A float operated switch (21) is actuated when the level of refrigerant fluid in a reservoir (11) is below a predetermined value to produce an electrical signal. The signal is delayed by 10 to 30 seconds and applied to a switch circuit (54) to disengage an electromagnetic clutch (63) and thereby de-energize a compressor (64). The clutch (63) is also disengaged by the action of a thermal switch (61) when the temperature in a space which is being air conditioned drops below a predetermined value. The operation of the float operated switch (21) is inhibited when the sensed temperature is below the predetermined value and also when a motor which drives the compressor (64) is not operating to prevent erroneous de-energization of the compressor (64).