A hydronic heating system employing comprises a water line, two or more boilers, and a control. In operation, the water line facilitates a circulation of water through a building and the boilers heat the water as the water flows through the water line. The control calculates a system energy load for operating boilers. The calculation of the system energy load is a function of a set-point water temperature for the hydronic heating system, a supply water temperature of water flowing relative to a supply point of the water line, a return water temperature of water flowing relative to return point of the water line and a flow rate of water flowing relative to a flow sense point of the water line.