the heated air may pass to atmosphere from the duct 11 or it may be used in a space heater 21 by ensuring that the temperature of the water in the heat exchanger is not less than 28 u00b0 c significant quantities of heat can be withdrawn from the hydraulic oil without lowering its temperature by more than 10 u00b0 c the arrangement shown in fig2 is an extension of that shown in fig1 in that a portion of the water leaving the coil 9 is diverted through the condenser 23 of a chiller not shown operating on the refrigeration cycle and a water water heat exchanger 25 before returning to the inlet to the coil 9 where it mixes with the water returning to the coil from the heat exchanger 5 water passed through the condenser 23 takes in heat from the refrigerant to raise the water temperature to 46 u00b0 50 u00b0 c depending on ambient temperature secondary water in the heat exchanger 25 absorbs some of the heat present in the primary water and the secondary water is passed through a mould 27 if necessary heat from an immersion heater or small boiler 29 is added to the secondary water to heat it sufficiently for it to be used for mould heating if additional heat is required the water leaving the water exchanger 25 can be passed through a heat exchanger 31 hydraulically in series with the heater 29 and the mould 27 as shown in.