Abstract: A method of defrost for an outdoor side heat exchanger in a heat pump having a refrigeration circuit in which a compressor, an indoor side heat exchanger, an expansion device and the outdoor side heat exchanger are connected together so that a heat collected in the outdoor side heat exchanger is radiated from the indoor side heat exchanger and that defrosting for the outdoor side heat exchanger is started when the outdoor heat exchanger is frosted, and a temperature sensor is provided to detect a temperature of the indoor side heat exchanger, wherein the defrosting is started when the temperature detected by the temperature sensor is not higher than a first predetermined level with a downward gradient, which is calculated on the basis of the temperature, of the same temperature becoming as sharp as, or sharper than, a predetermined gradient, and the first temperature level is increased after the temperature becomes as high as a second predetermined level wherein the second level.gtoreq.the first level.

Abstract: An air conditioner has a refrigeration system with a compressor, a motor for air-circulating fans, indoor and outdoor heat exchangers and a four-way valve connected through refrigerant conduits such that it can be operated selectably both for cooling and for heating. An electric heater is also provided for heating. An electrical circuit for the air conditioner has a series connection of (1) a first temperature switch which opens when the temperature of the motor becomes too high, (2) a thermostat of an ordinary kind which places its connector piece in an open or closed position by comparing room temperature with a user-set temperature level, (3) a relay which controls the switching of the compressor of the air conditioner on and off, and (4) a second temperature switch which opens when the temperature of the electric heater becomes too high.