler for gas before the town placed a 5 year moratorium on our street after paving (so no cuts for gas for 5 years). Had 5 places come out and not one of them does heat loss calculations as part of the estimate(didn't know about any of this myself til I looked about 1/2 way through calling people out). Since 3 of them provided Bosch Greenstar estimates I figure I can at least compare them more applies to apples. They range from the FS 100 to the FS 151. Current boiler is 151 MBTU. I'm guessing they are all going by rules of thumb and the like. My question is should I trust my fuel use analysis which suggests a lot less and push them to the lower sizes?

Quick background. The house is a 2 story colonial of about 2100 sq ft and was built in 1940 and is wood frame construction with aluminum siding over wood siding and I believe the walls are insulated. The attic has about 8-10 inches of rockwool and cellulose insulation. Windows are new double pane low E glass. Heat is via cast iron baseboard on the lower level and cast iron radiators on the 2nd floor. All 1 zone. The oil boiler currently supplies heat for the radiators as well as an indirect hot water tank.

Looking at my past oil use, even assuming 100% of the oil heats my house in December to March(which its not due to hot water tank) I don't see how I need more than the FS 100, but I could be missing some factors or other things hence this posting.

Based on the oil delivered to my house and following this guide (I've seen this method several places) I see needing maybe 51k BTU/hour assuming ALL the energy of the oil made its way into heat, and I know its a good bit less due to losses.

Full spreadsheet attached which includes my radiators for the curious.

I tried the Slantfin ipad app and if I gave my house 3.5 inches of wall insulation (I can only guess what may be in there I get a heat loss of about 55000 which is reasonably in line with the fuel use. I still cannot imagine that I lose more heat than I put in though so I'm thinking fuel use analysis despite its roughness has to be somewhat right, at least in giving the max you need.

Based on that I would imagine the contractors wanting to use the 131 and 151 sizes are probably oversized. I'm not sure I'd want to go below the FS 100 though. I also don't know what other factors I am missing that could throw this. So am I close to right? Any other thoughts?


    EBEBRATT-Ed Member Posts: 15,629
    No one can tell without doing an accurate heat loss. I trust the Slant Fin app if done correctly , some say it runs a little high.

    That being said, 55000 could be a little light but that's a wild guess and guesses don't count.

    Acouple of things jump out at me. You have a floor area of 2000+sq feet but the area over the basement is smaller. What is the floor over? Crawl space? Slab? Either way you need to add something for that.

    Attic same thing you have more ceiling square footage than attic?

    Windows and doors need square footage and heat transfer multiplier

    I commend you for doing your homework that a contractor should do.

    Just think you need to do some tweeking

    Most on here will tell you to think twicw about using a contractor who wont do a heat loss.

    What will his install be like?

    Check "find a contractor" on this site. Maybe one of the good ones is nearby.....or can make a recommendation

    Where are you located?

    Also remove some electrical switch plates and poke around & see if you can get a better Idea about insulation. maybe make a small hole in an outside wall...like a closet where it can be patched easily

    In my experience, @ 1940 would not have 3 1/2" insulation (but you could have some blown in)

  • Zman
    Zman Member Posts: 7,573
    I like your methodology. I have never seen a 2,100 sq foot house over 60k.
    I don't like the 4 to 1 turndown on the bosch. I would find a brand with 10-1 and go with a boiler around 80 k input.
    My personal preference would be Lochinvar.
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
    JUGHNE Member Posts: 11,077
    Sometimes the walls were blown with insulation before the new siding was installed. That way the hole plugs would be covered.
    1-2 holes per stud space, high and low. Outlets might show you what's inside the wall.
    My parents built a house in the early 40's and installed 1" of balsam wool batts. (they knew firewood/coal was not free).
    EBEBRATT-Ed Member Posts: 15,629
    Have seen old houses stud bay filled with crumpled up newspapers.

    Wonder what the homeowners insurance would cost if the insurance company knew that
  • glimmer00
    glimmer00 Member Posts: 2
    edited March 2019
    The missing square footage I think is a combination of me using the outside measures of the house for some basic wall calculations and the fact that I didn't count the small closets which have no exterior walls. Slantfin seemed a little iffy to work with for some odd shaped rooms as well.

    Windows and exterior doors I added in with square feet and filled out the app's questions for what they were made out of. Interior doors I did not add.

    Would I be right at least in assuming the degree day method is good enough for a max? Unless the degree day method doesn't properly account for max coldest day during my oil delivery period (which I thought that was the point of the degree days) then I see fuel analysis as likely over estimating things as you have hot water in there too. Could be wrong though.
    EBEBRATT-Ed Member Posts: 15,629
    edited March 2019
    I would just do a "block Load" of the house. measure the house and call it a "room". If you have 1 or more additions or odd shapes just make each space there own room. Then total it up. Use the Slant Fin. Do not add anything for DHW load