Charlie Z wrote:The question is, how does the AFUE (output efficiency) of the combo coal boilers compare to a modern, dedicated oil burner?
Always lower. How much lower and does it matter is open to debate. I would look at the combustion and operating efficiency of the primary fuel (coal) first then look at the efficiency of the backup fuel. The lower backup fuel efficiency might be acceptable since you are seldom using it. If however the backup fuel is heating the summer domestic hot water that's another story. I guess in that situation I would get a coal stoker that can run all summer and have an on-demand hot water heater as a backup. Or find an old "bucket a day" domestic hot water coal boiler. I just got one off Ebay for $10.