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Predicting fuel use from Manual J load calculations

John King | Posted in GBA Pro Help on


I’ve been attempting to develop a reliable method for converting a building’s heating load as determined by a Manual J load calculation to actual fuel utilization. In particular, I’ve been trying to convert this to electric use for cold climate air source heat pumps. I’m using an Excel spreadsheet to do so.

I’ve taken an hourly temperature data set for the nearest weather station. I used this to determine an hourly temperature difference between indoor design temperature and outdoor temperature. I use this and the buildings heat load to calculate the BTU requirements for each hour.

At that point, I use a quick index/match formula to determine the heat pump’s capacity & COP at that outdoor temperature (data pulled from manufacturer submittals; in this case, LG & Mitsubishi centrally ducted units). I then calculate the amount of BTUs that can be provided by the heat pump & the amount of BTUs that will need to be satisfied by any auxiliary heat. I then convert the BTUs to kWh using the heat pump’s COP at that outdoor temperature & a COP of 1 for the auxiliary resistance heater’s contributions.

The problem I’m arriving at here is that this method indicates that a modestly sized home would require 22-25,000 kWh annually for heating alone. When adding up the required BTUs, it comes to the equivalent of nearly 1,800 therms used by a 92% efficient furnace! That’s roughly double the actual demonstrated energy use of the home I modeled.

I plugged in a few other houses, only to receive similar results. 

Are Manual J load calculations that far off, or have I missed something obvious in my calculations?

Any help appreciated!

Thank you,

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  1. Matt F | | #1

    1) Manual J overestimates loads, sometimes significantly. Complete the manual J pretty much as aggressively as reasonable and it will still end up overestimating the load. Thermal mass provides a significant advantage during temperature swings.
    2) What are you using for a base temperature?
    3) What do you have for COP data? Most the capacity tables I know of show max output at a given indoor/outdoor temperature. The system will spend much of its time off the max output, where COP's may be much higher. I thought NEEP had a additional publicly available data I was looking at not long ago, I can't find it.

    1. John King | | #4

      Thanks for the replies!

      I was using 60°F as my base temperature. My design outdoor temperature is 0°F.

      For COP data, I pulled from the NEEP list. I'm going to refine it further at some point with the capacity/COP charts included in some of the manufacturer submittal data - but this was just a very rough first estimate.

      I'm using 5 years of hourly temperature data with the missing bits filled in to let me formulas work. When I go to make a final calculator for others to use, I'm going to find a way to bin that data to make the sheet less unwieldy.

      I use an INDEX(MATCH to pull the capacity multiplier and COP (simplified) for each outdoor temperature. When I go to make the final product, I'm going to have the high stage, rated, and low stage capacities and COPs listed so that the final product is a bit more refined - though that's probably false precision.

      I can't upload attachments of more than 3mb unfortunately. Since I haven't binned the data yet, it's a 15mb file. You can find it here instead:

      As I said, this is a very rough sheet. I'm trying to see if I can make a calculator that will give me reasonably accurate numbers before I bin the data and make something that will be user friendly to others. Once I've done that, I'm going to add hourly temperature averages for other nearby weather stations as well.

  2. Ethan Foley | | #2

    If you post your spreadsheet, some of the pros on here might be better able to assist you.

    1. John King | | #6

      Here you go!

      It's a doozy. I haven't binned the data yet, so it's got 5 years worth of hourly data. It's 56,000 data points for the time being. When I start to turn this into something for general use, I'm probably going to bin it into hours spent at certain temperature ranges per month over a five year average so that I only have a couple hundred data points to pull from or less (as in: 14 hours spent between 0° & -7°F, 5 hours between -8° & -13°F in an average February, etc.)


  3. Jon R | | #3

    > 1,800 therms used by a 92% efficient furnace!

    How does your seasonal total HDDs value compare to published numbers? Also be sure to use the right base temperature. Post your spreadsheet if you want.

    1. John King | | #5

      You can find the sheet there. It's too large to attach here. I pulled the hourly temperature data from NOAA. I haven't converted it to HDD, but I'm in Ithaca NY. We can see between 6,000-8,000 HDD per year depending on the weather.

      I made a calculator a couple of years ago that calculated performance based only on degree day data, but I don't think that it adequately captured extreme low temperature performance. We very frequently experience temperatures near the lower operating range of cold climate heat pumps around here, so I wanted an accurate picture of backup capabilities.

      1. Jon R | | #7

        Also consider using fuel use data to infer a design day load and use that for your calculations. Once you get the right design load, even picking a reasonable fixed seasonal average COP will do better than a 2X error. Also simplify by using only 1 year of data. Your COP of 3.22 at 60F seems low. When I total your kWh and divide by years, I get 13,167 kWh (about 1/2 of what you said).

        For extra credit, calculate the optimal heat pump vs backup heat sizing (assuming some cost increase for a larger heat pump).

  4. CollieGuy | | #8

    Our home is a fifty-one year old, 270 sq. metre Cape Cod located in climate zone 5, and the bulk of our space heating needs are met by two, eleven year old ductless mini-splits (4.0 kW, 9.3 HSPF). The heat pumps are individually monitored at the panel, and so I know precisely how much electricity they consume, and this data is recorded in a worksheet on an hourly basis, along with that of my weather station. The particulars for the past three years are as follows:

  5. Matt F | | #9

    Just a quick look, you are using a COP of 2.13 at 41 degrees. I would expect a COP much higher than that in at that temp in actual use.

    I believe those COP's are all at max output, which is going to be a real problem as the unit operates at a much higher efficiency modulation point most of the time until you hit the capacity balance point. I am not aware of any manufacture providing enough data to do what you want. See how your energy consumption comes out using the HSPF. You can convert HSPF to average COP:

    What are details of your house? You are show 438 BTU/Hour/Degree.

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