Hi, I just had a cast iron radiator removed from our radiant system in one room to make way for a new entrance. We want to add radiant PEX heating loop in the floor underneath to replace that room’s heat and ideally want to connect to the feed and return pipes from the old radiator to tie in with our current heating system/schedule. The pipes were capped off and the tankless system was drained and refilled afterwards. What would be the best way to connect the PEX loop to these pipe ends? They are 2” cast iron. My installer was not interested in doing this project so I’m willing to take it on myself. The boiler is an NTI tankless combi system that provides both our radiant and hot water needs.
If this does sound like a viable project what would be the best sequence of events to shut down and restart the system?
Thanks in advance, Jack.

the question is confusing??

you talk about a 2 inch feed and return to a radiator but say it is fed from your radiant system.
what temperature water was fed to the cast iron radiator?
radiant heat runs at 90 to 130 deg F.
cast iron radiators run at 160 to 180 deg F.
please try to be more specific as to what you have now.

Packy is correct with the temperature difference between radiant floor and cast rads

Your best bet is to buy 30/40gal water heater and use that for floor heat or Run the pex loop to the house heat piping on the tankless and install a mixing valve to the floor heat only to bring it to proper temperature limits, either option would require a new individual thermostat for the floor heat

The existing 2” piping would stay capped in either case

Or maybe find a spot for cast iron baseboard if you want to use the existing piping

Packy, it sounds like they have cast iron hot water radiators in the house and want to convert one room to radiant floor heating. We had similar in our 1917 house in NY, cast iron radiators plumbed to an old gravity loop. The house had been converted from coal to oil and we converted to gas and installed a new Bosch tankless combi for both heat and hot water.

Good point about the temp differences and I would not connect a radiant loop to the old cast iron. I would consider simply relocating the old cast iron rad to another part of the room since it was moved to accommodate a new doorway. You can simply extend the 2" cast iron with 1/2" copper to the new rad location. It sounds crazy going from 2" to 1/2" but we had two cast iron rads in our house that were connected like this and they got the hottest! Just remove the caps from the cast iron pipes and use reducers to get to get to 1/2" and then use a sweat adapter.

If you are sold on the radiant loop, then consider the suggestions from the pros who have responded here.