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Right off the bat, let me say that this question is strictly theoretical. The question is below, in bold. But first, some background.
I'm looking at the gas/oil heating cost comparison calculator at nwnaturalcompare.com.
The set of numbers I'm using is as follows:
The results I'm getting show an estimated usage of 613 gallons of oil (with the current oil furnace), 471 therms with a new 93% natural gas furnace, and 548 therms with a new 80% gas furnace. See attached screenshot.
With the new gas furnace, the overall heat load is 43.8 million btu per year, according to the calculator. (That's 471 therms at 93% efficiency, or 548 therms at 80% efficiency, assuming 100,000 btu per therm.)
For the old oil furnace, if we use that same heat load of 43.8 million btu's, and assume there's 138,000 btu of energy in a gallon of heating oil, I think that would mean 317 gallons of oil at 100% efficiency. Of course, oil furnaces don't operate at 100% efficiency, but to get to the 613 gallons that the comparison tool is coming up with, I believe the oil stove would need to be operating at 52% efficiency. (317/613 = 0.52)
(If I go back and select a highefficiency 90% oil furnace as the current equipment, the oil usage goes down to 571 gallons  or an actual efficiency of only 56%.)
So here's my question: can anyone point out to me if my thinking/math is incorrect, in figuring out what efficiency is being assumed for the current oil furnace?
Ignore the dollar figures that are being generated  I'm interested in fuel usage, rather than overall cost (which fluctuates based on gas/oil prices).
Thank you in advance!
I'm looking at the gas/oil heating cost comparison calculator at nwnaturalcompare.com.
The set of numbers I'm using is as follows:
 zip code: 97211
 electic provider: PGE
 square footage: 1800
 year built: 1970
 windows: average
 number of people: 4
 number of stories: 2
 current heating: oil  forced air  standard efficiency (89%)
 cooling: none
 compare to: natural gas furnace
The results I'm getting show an estimated usage of 613 gallons of oil (with the current oil furnace), 471 therms with a new 93% natural gas furnace, and 548 therms with a new 80% gas furnace. See attached screenshot.
With the new gas furnace, the overall heat load is 43.8 million btu per year, according to the calculator. (That's 471 therms at 93% efficiency, or 548 therms at 80% efficiency, assuming 100,000 btu per therm.)
For the old oil furnace, if we use that same heat load of 43.8 million btu's, and assume there's 138,000 btu of energy in a gallon of heating oil, I think that would mean 317 gallons of oil at 100% efficiency. Of course, oil furnaces don't operate at 100% efficiency, but to get to the 613 gallons that the comparison tool is coming up with, I believe the oil stove would need to be operating at 52% efficiency. (317/613 = 0.52)
(If I go back and select a highefficiency 90% oil furnace as the current equipment, the oil usage goes down to 571 gallons  or an actual efficiency of only 56%.)
So here's my question: can anyone point out to me if my thinking/math is incorrect, in figuring out what efficiency is being assumed for the current oil furnace?
Ignore the dollar figures that are being generated  I'm interested in fuel usage, rather than overall cost (which fluctuates based on gas/oil prices).
Thank you in advance!
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