a system for making modular all of the components of hydronic heating systems so that they can be assembled in various configurations with a minimum of error and labor . the system includes a modular transition adaptor that completes a first hydronic loop and provides a supply outlet and return inlet for servicing a second hydronic loop which is dependent from the first . the transition adaptor has standard spacing and fittings for simple and error free connection to other components such as boilers and mechanical modules . the system also includes a modular supply - return connector that can be connected to the branch outlet and branch inlet of the transition adaptor and to other supply - return connectors . each supply - return connector includes further branches to provide heated liquid to hydronic radiator loops . the supply - return connectors may include pumps or valves .