PURPOSE: To prevent the decrease in heating efficiency due to cold-water overcooling by a residual refrigeration effect and due to excess heat dissipation to cooling water by bypass-controlling the quantity of cooling water passing through an absorber and a condenser by a three-way valve in response to cooling load.

CONSTITUTION: When the lowering of the outlet temperature of cold water 3 is sensed by a temperature detector 13, a control board 14 issues a command to a three-way valve 15 for bypass of cooling water 4, and reduces the quantity of cooling water passing through an absorber 5 and a condenser 10. Consequently, a temperature rise at a time when cooling water 4 passes through the absorber 5 and the condenser 10 is increased remarkably, and the temperature of cooling water 4 is brought close to the temperatures of a solution and steam in the absorber 5 and the condenser 10, thus lowering the capacity of heat transfer, then diminishing the quantity of heat transfer. When the outlet temperature of cold water is further lowered, cooling water 4 is bypassed completely to the absorber 5 and the condenser 10, thus stopping a refrigeration effect, then preventing the overcooling of cold water 3.