The quasi-empirical formula that I use to calculate theoretical daily coal consumption is as follows:
Daily Coal Consumption = (Daily HDD's x 1.34) +4.33
where I assume 4.33 lbs. per day to be used for the homes DHW needs, and not for home heating
and where: daily HDD's = 65 - (days high temp + days low temp)/2
and where I burn a flat 14.75 lbs. on all warm days with daily HDD's that are at or below 7.78
[the above 14.75 lbs. figure is because (7.78 x 1.34) + 4.33 = 14.75 lbs., and because my boiler needs to burn at least 14.75 lbs. per day to maintain its fire]
Here is my theoretical vs. actual coal consumption this season by month using the above empirical formula:
October: Theoretical = 775 lbs., Actual = 756 lbs.
November: Theoretical = 1,222 lbs., Actual = 1,110 lbs.
December: Theoretical = 1,351 lbs., Actual = 1,303 lbs.
January: Theoretical = 1,694 lbs., Actual = 1,705 lbs.
February: Theoretical = 1,598 lbs., Actual = 1,595 lbs.
March: Theoretical = 1,443 lbs., Actual = 1,466 lbs.
April (so far): Theoretical = 290 lbs, Actual = 270 lbs.
Totals to date: Theoretical = 8,373 lbs., Actual = 8,205 lbs.