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Extrapolating Yearly Energy Usage from Manual J Calculations

jadziedzic | Posted in Energy Efficiency and Durability on

I have an HVAC design that was created by a firm specializing in that work; the design includes Manual J and S (plus D and T) reports plus the data used to perform the calculations (e.g., wall construction and R value).  I now know the correct size of equipment (BTU-wise) to ensure I’ll have heat at the lowest temperatures expected to occur in my area.  I also know the AVERAGE low temperature my area will likely see based on historical data.

How do I take the information from the load calculation and come up with how many BTUs I would use in an average heating season?  It seems like there’s a calculation using Heating Degree Days lurking about.  Simplistically it seems I’d take the maximum BTU requirement and multiply that by a fraction expressed by (design base – average temperature) / (design base – lowest temperature expected) * 24hours * HDD, but that results in really LARGE values which don’t seem to make sense.

Thanks for any pointers you can provide!

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  1. paul_wiedefeld | | #1

    24 x annual hdd65 x Heat loss / (65-design temp) would get you total output. Divide by efficiency to get total input. Manual Js will come in high most often, so discount it.

    1. jadziedzic | | #4

      Hi Paul - I'm sorry, I don't quite grasp the "(65-design temp)" part of your equation. With real numbers, my heat load at -4F worst-case temperature is shown as 33,519 BTU/hr, with indoor design temp 70F (thus temperature (delta-T = 74F)). The average winter temperature in January is around 20F, with 6008 HDD(65). With a rough assumption that heat loss is linear with temperature change, it seems I would need about 2/3rds of the heat load number at the average 20F temperature.

      Thanks for the note regarding input/output loads; I forgot to factor in that data. Certainly improves the numbers, but I'm still off in the weeds.

  2. Expert Member
    DCcontrarian | | #2

    This article explains how to calculate heating system size from your fuel bill and weather records:

    The process works in reverse, given a heating system size and weather records you can estimate what your fuel bill will be.

  3. mr_reference_Hugh | | #3

    You mention
    that results in really LARGE values which don’t seem to make sense.

    Yes, the numbers look high in terms of BTU. The cost of BTU/hr of energy is relatively low, which explains our problems with co2 in the atmosphere and climate change.

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