tjzmc wrote: There is two inch iron pipe supply and returns in the basement that have 3/4 inch supply and returns branching off to each radiator. The problem is that when it calls for heat the circulator runs for about maybe a minute and it sucks all of that cold water into the boiler and then shuts off. The boiler has to run for awhile to heat the water up and then the proccess starts over again until it gets caught up. My plan is to split the system into two or possibly three zones.
Why split it up - to add zoning for comfort or to solve the "problem"?
What is the problem? - cold water into the boiler could be. If a steel boiler, thermal shock is not as big of a problem as cold return and condensation in the fire box. (increased corrosion). You want to maintain at least 140 deg. return water temp to a boiler unless you have a condensing type gas fire boiler.
It sounds like the system is a converted gravity system. Large supply mains with individual branches to radiators. I bet the mains and branches are precisely sloped. I have had no problem running forced circulation into gravity systems but one must understand that most old gravity systems have large mass and ample heating capacity.
I suspect that the thermostat calls for heat, the pump starts and the large mass of the gravity system overwhelms the boiler mass. The boiler then has to fire hard to catch up.
Look at your pump sizing. The gravity system will have very low head requirements. Use a small pump to supply the system. Low flow with high deta T. will move enough btu's to keep you comfortable and smooth out the boiler operation.. Maybe look at primary secondary pumping for the boiler and gravity system.