K factor is Degree Days divided by gallons of oil. Higher is better for the homeowner!
Where I live we have had 4352 DD in the past year, I have used about 550 gallons of oil.
K Factor 7.9
I have a formula somewhere, have to look for it. I condensed it so I could just multiply the
number of gallons used in a year by the number 55 to get the heat loss in my particular climate.
It's rough but close enough to size a boiler.
Just remember smaller is better with boilers, if your going to oversize something, oversize the indirect.