All back boilers are rated differently. You will have to find out the output from it. If your thinking of putting a back boiler on an open fire and heating a 2000sq ft house with that GOOD LUCK. A back boiler on a stove is a very good option but you have to find out the kW output is first. For 2000sq you need a min (mad guess) 20kW or 70,000 BTU.
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see that the Erin will do 9 rads, output is 13.2kW (45,000 BTU).
Compare that to an oil boiler for most oil burning houses which is a Grant 70/90 which means that it has an output range from 70,000 to 90,000 BTU or 20kW to 26kW.