I have cast iron radiators. I am replacing the pipe in the basement with pex tubing. What size pex tubing should I use? I am going to have 5 runs with two radiators to a run. One large 30" by 10" with 17 to 19 sections and a smaller 30" by 10" radiator with 7 to 9 sections on it. I was thinking 3/4". Also what kind of manifold do you recommend?
Thank you any advice would be appreciated
Paul

I would calculate exactly what you need instead of taking a guess. You might have to split the cast iron radiators into different zones so the PEX tube can handle it. There is a special grade of PEX for HVAC applications. Do not use regular PEX.

You will need to calculate the following.

Flow rate from pump into PEX pipe. GPM and at what temperature.(system boiler water design temp)
Required BTU's of all areas serviced by the old cast iron radiators. (heat loss calculation)
Flow rate of the brand of PEX you choose to use including and bends and fittings.

Most if not all this info is available on the PEX manufactures web sites. Use Google for your search.

You can spend much $$$ only to find out that your system does not perform as it should. Do not make a mistake but calculate exactly what you have to have to do the job correctly.