Harold Kestenholz
There is rarely a problem with a pump being able to serve a zone when the other zone valves are closed. The problem appears when all the zones are open at once. The pump has to be able to pump all the water needed to provide heat in all the loops when they are all open. This is determined by knowing the amount of heat required by each zone then adding them up. The total heat required in btuh, divided by 10,000 is the required gpm the pump must deliver at the highest resistance the pump must encounter. This gpm capability is found in the pumping charts published by pump manufacturers.
There are helpful design computer programs online to give you the maximum gpm flow of pumps through 3/4 and 1/2-inch tubing. There are also helpful design procedures that lead you through the steps to properly design multi-zone systems using zone valves and circulators at: http://www.hydronic.net
Sorry there is no simple answer to the question, the answer depends upon the heat requirements of each zone and the pumps you have available or plan to use. This is the work that an experienced heating plumber learns to estimate and install hot water heat properly.
