I am remodeling my basement and want to replace my old head-height radiator system pipes with copper pipes that I can run between the joists and out of the way. The system is an old gravity system, but now has a new-ish boiler with a pump. The return pipes into the boiler are converted to 3/4" right at the boiler, and the outgoing pipe from the boiler is 1 inch, but quickly converts to 3 separate 2 1/2" systems that each feed 2 - 4 radiators.
Can I use 3/4" copper pipes and run them through the joists to the 1 1/4" pipes that head up to the radiators? I am worried about balance to the first and second floors and wonder if temperature loss in returning water will be an issue.
Thanks!

The smaller diameter pipe will not carry the same amount of heat as the larger pipes. Since the boiler discharge is 1 inch you can probably get by with 3/4 inch pipes BUT I would come off the 1 inch boiler line with a 2 tee setup and feed the 3 separate 2 and 1/2 lines with its each 3/4 line just to be sure of capacity. You can also put valves in the 3/4 lines to be able to adjust each run of heat to balance the system. Make sure all the air vents in the system are working correctly and if you need more now is the time to add them at the highest piping location. If you put auto bleed valves at the top of the system it will be a snap to get and keep the air vented out of the system.

In the real world I would do a calculation on all the needed BTU's and calculate the pipe from there but the system I described above should do the trick.