Extrapolating Yearly Energy Usage from Manual J Calculations
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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Replies
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.
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.
This article explains how to calculate heating system size from your fuel bill and weather records:
https://www.greenbuildingadvisor.com/article/replacing-a-furnace-or-boiler
The process works in reverse, given a heating system size and weather records you can estimate what your fuel bill will be.
You mention
that results in really LARGE values which don’t seem to make sense.
Comment:
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.