You are reading the Keystoker information incorrectly (Don't worry it's confusing and many do) What you need to go by is the Net Output column so, you you would probably be looking at a KA-6 which, at max, puts out 122,000 btus/hr. Remember also that the burner on your oil fired boiler is (hopefully) not running for more than 10 or so minutes then shutting off for a time during cold weather so the btu/hr rating is higher than what your house is losing at a constant rate, as opposed to a coal burner which continues to burn (at a lower rate but still produces heat) when your thermosat is satisfied. So the best way to size a coal boiler is to know what the heat loss of your house is at a "worse case" basis.(Say -20 degrees outside and 72 degrees inside). If you plan to operate the coal boiler in tandem with your current boiler, it's okay to to slightly undersize the coal boiler since you won't see too many -20 degree days and if you do, your oil fired boiler will come on to "help" the coal boiler at such time if needed. A KA-2 may work for you if you have a tight, well insulated house. A quick estimate for how much coal you will burn is that one ton of coal is approximately equal to 180 gals. of heating oil. I own (For full disclosure) CSM Stoves and Boilers in Monson MA and our price for bagged rice coal is currently $270/ton picked up and most others in the area are within $15+/-.