Larry, you need to specify the baseline. Size, condition, exposure, age, and updates to the structure. I burn 9 tons, but I heat a large area and use lots of hot water for pressure washing . I'd say 4 to 7 tons is a fair average, assuming whatever average means. I know one customer who burns 18 tons in a 130. Needs the btu's, and is satisfied so he won't go to a bigger boiler; doesn't need to. It's all relative, IMO. 16 here this am. It's coming!
Wow, I generally get lightly roasted for being too technical. Now I see that it works both ways.
OK, our house was built in 1964, and it is at an elevation of 1,180 ft., which places it at an elevation that is about as high as it gets around here. It has 1,680 sq-foot upstairs, and 840 sq-ft in our walk out basement that was converted into the family room. If the family room requires only half of the heat of the upstairs, then for heating purposes lets call it 420 sq-ft, and thereby assume I'm '"effectively" heating about 2,100 sq-ft. The house has been insulated and upgraded to about the typical level of a 1980's constructed home.
Our area is exposed to about 6,300 annual heating degree days on average, with about 5,900 of those coming during the months when we are actively heating our home (which means when the T-Stats are turned on).
Combining these, I calculate that we are heating 2,100 sq-ft x 5,900 HDD's = 12,390,000 Sq-ft-HDD's (this being the average heat load requirement for our home)
4.6 tons of coal = 9,200 lbs. of coal, but of this roughly 800 lbs. are burned for the homes hot water over the heating season, so that means home heat by itself is consuming about 8,400 lbs.
8,400 lbs. burned/ 12,390,000 sq-ft-HDD's = 0.00068 pounds of coal burned per square foot per HDD to heat our home during the heating season.
Heating our house therefore requires approximately 0.00068 lbs. of coal burned per square foot per heating degree day.