TEMPERATURE CONTROL APPARATUS AND A CENTRAL UNIT FOR TEMPERATURE CONTROL APPARATUS

Abstract: Temperature control apparatus (1) for controlling temperature in a building comprises a central unit (2) for supplying a heat transfer medium, namely, water to a remote unit (3) through a circulating circuit (4). The central unit comprises a reversible refrigeration circuit (8) comprising a main heat exchanger (11) which exchanges heat between the refrigerant medium and the heat transfer medium. A return temperature sensor (32) monitors the return temperature of the heat transfer medium to the main heat exchanger (11), and a differentiating circuit (34) determines the rate of exchange of change of the return temperature with respect to time. A microprocessor (26) controls a compressor (12) of the refrigeration circuit (8) for varying the energy output of the refrigeration circuit (8) in response to the rate of change of the return air temperature.

Description: "Temperature control apparatus and a central unit for temperature control apparatus" Field of the invention The present invention relates to multi-zone temperature control apparatus, and in particular, though not limited to multi-zone space temperature control apparatus for cooling and/or heating respective zones of a building or the like independently of each other. The invention also relates to a central unit for supplying a heat transfer medium to at least one remote unit of temperature control apparatus. Further, the invention relates to a remote unit for receiving a heat transfer medium from a central unit for controlling temperature. The invention also relates to temperature control apparatus which comprises at least one central unit and one remote unit. The invention also relates to a method for controlling the energy output of the central unit. Background to the invention Temperature control apparatus for space heating and/or cooling for controlling the temperature in one or more zones of a building is known. One type of temperature control apparatus comprises a central unit which comprises a refrigeration circuit, which may be reversible and operated in a chilling mode and a heat pump mode for delivering cooling and/or heating to one or more remote units mounted in . the zones of a building for controlling the temperature of one or more zones of the building. However, in general, such apparatus are restricted to either supplying cooling or heating at any given time. In other words, the remote units would normally all be delivering heating or cooling at the same time. It is not possible, in general, to provide a plurality of remote units connected to a single central unit where the remote units can simultaneously, independently of each other supply heating and cooling to control the temperature of respective zones. A further problem with such apparatus for heating and/or cooling a building which comprises a central unit for supplying heating and/or cooling to a remote unit or units is that such apparatus tend to be relatively inefficient in use. A particular problem with such apparatus is that, in general, the central unit generates heating or cooling at a rate which is largely independent of the rate at which the remote unit or units is or are demanding heating or cooling. This it will be appreciated leads to considerable loss and wastage of heat or cooling energy which is undesirable. There is therefore a need for multi-zone temperature control apparatus for controlling the temperature in one or more zones. There is also a need for temperature control apparatus for controlling the temperature of a single zone. Further, there is a need for a central unit for supplying a heat transfer medium to one or more remote units of such apparatus, and there is also a need for a remote unit for such apparatus. There is also a need for a method for controlling the energy output of the central unit. The present invention is directed towards providing such a multi- zone temperature control apparatus, such temperature control apparatus, such a central unit and a remote unit and a method. Throughout this specification, where reference is made to heat being transferred between a central unit and a remote unit, it is to be understood that the heat may be transferred both ways or one way only between the central unit and remote unit. For example, heat is transferred to a remote unit when a central unit is supplying heating energy to the remote unit, and heat is transferred from the remote unit to the central unit when the central unit is supplying cooling energy to the remote unit. In other words, when a central unit is operating in a chilling mode, cooling energy is supplied to a remote unit, and accordingly, heat is being transferred from the remote unit to the central unit. On the other hand, when a central unit is operating in a heating mode for supplying heating to the remote unit, heat is being transferred from the central unit to the remote unit. The central unit may be provided to operate in a chilling mode only, or in a heating mode only, or both. Further, reference to a refrigeration circuit is to be understood to mean reference to any such circuit which may operate in a chilling mode for providing cooling or in a heating mode for providing heating or in both modes. Objects of the invention One object of the invention is to provide multi-zone temperature control apparatus which permits independent control of the temperature of different zones, and further, permits heating of one zone while at the same time another zone is being cooled. It is also an object of the invention to provide such multi-zone heat control apparatus which operates relatively efficiently, and which can be installed at relatively low cost and with minimum inconvenience. Another object of the invention is to provide temperature control apparatus for controlling the temperature of at least one zone which operates relatively efficiently, and which may be installed at relatively low cost with minimum inconvenience. A further object of the invention is to provide a central unit for such temperature control apparatus or multi-zone temperature control