Method and device for defrosting of an evaporator (1) in a heat pump (2) connected to an air handling unit (9), which comprises a controllable heat recovery device (10). The heat pump (2) comprises a refrigerant system (3) with a refrigerant (4), a compressor (5), a condenser (6), a first expansion valve (7) and an evaporator (1). A four-way valve (8) is arranged after the compressor (5) and before the condenser (6), for changing a flow direction of the refrigerant (4) to the evaporator (1) instead of the condenser (6). The heat recovery device (10) recovers energy from first air stream (11) and transfer energy to second air stream (12). When frost formation occurs, the refrigerant temperature is increased and the recovery device (10) is regulated down so that heating of the evaporator (1) is accomplished, both from inside and outside to defrost the evaporator (1).