Abstract: A control system for a hydronic heating system for an enclosed structure which includes a boiler and at least one heating zone controlled by a thermostat and having a circulator or zone valve for controlling delivery of heat from the boiler to the heating zone(s). The control system senses the ambient temperature external to the enclosed structure, and also the water temperature of the boiler. Responsive to those two sensed values, it determines an upper temperature for the boiler at which the boiler is shut off. The control system further periodically redetermines the upper temperature for the boiler in accordance with the heating demands of the enclosed structure, and accordingly is heuristic in nature. In the preferred embodiment, a central processing unit forms the heart of the control system, and digitally stores in memory words representative of the sensed temperatures and other pertinent parameters relating to timing.