Guest
zone valves or individual pumps would be the best way, but either requires an additional thermostat. If it would be impossible to wire that second thermostat on the upper level, then balancing cocks at the return lines to the boiler would allow you to restrict the flow to the lower level and force more flow through the upper zone. However, if your pump is not on the boiler supply side, then you might consider putting the balancing cocks on the feed lines to the zones in order not to starve the pump and create cavitation if you closed the valves too far.
