Dead Rad Pipe Question

oldhousenut

I just recently removed a working rad(2 pipe HW converted from steam) from a converted attic space and installed fin tube which is being fed from a new run to the boiler. Question is, the old steel supply and return also feeds a working 2nd floor rad. I capped the two pipes at the elbows just under the attic floor. My plumber has gone back and forth as to whether to just cap them or connect them to allow a loop. He says that if they are capped they will maintain air in them and act as an expansion tank. In another part of the attic there is are supply/return terminations(old expansion tank?) that are connected to a different rad on the second floor and these pipes are conneected w/ a bleeder attached. Any thoughts?? Thanks!