by the inner tubes 53 54 serve as reservoirs for liquified working fluid and the outer paths provided by the outer tubes 55 56 have circulated through them either a hot fluid if the associated tube is to provide heated working fluid to an ejector 57 or a cold fluid if the associated inner tube is to provide a collector for liquified working fluid from the primary circuit as with previous embodiments the reservoirs are substituted for one another when the heated reservoir is almost empty and the cooled reservoir is almost full the upper ends of the inner tubes 53 54 are connected through respective non return valves 58 59 to a drive inlet 60 of the ejector vapourised working fluid is fed from the ejector to a reversing valve 61 supplying in accordance with its operating position one of tweo heat exchangers 62 63 the two operating positions of the valve 61 are respectively shown in fig8 and 9 in fig8 the vapourised working fluid passes from the valve 61 to the heat exchanger 62 which as providing heat used to warm a stream of air supplied tby a fan 64 the working fluid condenses in the heat exchanger 62 and is fed through a non return valve 65 to a cooling tank 66 this is kept at a low pressure by part of its contents being drawn off through an expansion valve 67 which discharges it adiabatically into the second heat exchanger 63 this acts as an evaporator and is connected via the valve 61 and the non return.