Cast Iron Radiator for New Room

[1890farmhouse]

My house has a two-pipe, single-loop hydronic heating system with cast iron radiators. I want to add a small cast iron radiator to a new 2nd floor bathroom. The pipes are all black metal with cast iron fittings. How do I tap into the 2.5" O.D. pipes in the basement to run new lines to the 2nd floor. I could tap into nearby lines to other radiators, but I'm thinking that may not be very efficient. I could also remove ten-foot sections between 2 other radiators and tie in a T with 2 shorter lengths on either side, but am concerned about frozen and brittle cast iron fittings. Any suggestions?

[Marlin336]

How old is your system? I never touch real old black iron pipes like that without first warning of the worst case scenario. That is you break a pipe or fitting, none of the fittings will come loose without breaking and the pipe is to worn or you don't have the room to thread in place. Meaning it's possible for a simple repair to turn into a re-pipe.
As for teeing off I wouldn't tap into other radiators lines. Each radiator should have it's own line coming off the main pipe.

[1890farmhouse]

The house is circa 1890, so the system was installed later, but is still very old. I have moved some radiators successfully within the same room, but was working off an existing Tee. I understand the worst case, but would still like to proceed. I'm wondering what approach a professional plumber would take if requested to do this? Based on that and the obvious risks you mentioned, I can decide if I should attempt myself. Thanks!