Abstract: Method and apparatus for remotely monitoring the condition of a heat transfer fluid in the liquid line of a heat transfer system. The operation of the system is controlled in response to the results of the remote monitoring which may be used to indicate excessive moisture in the heat transfer fluid, low levels of heat transfer fluid, and non-condensed transfer fluid in the liquid line. Based upon the monitoring of the transfer fluid for non-condensed fluid, the rate of movement of a cooling fluid past a condenser for the transfer fluid is varied so that the temperature of the heat transfer fluid is kept as low as possible without formation of bubbles of non-condensed transfer fluid in the liquid line.