apparatus which operates relatively efficiently, and which can be installed at a relatively low cost and with minimum inconvenience. It is also an object of the invention to provide such a central unit in which the energy output delivered by the central unit can be matched to the demand for energy by a remote or remote unit. A further object of the invention is to provide a remote unit for such temperature control apparatus or multi- zone temperature control apparatus which operates relatively efficiently, and which can be installed at relatively low cost and with minimum inconvenience. Another object of the invention is to provide a method for controlling the energy output of a central unit of temperature control apparatus so that the energy output can be matched to the demand for energy from a remote or remote units of the temperature control apparatus. Summary of the invention According to the invention there is provided a central unit for supplying a heat transfer medium to at least one remote unit of temperature control apparatus for transferring heat between the central unit and the remote unit, the central unit comprising a refrigeration circuit having a refrigerant medium therein, the refrigeration circuit comprising a master heat exchanger for exchanging heat with the refrigerant medium, and a main heat exchanger for exchanging heat between the refrigerant medium and the heat transfer medium, a compressor means for compressing the refrigerant medium, and an expansion means for expanding the refrigerant medium, return temperature monitoring means for monitoring the return temperature of the heat transfer medium returning to the main heat exchanger, differentiating means for determining the rate of change of the return temperature of the heat transfer medium with respect to time, first control means responsive to the differentiating means for controlling the energy output of the refrigeration circuit in response to the rate of change of the return temperature of the heat transfer medium with respect to time. By providing a control means which is responsive to the differentiating means for controlling the energy output of the refrigeration circuit in response to the rate of change of the return temperature of the heat transfer medium with respect to time, the energy output of the central unit can be substantially matched to the demand for energy being placed on the central unit, by one or more remote units. This leads to a relatively efficient device and minimises energy wastage. Preferably, the first control means varies the energy output of the refrigeration circuit in response to a change in the rate of change of the return temperature of the heat transfer medium with respects to time. This permits relatively accurate matching of the energy output of the central unit to the demand for energy. Advantageously, the first control means is responsive to the rate of change of the return temperature of the heat transfer medium with respect to time moving from one predetermined range of rates of change of return temperature to another range. By having the first control means responsive to the rate of change of the return temperature moving from one predetermined range to another, an apparatus which operates relatively efficiently is provided and frequent variations in the operation of the central unit are avoided. In one embodiment of the invention the first control means is responsive to the rate of change of the return temperature of the heat transfer medium with respect to time reaching a predetermined value. By having the first control means responsive to the rate of change of the return temperature of the heat transfer medium with respect to time reaching a predetermined value, particularly advantageous form of the invention is provided. The central unit according to the invention operates particularly efficiently and frequent variations in the operation of the central unit are avoided. In one embodiment of the invention the first control means comprises compressor control means for controlling the compressor means for varying the energy output of the refrigeration circuit. By controlling the compressor means of the refrigeration circuit relatively effective control of the central unit is provided and matching of the energy output of the central to demand is relatively efficiently obtained. Preferably, the compressor control means comprises means for controlling the mark/space ratio of a power supply being delivered to the compressor means. By controlling the mark/space ratio of the power supply being delivered to the compressor controller relatively efficient and effective control of the refrigeration circuit is achieved. Advantageously, the compressor control means varies the mark/space ratio of the power supply to the compressor means inversely to the rate of change of the return temperature with respect to time. This provides relatively efficient and accurate means of controlling the central unit. In one embodiment of the invention a circulating pump means for circulating the heat transfer medium through the main heat exchanger is provided, the first control means comprising pump control means for controlling the delivery of the pump means, the pump control means being responsive to the differentiating means for controlling the delivery of the pump means in response to the rate of change of the return temperature of the heat transfer medium to the main heat exchanger with respect to time. Controlling the pump means further facilitates control of the central unit for matching the energy output of the central unit to the demand. In one embodiment of the invention the pump control means varies the delivery of the pump means inversely to the rate of change of the return temperature with respect to time. This provides relatively efficient means of controlling the circulation of the heat transfer medium which in turn facilitates matching the energy output from the cental unit to the demand. In one embodiment of the invention the first control means is responsive to the return temperature of the heat transfer medium. By having the first control means also