Sizing boiler allowing for future expansion

Discussion in 'Boiler Forum' started by DPJ, Sep 10, 2012.

  1. SD4US

    SD4US New Member

    Oct 14, 2012
    Northern New Jersey
    I'm trying to follow Dana's very nicely detailed (many thanks for it) heat load calculation in post #5 (using my own numbers) but there is a step there, where something seems wrong:

    "so with a maximum of 780 BTU/degree- hour, that means your heat load at + 5F is less than or equal to 60 x 780= 78,000BTU/hr as an absolute max, which is pretty close to your online heat load numbers"

    Either you factored in something unknown into the above equation or it could be a simple mistake, as 60 heating degrees x 780 BTU per degree-hour = 46,800 BTU/hr, which is no longer very close to DPJ's online heat load figure (75,000).

    You felt (I guess correctly) that 78,000 BTU/hr was a significant over-estimate and further lowered it based on DHW and old boiler efficiency and came up with an end result of 55,000 BTU/hr (which you felt was still on the high side). If the quoted calculation was indeed incorrect, that final number would be only 33,000 BTU/hr.

    Does this make any sense?
  2. BadgerBoilerMN

    BadgerBoilerMN Hydronic Heating Designer

    Mar 4, 2011
    hydronic heating designer/contractor
    Better than nothing but nearly useless in renovation work, as the variables are many and the options with such software few. Let's start with a proper heat load. You have enough baseboard to support the larger boiler but that is only a one factor. The indirect water heater should not be added in the the heat load and a 40 is probably the same cost as 35-see MegaStor or Trin&Stor indirect water heaters. Foamed your rim joist yet?
  3. Dana

    Dana In the trades

    Jan 14, 2009

    Yup- it was indeed a typo/math-error (I need an editor! :) ), and...

    yes 46.8K is a realistic combined-use number. With the oversizing factor contributing to lower as-used efficiency of 75%, not 85% you're then looking at (46.8K x 75/85)- 41.3K for the combined use. If hot water was 20% of it, then the heat load portion is actually only 33K.

    But running the numbers on the actual fuel use against the actual heating degree days is important. If the oil supplier stamps a "K-factor" on the billing, it's the same information expressed differently: HDD/gallon rather than BTUs (or gallons)/HDD, but the conversion is 5th grade arithmetic (which I apparently need to re-take! :) )
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