Baseboard copper return pipe to PAP change?

dgoldstein

I'm going to have to do some pipe work on my living room's copper baseboard heater's feed and return lines. The living room heater is 3/4" tube/fin with a 3/4" copper return line running under the tube/fin. It "outputs" up and into the remainder of my 1st-floor heating that's several inches above the grade of the living room heater.

I need to cut the lines at the transition to the higher part of the house since the previous installer didn't use a long enough connection to allow the living room baseboard to sit at floor level. It currently angles up the wall to where it goes into the next section of the house - it's weird and looks terrible =\

Since the living room heater is the first in this series of elements, I was wondering if there was any merit to changing out the exposed run of copper return line to PAP so as to keep more heat in the water despite it being on the return side?

That aside, I was thinking of making the transition to the higher part of the house using PAP and some copper-to-PEX elbow couplers. Am just not sure whether or not I should match the PEX diameter to the copper or if I can or should go smaller?