@Patrick, as you appear to be THE go to guy on this stuff, I hope you don't mind me asking...
Goal:
I need to extend the (vertical) 3/4" copper pipe that goes through the living floor and connect one of my baseboards to the rest of the system.
Why:
The plumber that did the connection (the baseboards were all disconnected when the subfloor was rebuilt) decided to crimp the pipe under the floor and sized the pipe sticking out the floor using as a reference the new bare subfloor instead of the adjacent dining room floor. As it happens, he did not leave any slack in the pipe. Now that I am going to tile the semi-rough living room floor that means that I don't have enough clearance to place the cement board and tiles (in addition to an extra layer of plywood, but that is another complicated story) under the baseboard, and even if I had any it would look weird since the baseboard now sits at least 2" lower than the next section just across the door (living room and dining room are contiguous and not separated by any wall. Let me add that I fired this plumber in the middle of the renovation due to one too many glorious screw ups he performed on the kitchen sink vent and laundry water feed and drain, but this issue had been flying under my radar until I gathered the energy to start thinking about fixing the living room...
Problem:
I would ordinarily get my torch and add a section of copper pipe to that sucker to level it with the rest of the baseboards, but I am resisting ding that because of the rubber pipe crimped to the pipe under the floor in the crawlspace, which really doesn't make want to go down there to fix a leaky heating pipe.
Alternative Solution:
I was thinking about using a compression fitting but I have no idea if this would be acceptable on a baseboard heating system. I don't see why not but...
Thanks for any input.