# Extrapolating Yearly Energy Usage from Manual J Calculations

| 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!

## Join the leading community of building science experts

### Replies

1. | | #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. | | #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
| | #2

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

3. | | #3

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.

• |
• |
• |
• |