responsive to the return temperature of the heat transfer medium, matching of the energy output of the central to the demand from the remote unit or units is further facilitated. In one embodiment of the invention the first control means is responsive to the return temperature of the heat transfer medium moving from one predetermined range of return temperatures to another range. This provides a relatively efficient means of controlling the pump means, and frequent variations in the operation of the central unit are avoided. In one embodiment of the invention the compressor control means varies the mark/space ratio of the power supply to the compressor means proportionately to the temperature difference between the return temperature of the heat transfer medium and the flow temperature of the heat transfer medium flowing from the main heat exchanger. This further facilitates matching of the energy output of the central to the demand by the remote unit or units. In a further embodiment of the invention the pump control means varies the delivery of the pump means proportionately to the difference between the return temperature of the heat transfer medium and the flow temperature of the heat transfer medium flowing from the main heat exchanger. This provides a relatively efficient control means for the pump means. In one embodiment of the invention the refrigeration circuit is reversible and is operable in a chilling mode and a heat pump mode, and means for reversing operation of the refrigeration circuit between the two modes is provided. The advantage of providing a reversible circuit is that a single central unit may be operated in a chilling mode for providing cooling to the remote unit and in a heat pump mode for providing heating to the remote unit. Advantageously, the compressor means is a scroll compressor. It has been found that a scroll compressor is a relatively efficient compressor and is particularly suitable for control for varying the energy output of the refrigeration circuit. Preferably, the heat transfer medium is water. Where the heat transfer medium is water a relatively low cost and efficient apparatus is provided, and furthermore, the apparatus is environmentally friendly and does not provide a health hazard. Additionally, the invention provides a remote unit for receiving a heat transfer medium from a central unit of temperature control apparatus for transferring heat between the central unit and the remote unit, the remote unit comprising a secondary heat exchanger for exchanging heat with the heat transfer medium, a booster heat delivery means, and a heat transfer means for transferring heat to or from the secondary heat exchanger and the booster heat delivery means, air temperature monitoring means for monitoring the return temperature of air to the remote unit, and second control means responsive to the air temperature monitoring means for controlling the heat transfer means and for delivering a signal to the first control means for activating the central unit. The remote unit provides a relatively efficient apparatus for providing heating and/or cooling to a zone, and by virtue of the fact that a signal is transmitted from the remote unit to the central unit, the central unit reacts quicker than if the central unit were reliant solely on the flow and return temperatures of the heat transfer medium. Further, the invention provides temperature control apparatus comprising a central unit according to the invention and a remote unit also according to the invention, the remote unit being connected to the central unit by a circulating circuit for circulating a heat transfer medium between the central unit and the remote unit. The temperature control apparatus according to the invention is a particularly efficient apparatus. Further the invention provides multi-zone temperature control apparatus comprising a plurality of remote units, one remote unit being provided for each zone, a central unit for supplying a heat transfer medium to the remote units for transferring heat between the central unit and the respective remote units for controlling temperature of the zones, each central unit comprising a main heat exchanger for exchanging heat with the heat transfer medium, and first control means for controlling the central unit, each remote unit comprising a secondary heat exchanger for exchanging heat with the heat transfer medium, a heat transfer means for transferring heat between the secondary heat exchanger and the zone, air temperature monitoring means for monitoring the temperature of air in the zone, and second control means responsive to the air temperature monitoring means for controlling the heat transfer means and for delivering a signal to the first control means for activating the central unit in response to a change in temperature of the air, the apparatus further comprising a plurality of circulating circuits for communicating the secondary heat exchangers of respective remote units with the main heat exchanger of the central unit for circulating the heat transfer medium between the main heat exchanger and the respective secondary heat exchangers, circulating means being provided in respective circulating circuits for circulating the heat transfer medium. The advantage of the multi-zone temperature control apparatus according to the invention is that it permits the temperature in different zones to be controlled at different levels. It also permits independent control of the temperature in the zones relative to each other, and under certain condition, enables some zones to be heated while at the same time others are being cooled. In one embodiment of the invention each circulating means is responsive to the first control means. The advantage of having the circulating means responsive to the first control means is that relatively efficient control of the apparatus is achieved. In another embodiment of the invention the heat transfer medium is water. This provides a relatively low cost and efficient apparatus which is also environmentally friendly and does not present a health hazard, and furthermore may be relatively easily installed. In