I've a customer who has a gutted house and wishes to keep the old cast radiators (hot water heat). Since new pipe has to be run to them, does anyone have a good idea on how to size the pipe required?
New boiler has a circulator and if all is equal 3/4" should work. However downstairs zone has 11/4" pipe.
Anyone have thoughts?
Joe

It depends on the heat load of the finished house, radiator size and distance from boiler to radiator.

I re-pipe old gravity systems dozens of times a year, using PEX tubing mostly. Many are home run systems and many also get TRVs on the master bedroom radiator at least.

I design the system from the condensing boiler up.