oe the “tricks of the trade” have a lot to do with the kind of heating system we encounter.
For example if it is a steam system properly installed the fittings would be cast Iron and easily cracked with a heavy hammer with a wrench holding back.
If it is hot water system the installer could have also used cast fittings Or black malleable which will not break when hitting them.
I normally just take a sharp chisel and possibly a reciprocating saw and cut out the piping at the boiler very carefully and start my new piping from there.
Just be careful as not to crack the cast Iron bossing at the boiler.
It actually takes me longer to explain how to do it then actually cut it out or cave in the threads with the chisel CAREFULLY.