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Slant Fin Heat Loss Calc Questions

MSmithHH Member Posts: 14
edited August 2019 in Gas Heating
Hi folks,

Trying to use the Slant Fin Heat Loss calculator to properly size a new boiler for my 1989, 4k sq ft house that I've owned for about 1 year. However there's a lot of ambiguity in the app that I'm hoping someone can help me with. Here's my questions:

1) What do you do when you don't know the "Factors". I don't know what windows or doors are single vs double glazed vs triple glazed, etc. Should I choose the lowest numbers, highest numbers, or ones in the middle?

2) What doors should be counted? Exterior only? Interior as well? Open entryways? If interior and open entry way as well, do I double count them for both rooms that touch them?

3) Should skylights be counted as part of glass sq ft and how should they impact selections on the infiltration factor?

4) What is exposed wall length? Is that exterior walls?

5) What is cold partition length?

6) When a door is a patio door that is almost all glass, should I count that as a window instead of a door?

7) What do I set for Indoor temperature? Is that the desired temperature in the winter time?

8) How do I know if I have Fine/Line or Base/Line baseboards?

9) I'm assuming I don't do this for non-heated rooms such as finished basement (currently use space heaters but will switch to a separate mini-split system) or non-finished attic/basement spaces? Or do I, but I set baseboard length to 0?



  • Jamie Hall
    Jamie Hall Member Posts: 15,845
    On a few of your questions. On windows. You're going to have to go around and find out which windows are single, double (or good storms) or triple. You can't fudge on that one. And a patio door which is almost all glass is best counted as a window (most are double glazed); if there is a large window in some other door, add it in as a window.

    Skylights are windows -- and I would be very conservative on their values; perhaps go to single glazed. They are terrific heat losses at night if they don't have opaque shades.

    Interior doors can be iffish. When I use the calculator, I don't count a door if it opens into a space which is heated in the same way as the space I'm calculating. If it opens into a cold space, it's an exterior door. Note that large openings from space to space don't divide the two spaces from a heating point of view -- the two spaces must be treated as one large room.

    Exposed wall length is exterior walls; cold partition length applies to any wall which is heated on one side and not the other.

    Indoor temperature. Well, to find out what your heat loss will be at, say, 65, use that. However, most building codes will require that you demonstrate that you can heat the structure to a code specified temperature -- usually 75.

    Baseboard type doesn't come into play when calculating heat loss. The unheated basement does, however--it affects what you pick for the floor in the spaces above it.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • MSmithHH
    MSmithHH Member Posts: 14
    edited August 2019
    Excellent! Thank you very much!
    JUGHNE Member Posts: 8,316
    Usually you can see how many layers of glass are in a window with a candle flame at night. The number of reflections of the flame tell you 1, 2, or 3.

    The simple single page whole house worksheet I mostly use has sliding doors-double glass at a U factor of 2.....Other doors w/storms at U factor of 1.3. Sliders lose more because of having to be loose for the sliding operation.
    Single glass windows show 1.5....double glass at .9....triple at .7.

    This is an old form that has air infiltration built into each factor IIRC.

    If you have an aluminum slider patio door, they are usually a good candidate for change out.

  • hot_rod
    hot_rod Member Posts: 14,786
    Most common indoor design is 70°F, certainly you could run it with a higher temperature.

    Run the calc a few ways, best and worst case maybe, see how much of a difference.

    A heat load is always a "best educated guess"

    The dimension of the actual fin tube and cabinet would help determine which model you have.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • Robert O'Brien
    Robert O'Brien Member Posts: 3,326
    You can back into the number if you know the annual fuel usage.
    To learn more about this professional, click here to visit their ad in Find A Contractor.
  • SlamDunk
    SlamDunk Member Posts: 1,001
    Ultimately, you will need to figure out the heat capacity of your existing baseboards/radiators.

    Hopefully, their btuh rating will match your boiler's output rating and both will match what you come up with on the slant fin app. Product Data Sheet.pdf
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