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  1. #1
    Join Date
    Aug 2012
    Posts
    53

    Help Determining Boiler Size!

    So, I have had something like 8 estimates for a new oil boiler at this point. Only one took measurements for a load calc and, although that won him great favor in my mind, it took him nearly two weeks to get me an estimate and then the estimate didn't even say what size boiler he would recommend and, to date, he has not answered any of my follow up questions. But I digress...

    One guy told me he could determine the boiler size if I got the average K factor from the oil company, which I did. Actually, I got two: the prior owner's oil company said 3.93, and my company from last year said 4.35.

    So, what does that tell me about boiler size? (and, yes, I already know I should probably insulate my house better.) First and foremost I need to know whether to get 3, 4, or 5 section boiler, but also how many BTUs? Planning to get a Buderus G115 with 40 gallon indirect, and logamtic control.

    Additional info: House is about 2400 square feet (that's a real estate measurment, not necessarily interior), thick stone walls, built in 1940.

    Thanks.

  2. #2
    Join Date
    Nov 2011
    Location
    Long Island, NY
    Posts
    284
    K factor is Degree Days divided by gallons of oil. Higher is better for the homeowner!

    Where I live we have had 4352 DD in the past year, I have used about 550 gallons of oil.

    K Factor 7.9

    I have a formula somewhere, have to look for it. I condensed it so I could just multiply the
    number of gallons used in a year by the number 55 to get the heat loss in my particular climate.
    It's rough but close enough to size a boiler.

    Just remember smaller is better with boilers, if your going to oversize something, oversize the indirect.
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  3. #3
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    68,337
    Using K factor to determine size of boiler needed, over sizes them most of the time, if not always.

    2 houses using the same amount of oil in the same neighborhood have the same K factor. But one has a boiler/furnace that is only operating at 60% efficiency, while the other is at 80% would size both the same, while the first house needs a smaller replacement boiler/furnace. An old boiler that has a tankless coil and is being swapped out for a new boiler with an indirect can be sized much smaller then the old boiler was. K factor won't tell you how much.
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  4. #4
    Join Date
    Aug 2012
    Posts
    53
    Thanks, beenthere, that is helpful. The guy I am planning to use did not do a heat loss yet, but said he would come back and do it if I wanted, so it sounds like I should make him stick to that, rather than just rely on k factor. He estimated about 89000 btu based on k factor, which sounds about right to me based on my own feeble attempt to calculate heat loss. Current boiler has net output of 135,000.

  5. #5
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    68,337
    The smallest section boiler for the load calc will save the most on your heating and hot water cost.
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  6. #6
    Join Date
    Nov 2011
    Location
    Long Island, NY
    Posts
    284
    Quote Originally Posted by beenthere View Post
    The smallest section boiler for the load calc will save the most on your heating and hot water cost.
    X2

  7. #7
    Join Date
    Nov 2011
    Location
    Long Island, NY
    Posts
    284
    Have you called Steve yet?

    Just curious.

  8. #8
    Join Date
    Aug 2012
    Posts
    53
    No, I did not call Steve because I did not see his company listed on the Buderus website as an authorized dealer.

  9. #9
    Join Date
    Nov 2011
    Location
    Long Island, NY
    Posts
    284
    I would call him anyway. He frequents websites other than this one and is obviously committed to his trade.

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