Retro-fitting an oil-fired hot water, iron radiator system-Is this possible?

Henry, all you have to do is dedicate a few or install a few new radiators to work with solar hot water. I assume the radidators are the old cast iron type. Basically, if they are already converted to hot water, just cut them out of the loop, bypass, and repipe them so that solar hot water goes through them. As mentioned you can either use the existing heater or add some new ones. If you do add some SHW panels you may want to install a tank to be used for domestic hot water the rest of the year. You can even combine the SHW loop for domestic with the radiators. Just install the radiators on the return line going back to the panels for residual heat or add a second controller or aquastat and pump dedicated to the radiators.