Don I just finished a house last week where I had to repipe almost the whole house as vandals had stolen all the copper and steel piping off the boiler and the basement runs. They left the radiators and stubs to the first and second floors. They would have gotten them also but a neighbor called the police and they walked in with one guy holding copper in his hands and the other guy had a hack saw...StokerDon wrote:I guess I wasn't considering 3/4" "big". Now that I think about it, the 1/2" PEX should be able to deliver the 12,750 BTU to those two radiators. The 1/2" just seems small! Real easy to work with though.Rob R. wrote:Don, there is no need for 3/4" pex to feed a single radiator. It will certainly work fine, but depending on the system design you might need to throttle some of the loops. Radiators actually put out less heat if the flow rate through them is too high - it disrupts the convection currents within the radiator.
O.K., all the rest will be 1/2".
-Don
The jails are not full of smart people.. 
The stubs were still connected to the existing radiators and they were 1/2 inch copper with no radiator valves or any way to regulate flow from the boiler. I used 1 inch copper and made separate supply and return headers for the front two radiators and another set for the the back two radiators. I stubbed off the headers and ran all the supply and return runs to the radiators in 1/2 inch orange oxygen barrier pex. I was kinda leary at first about the BTU delivery but said "It was 1/2 inch before so I'll put 1/2 inch back."
It worked great. I followed up when we had our first cool spell where it hit 35F OAT and the folks asked how do we turn down the radiators as they are getting the rooms too hot!!! This is a small row home probably about 900 sq ft heated by a rad in the front living room, a rad in the front bedroom on the second floor then a rad in the kitchen at the rear and another rad in the rear bedroom on the second floor. Nothing in the bathroom. I split it with two zones one being the front zone and the other being the rear zone. There is only one thermostat in the living room and I changed it out with a 7 day programmable unit. With the homes on each side being occupied and I figured they would pick up heat from them through the common walls so I lowered the water temp to 150F high 130 low with a 10 differential. Your 1/2 inch pex should work just fine.
The folks had never lived in a hot water boiler heated home before and had been in all electric before. So they put a sheet up across the living room arch way and heated that room up to 80F with an electric oil radiator which they had done in their previous houses.
Same room where the thermo is at. They called me that the rest of the house was cold. 
I had to explain that they couldn't heat up only the room where the thermostat was at and expect the boiler to heat the rest of the house. A short explanation and another no heat call visit and they now understand. Take the sheet down, put the thermostat on 70 and leave it alone. I started to explain the set back function but decided to leave well enough alone for now. We'll get into setbacks later.
