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Is this correct?

Kevin_54 Member Posts: 30
Does a 1 degree tempature drop in a boiler represent a savings of 3 BTU's? I thought i heard this once but i can't remember where.


  • Brad White_9
    Brad White_9 Member Posts: 2,440
    Only if

    you are dealing with three pounds of water and you are only speaking of net BTU's available after combustion.

    Otherwise I am not really sure of the context.
  • Kevin_54
    Kevin_54 Member Posts: 30

    It would be a radiant system running @ a lower tempature using a condensing boiler. So running the boiler @ say 120 opposed to 180 just trying to figure out what kind of potential savings.
  • ALH_4
    ALH_4 Member Posts: 1,790

    Does this chart help? It's not linear.
  • Steve Ebels_3
    Steve Ebels_3 Member Posts: 1,291
    It's been said.......

    The folks at a Viessman class I attended referred to the fact that for every 3 degree drop in boiler water temperature, below 140*, you pick up 1% in boiler efficiency. I think Andrew's chart bears this out. Low temp systems with condensing boilers ROCK!!! Nothing beats looking at exhaust temps that are 10-20* LOWER than your supply temp.
  • Mike T., Swampeast MO
    Mike T., Swampeast MO Member Posts: 6,928

    This is a digest of information--theoretical, manufacturer supplied and as recorded in my "wired" system.

    Using a condensing/modulating boiler

    Presuming 120F supply temperature at design load and presuming some form of reset and presuming a well-sized boiler you can sincerely count on 98% gross system efficiency for the heating season as a whole.

    If the boiler and all piping are in the conditioned envelope, you can consider seasonal net system efficiency very close to 98% as well.

    Significant daily setback is really the only way to reduce this seasonal efficiency as it will require higher water temperatures and/or higher boiler output during the recovery phase. Granted there is a savings from reduced load from lower room temperatures during the setback phase, but my personal opinion and measurements (substantiated here by someone else with a "wired" system) are that these essentially cancel each other out and there is no net savings. My personal opinion and measurements (not yet substantiated here) is that not using setback results in a savings if you maintain space temperature lower than the high setting when using setback.

    Using a conventional boiler with a fixed operating temp of 180F in the same system

    If you achieve 70% gross seasonal efficiency, consider yourself to be among the finest hydronic engineers--and pray that the always problematic vent damper isn't abandoned...

    More typically, you can expect 40% - 60% gross seasonal efficiency. I say with absolute sincerity that the more the customer will try to save with such a system, the lower the gross seasonal efficiency will be. The fuel savings will still outpace the efficiency hit, but not by much!

    If you operated the condensing/modulating boiler at a fixed temp of 180F in the same system

    Yes, it's possible even if it is rather stupid. You could expect a gross seasonal efficiency in the range of 85% - 92%. My personal opinion is that efficiency would wind up nearer the low end of this range than the higher.


    Yes, I know these numbers are at complete odds to AFUE. Condensing/modulating boilers and conventional boilers simply cannot be compared via AFUE. Search here and you will find innumerable reports from both homeowners and contractors to verify.
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