Load calculations vs. actual fuel use
I’ve done my own fairly in-depth heating load calculation, tweaking and re-thinking certain aspects over time as I’ve learned more, and my result is 10,800 BTU/hr.
Today I decided to analyze my electric bills to reconcile theory vs. reality. What I found is that, despite a lot of attention to accuracy, I think I still over-sized by a lot. My heating load appears to only be about 5,800 BTU/hr.
Some details up front: I’m in zone 4c. The house is about 900sqft, built in 1950, single story, heated with a Mitsubishi FE18NA. During the period I analyzed, the basement had no heat of its own. (We just very recently put another mini split down there.) The attic is air sealed with R-50 of cellulose, and the walls are 2×4 construction with dense packed cellulose. The basement is uninsulated, but reasonably well air sealed at this point.
Here was my process:
- Looking through my electricity usage history, I found a very mild summer week where we used almost no heating or cooling. During this week we used 12.57 to 17.31 kWh/day. Based on this, I roughly guessed my baseline load (i.e. a day with zero heating/cooling) to be about 12 kWh/day.
- I found the harshest day in winter. During this day we used 58.52 kWh. It was Christmas Eve and heavy cooking clearly padded this number, but not by a huge amount.
- I subtracted 12 from 58.52 to find out roughly how much energy was spent solely on heating on that day.
- I found the Heating Degree Days for that day.
- I divided the kWh-used-for-heating by the HDD to find out how many kWh were spent on heat per HDD on the worst day.
- I multiplied that last number by the HDD expected on a design day (i.e. at 25F) to see what my kWh would be on a design day.
- I converted that to BTU and then divided by 24 to find the BTU/hr I need on a design day.
Is that a reasonable way to calculate what I’m looking for? 5,800 BTU/hr still might be a little too high, given that we did so much cooking on that worst day. But it seems surprisingly low. And now that my basement is heated, this upstairs unit will have even less load than that. So my upstairs heat pump is probably 3x or even 4x oversized. How embarrassing!
Martin: If you’re looking for article ideas, a guide to performing this kind of calculation could be a great companion to your heat loss calculation series.
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