I have recently converted from oil to gas in my 2500 sqft 1930s era Home. We have a hot water radiator system. We left the original radiator main pipes and risers. They are large pipes 3” mains. and as such the plumber used a rather large 1/8 HP pump model taco 0011-f4. We would like to move to three zones in the House, and also lower the electricity usage of the system. That pump uses 202 watts while running. I can’t seem to find information about how to size the main pipes and how that relates to pump size.
1> install a constant delta-P pump ERP rated circulator (it'll adjust the power to the required delta-P instead of running fixed RPM)
2> install constant-delta-P thermostatic preregulation valves (Caleffi Dynamical)
3> Calculate each valve maximum flow allow no more than the needed flow to heat each room
This configuration can let your (GAS) consumption go down by at least 10% if you have a condensing boiler and you set properly the delta-T, also can push down the electric consumption by about 40% or more (up to 95%), (Pump will run maximum just for the first hours, when valves start to throttle down radiator flow, pump 'reads' less need and slows down it's speed accordingly, pumps are also 'intrinsically' much more energy efficient (EEI<0,23 when older 'A-class' had EEI of about 0,40)
Similar configuration may be achived (knowing diameters and lenght of each pipe) using delta-p compensation before zone valves and using 'standard' preregulation valves on radiators (But in this case, calculating each valve is much more difficult, with dynamical calculation is quite easy)
If in fact the main delivery pipes are 3", then the old piping system was probably a gravity system. They usually worked well but were slow to respond to temperature changes and used a little more fuel than a pumped system. The contractor should have done a better job by re-engineering the whole system and replacing the oversized piping. As far as wanting to zone the system there are a lot of options depending on how the existing radiators are piped. You could just add a "Danfoss " type valve with an integral thermostat on each radiator. This would be the most simple zone system I can think of. Some type of thermostat would be used to cycle the boiler or you could run the boiler on an outdoor reset type device. The pump would probably run 24/7 as long as the water in the loop piping was above a minimum set point. Other than that you may have to re-pipe most of the delivery mains to achieve zoning. For that I would hire a heating engineer.