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Mod cond boiler oversizing

by some who actually do heating work for a living,,some only have a M/C in their own home.<BR>But I agree, regardless of the HL program you use, stay with the smaller #s.<BR>Besides, you`re the one paying the fuel bills. <BR><BR>Dave


  • andy_21
    andy_21 Member Posts: 42
    Mod cond boiler oversizing

    with the new Mod Cond. boiler, is it really wasteful to oversize a boiler by 25/ 30K BTU. Won't the indoor/ outdoor reset compensate & only allow the boiler to fire to the necessary load demand?
    Planning a new boiler but keeping in mind a possible room addition in the future, that may not happen if the wife has her way.
  • Ken_40
    Ken_40 Member Posts: 1,320

    What would you prefer:

    1) A boiler that cycles all its electrical contacts and moving parts 1,000 times a heating season, or

    2) One that only cycles 800 times a heating season?

    (both will make the house exactly the same comfort level.)

    3) A boiler that only cycles 1,000 times that will definitely handle an additional load some time, or

    4) One that will cycle 800 times and probably STILL handle an addition - some day.

    Keep in mind: Most additions are "super-insulated," replace an older and less efficient envelope wall/ceiling/roof system - in favor of extremely well insulated and glazed additions that may actually diminish the overall heat load - not increase it - by virtue of being "added" to an old wall/ceiling/roof that had a relatively "poor" envelope?

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  • ALH_4
    ALH_4 Member Posts: 1,790

    I am not yet married, but from what I can tell there is a distinct chance that your wife may get her way. Is there a boiler available that exactly matches your existing load? You may end up slightly oversized simply because of the available sizes of your chosen boiler. That sounds like a substantial addition...1500sf or so?
  • Mike T., Swampeast MO
    Mike T., Swampeast MO Member Posts: 6,928

    Presuming a proper heat loss calculation has been made (and that's the only way to size the boiler), and presuming you find a good match, I would not increase the size any further.

    Heat loss calculations are by their very nature overstatements, so it's unlikely you'll ever need that much heat to maintain indoor temperature in any weather.

    You're right that the boiler will at least attempt to fire to the actual load, but that load drops as outdoor temp rises. 20,000 btu/hr is about the minimum modulation for the smallest available mod-cons, but ALL have a turndown ratio, e.g. 4-to-1. This means as the rated maximum output increases, so does the minimum.

    Once the load is less than the minimum modulation ability, the boiler will begin to cycle and efficiency will drop somewhat.

    The mod-con in my own house is technically undersized. DOE rated output of 81,000 btu/hr with a calculated heat (Manual J) of right at 100,000 btu/hr. I have NEVER run out of heat and the outside temperature has to drop into the upper 20s before the boiler begins what I consider "true" modulation (e.g. continuous operation that can continue for days at a time).

    If you're reasonably certain that an addition will be made within 10 years or so, I'd do my best to estimate its size and configuration. Then add it to the heat loss calculation. If it does not increase by more than 20%-25%, I still would not increase the size of the boiler.

  • Plumb Bob
    Plumb Bob Member Posts: 97

    > Won't the indoor/ outdoor reset compensate & only

    > allow the boiler to fire to the necessary load

    > demand?

    Outdoor reset has nothing to do it; that just sets the water temperature. But modulation does help. Still, as already stated, you probably don't even need what the heat loss calc says you need.
  • andy_21
    andy_21 Member Posts: 42

    Thanks for all the feedback. Heres where I'm @ Have been leaning toward the Knight unit w/ 80 gal. Indirect DHW. 3000Sq house resent total renovation all update thru out windows Insulation, etc., 1 fl- radiant, 2nd. fl Hot water coils in air handler.
    I did the Slantfin heat loss calc. & came up w/ 73k looking toward a possible addition w/ a heat loss of 14k for a future total of 88K.
    The Knight unit net IBR go from 63,83,or 118 . I was leaning toward the KBN 105 w/ 83 net IBR, the installer wants to use the KNB150 w/ 118k for a little cushion. said the outdoor reset would make up for the oversizing.
    Since I'm spending the extra for some hopeful future savings, would I be fine w/ the 83k unit & is there anything else I should include to get a more effecent set up?
    Once again Thanks for all the great Info!
  • Utopia,,

    difficult decision, at this point in time as M/C`s are manufactured to a max demand, modulating down to the lower(based on outside temps), the residence requires.

    Over,,well some people opt for a buffer tank(artificial-load).

    Under, you`ll likely freeze.

    Difficult to be exact as weather broadcasters have been trying for years.

  • Floyd_46
    Floyd_46 Member Posts: 2
    Go with the 83K....

    If the heat loss program says 88K then you will actually be okay with 70 or less. If it was my place, I would even push it down to the 63.... The 83 will still have some "cushion" built in, no need to pad it more...Stick to your guns the intaller probably won't like being told what to do by you.. :-)

  • Uni R_2
    Uni R_2 Member Posts: 589
    Don't use IBR

    Your boiler's not going to be installed outside so don't use the IBR. Oh... and your Manual J will be way overstated.

    If a conservative proper Manual J has you at 73K, then don't be surprised to find out that your true heatload at design is that divided by 1.5 or 49K. The modcon will give you a pretty good idea of the true maximum heatload.

    That said, the 80K Knight is plenty big enough for the job and to cover an addition as well.
This discussion has been closed.