Disclosed is a method for detecting a leak in a closed loop heat transfer system comprising monitoring the pressure of the heat transfer composition inside said heat transfer system, wherein a drop in pressure indicates a leak. Also disclosed is a heat transfer system comprising an evaporator, a compressor, a condenser, an expander and a device for measuring internal system pressure. The system pressure measuring device is disposed inside the closed loop heat system. The internal system pressure measuring means may be located either between the evaporator and the condenser, between the expander and the evaporator, between the compressor and the condenser, or between the condenser and the expander.