A device for use in improving the heating efficiency of a water heater comprises a housing mounted over the heat collecting pieces of the water heater and provided with at least a set of ventilating ports, each of which is composed of a predetermined number of ventilation holes arranged in a geometric pattern. Pivoted to each of the ventilation holes is a damper, which can be triggered by a heat sensing element to turn so as to open or close automatically the ventiation holes. The heat sensing element is caused by heat to deform in order to bring about a displacement of the damper. The extent of the opening of the ventilation holes depends on the extent of the displacement of the damper.