This paper presents a new algorithm for combined heat and power (CHP) economic dispatch. The CHP economic dispatch problem is decomposed into two sub-problems: the heat dispatch; and the power dispatch. The sub-problems are connected through the heat-power feasible region constraints of cogeneration units. The connection can be interpreted by the unit heat-power feasible region constraint multipliers in the Lagrangian function, and the interpretation naturally leads to the development of a two-layer algorithm. The outer layer uses the Lagrangian relaxation technique to solve the power dispatch iteratively. In each iteration, the inner layer solves the heat dispatch with the unit heat capacities passed by the outer layer. The binding constraints of the heat dispatch are fed back to the outer layer to move the CHP economic dispatch towards a global optimal solution.