I am heating about the same size house with a KB-6. My house is about 100 years old, no insulation and original windows; it’s a bit drafty to say the least. Keystoker just ads a section of grate for each size larger stoker they make. My stoker has three grates. On these cold days the circulator may run for hours almost non stop and the stoker will turn off because it has reached its upper limit (180) on the aquastat. This tells me the stoker is putting out plenty of heat but my house is loosing heat about as fast as the baseboard puts it back.
With proper draft the Keystoker runs one minute out of a fifteen minute rotation of the timer to maintain fire. This timer is adjustable to maybe three out of fifteen or maybe more but one minute is their recommendation if everything is set up properly. The gearbox gives one push of coal every minute.
I found that rice and buckwheat burn about equally as well. I use buckwheat. I have an uncle who uses rice and know of others that use a mixture of both. I use between 2 to 2 ½ 5 gallon buckets of coal a day in this cold weather, one bucket when it’s not so cold. One bucket weighs about 40 pounds I think. I just started keeping records in January so I don’t know what actual usage for fire maintenance would be.
Maybe someone knows for sure about the square feet thing. My thinking always was that they are referring to the cast iron radiators that are sized by square feet per section. One square foot gives off 150 btu/hr at 170 avg. temperature. 150x900=135,000 btu/hr for steam 210 degrees gives 230 btu/hr per square foot. 230x560=128,800 btu/hr. At least the math works. I hope someone who knows for sure chimes in then we will all know for sure.
Give me a PM if you want to know more specifics about my Keystoker.
Information on cast Iron radiators and figuring sqare feeet can be found at http://www.burnham.com
look for product info under slenderized radiators