gravity boiler retrofit

Bry_2

I have an old gravity boiler with black pipe feeding 6 rads which I plan on switching to a pump fed pex system with two loops. I would like to tie into the existing pipe just below the rads and run 1/2" pex supply and returns tight to the ceiling with pex-al-pex. Can I grab a simple distribution manifold with a couple valves to supply this system and will 1/2" pex be sufficient to supply the radiators instead of 3/4" black pipe?

Ironman

Normally, 1/2" will be enough for most rads, but it does depend on the EDR (square footage) of the rad. A 1/2" line is good for about 15k btus, but when connected to a manifold, you'll only get about .75 gpm max. That's only good for 7500 btus.

There are radiation tables in the resources section of this site from which you can determine the EDR of your radiators.