Reversible heat pump system

The system comprises a compressor (10), an outside heat exchanger (24), and an evaporator (20) and condenser (22) both positioned internally of a space such as a vehicle cabin. In a cooling mode, the compressor (10) circulates refrigerant around the outside heat exchanger (24) acting as a condenser and the evaporator (20). In a heating mode, refrigerant is circulated around the outside heat exchanger (24) acting as an evaporator and the condenser (22). In a third mode, the outside heat exchanger (24) is isolated and refrigerant is circulated around the condenser (22) and evaporator (20). The third mode is useful for providing defogging and heat at the same time. Choice of mode is controlled by a control valve (30) acting in conjunction with reversing valve (14), evaporator valve (34) and condenser check valve (28).