Berlin wrote:A couple of things, as was mentioned the ash drop door on the stack needs to be sealed; also, the coal boiler needs to enter higher on the stack than the oil boiler. I would also be looking at the kitchen area in the house, there is usually a thimble opening for a wood cook stove- these are often not even sealed up when they're covered over. Be sure the pipe from the coal boiler to the stack is not obstructed with soot/ash etc. I would strongly recommend an inline automatic electric stack damper in the connecting pipe of the oil boiler, not only will this increase the efficiency of the oil boiler, but, it will reduce air entering the stack, these can be wired in very easily.
Thanks Berlin, The liner is intact from top to bottom with the exception of the two openings for the oil and coal boilers. The cleanout door at the bottom of the chimney is sealed with a bead of RTV.
I plan on running the coal boiler with the oil boiler flue piping disconnected and its flue opening blocked as a test to see if there is an improvement. That will help me decide if an electric stack damper on the oil boiler is worth consideration.