I am performing a bathroom remodel. Part of my scope of work requires that I remove a 2.5'x2.5' cast iron radiator, and replace with a 5' baseboard radiator. The system is hot water.

The 5' baseboard (and the cast iron) are set up for 3/4" plumbing. The previous remodel employed 1/2" copper supply and return to the cast iron radiator. It seemed to work okay (with the c.i. radiator).

My question is since I have it all ripped up, should i spend the time and money (~$50) to run new 3/4" copper service to the main system? The main system is 1" iron (i think? - the o.d. is 1.25"). I don't want to go through the effort if this will work just fine. I am concerned about head losses due to the changing diameters, and I don't want to discover a problem in January.

The new baseboard is the 5' Slant/Fin model (3/4" copper tube w/ aluminum fins shrouded by a steel cover.

Finally, the bathroom radiator is the only radiator supplied by these lines, which split off the main trunk line in the basement.

Thanks for your help!