I lose only 3* on my water temp with the insulated, buried pipes that are underground for 120 feet
You want the pipes to be not only insulated, but enclosed in a WATERTIGHT tube, Ground Water will conduct heat away from the pipes better than any loss of insulation.
Hot water baseboard systems [finned tubing] are designed to work at 190* water, if your water temp is only 140-150* you will not keep your house warm
Between the heat loss in your underground pipes, and significant load on the system, I think you need a bigger boiler, or ??
What is the coal boiler rated at for BTU ??