Zone control by circulator control involves using a circulator pump to control the flow of hot water to different zones or areas of the building. This approach is often used in larger buildings with multiple zones. Zone control by three-way valve control, on the other hand, involves using a three-way valve to divert flow between the boiler and the heating circuit. This approach is often used in smaller buildings or where there are fewer zones to control. Both approaches can be effective, but the choice of which to use depends on the specific application and system design.