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  1. #1
    Join Date
    Aug 2008
    Location
    Lindenhurst, NY
    Posts
    6

    Confused Do heat-up losses warrant a larger boiler?

    In reading this forum, it seems that the optimal configuration for efficiency is a mod/con boiler + outdoor reset + indirect DHW that is sized to provide just enough output to match the home's heat loss.

    It seems to me that using a boiler that just meets heat loss would be less efficient than an oversized boiler that can get the house hot quickly. Let us say the ideal temperature profile for the house during the week is:

    Outside temp: 10
    Desired temp:
    6am - 9am 70
    9am - 6pm 40
    6pm - mid. 70
    mid - 6am 60

    and the thermostat set points are adjusted so that the profile is actually achieved by turning on the boiler earlier. If I had a smaller boiler, then it would take much longer to achieve the desired temperature in the morning and evening; thus the boiler would turn on earlier, say:

    Actual temp w/ small boiler (heat loss = 10/hr @ 60; output = 12/hr)
    1am - 6am 60->70
    6am - 9am 70
    9am - 10:30 70->55
    10:30 - 6pm 55->70
    6pm - mid 70
    mid - 1am 70->60

    Avg temp: 65 for 6hrs, 63 for 9 hrs, 70 for 9 hrs = 66

    A larger boiler would heat up faster and thus the average heat loss of the house would be lower (since it is on average cooler than the house with the small boiler). With a modulating boiler, short-cycling losses are less of an issue, wouldn't it make sense to "oversize" it to reduce the need to heat-up the house early?

    Actual temp w/ small boiler (heat loss = 10/hr @ 60; output = 20/hr)
    5am - 6am 60->70
    6am - 9am 70
    9am - 12:30 70->40
    12:30 - 3:30 40
    3:30 - 6pm 40->70
    6pm - mid 70
    mid - 1am 70->60
    1am - 5am 60

    Avg temp: 65 for 2hrs, 70 for 9 hrs, 40 for 3 hrs, 55 5 hrs, 60 for 5 hrs = 60

    At 10 outside, this would mean the smaller boiler would use 12% more energy, even though the occupied temperature is the same! Please tell me where this analysis fails.

    Thanks for sharing your knowledge!
    - Bryan

  2. #2
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    68,064
    If you have radiators sized for 60,000 BTUs an hour.
    Weather you have a 80,000 BTU boiler, or a 1,000,000 BTU boiler. The rads can only emit 60,000 BTUs of heat in an hour.
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    How-to-apply-for-Professional

    How many times must one fix something before it is fixed?

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