. The flo-check on each heat zone prevents the hot water that is circulated between the PB and OB from traveling to each zone. A call for heat on a given zone starts the circulator for the zone, thus the flo-check for that zone opens , hot water flows to supply the heat demand, heat demand meant, circulator shuts off, flo-check closes.Wayne64SS said:yea i'm good with that,, but what prevents the water from traveling through the zone instead of just circulating through the two boilers.