Sizing Pump for Hot Water Radiators.

JStar

How do you figure your head pressure for a system with all converted hot water radiators?

Alan (California Radiant) Forbes

Pressure drop

If they are old-style column-type radiators, the pressure drop is minimal since the passageways are large. 



Figure your pressure drop through your boiler and piping and then add a couple of feet of head.

Steamhead

Here's an article that may help

especially if this is a converted gravity system:



http://www.heatinghelp.com/article/11/Hot-Tech-Tips/238/Sizing-Circulators-for-Hot-Water-Heating-Systems