another embodiment of the invention the circulating circuits are connected to the main heat exchanger independently of each other. This permits operation of the remote units independently of each other. In one embodiment of the invention each remote unit comprises a booster heat delivery means, the heat transfer means co-operating with the booster heat delivery means for transferring heat between the booster heat delivery means and the zone. This permits some of the remote units to provide heating at the same time others of the remote units are providing cool ng. For example, where the central unit provides heating or cooling and the booster heat delivery means provides the alternative form of energy, the central unit may thus supply the remote units requiring the type of energy being supplied by the central unit, while the other remote units can supply the alternative form of energy by means of the booster heat delivery means. Additionally, the booster heat delivery means may supply additional energy if the central unit is unable to meet the demand. Preferably, the booster heat delivery means is responsive to the second control means. This provides efficient control of the apparatus. In another embodiment of the invention each booster heat delivery means comprises a heat source. This permits the remote units to provide additional heat from the booster heat delivery means in the event that the central unit is unable to meet the demand for heating by the remote unit, either as a result of lack of capacity, or the central unit being in a chilling mode supplying cooling to another remote unit. Advantageously, each booster heat delivery means is provided by an electrically powered heat source. This provides a relatively efficient and easily installed remote unit. Preferably, each secondary heat exchanger is provided by a coil heat exchanger. This leads to a relatively efficient remote unit. Advantageously, each heat transfer means comprises a fan. This provides a relatively efficient remote unit. Preferably, the fan is electrically powered. This provides a relatively efficient remote unit. Advantageously, each circulating means comprises a circulating pump. This provides a relatively efficient apparatus. Preferably, each circulating pump is an electrically powered variable speed circulating pump. This provides a relatively efficient apparatus. In one embodiment of the invention the air temperature monitoring means are mounted in the respective remote units for monitoring the return air temperature of air returning to the respective remote units. This provides a relatively efficient remote unit with a relatively quick response time. In one embodiment of the invention the central unit comprises a refrigeration circuit having a refrigerant medium therein and comprising the main heat exchanger for exchanging heat between the refrigerant medium and the heat transfer medium, a master heat exchanger for exchanging heat with the refrigerant medium, a compressor means for compressing the refrigerant medium and an expansion means for expanding the refrigerant medium, the refrigeration circuit being responsive to the first control means. This provides a relatively efficient construction and operation of apparatus. In another embodiment of the invention the refrigeration circuit is reversible, and means for reversing the refrigeration circuit is provided. The advantage of providing a reversible refrigeration circuit is that a single central unit may provide cooling and heating energy. Preferably, the multi-zone temperature control apparatus comprises a central unit according to the invention. The advantage of providing the multi-zone temperature apparatus with such a central unit is that the energy output of the central unit can be matched to the demand of the remote units, and an efficient apparatus is provided. In one embodiment of the invention flow temperature monitoring means for monitoring the flow temperature of the heat transfer medium from the main heat exchanger is provided in each circulating circuit. In a further embodiment of the invention flow measuring means is provided in each circulating circuit, the flow temperature monitoring means and flow measuring means being connected to the first control means for enabling computation of the energy delivered to the secondary heat exchangers of the respective remote units. The advantage of providing flow measuring means in the circulating circuits is that it provides for computation of the energy being supplied to the respective remote units. The invention also provides a method for controlling the energy output of a central unit of temperature control apparatus, wherein the central unit is of the type which supplies a heat transfer medium to at least one remote unit of the temperature control apparatus for transferring heat between the central unit and the remote unit, and the central unit comprises a refrigeration circuit having a refrigerant medium therein, the refrigeration circuit comprising a master heat exchanger for exchanging heat with the refrigerant medium, and a main heat exchanger for exchanging heat between the refrigerant medium and the heat transfer medium, a compressor means for compressing the refrigerant medium, and an expansion means for expanding the refrigerant medium, the method comprising the steps of determining the rate of change of the return temperature of the heat transfer medium returning to the main heat exchanger with respect to time, and controll ng the energy output of the refrigeration circuit in response to the rate of change of the return temperature of the heat transfer medium. The advantage of the method is that it permits the energy output of the central unit to be substantially matched to the demand from a remote or remote units. In one embodiment of the invention the method comprises the step of varying the energy output of the refrigeration circuit in response to a change in the rate of change of the return temperature of the heat transfer medium with respect to time. In another embodiment of the invention the energy output of the refrigeration circuit is varied in response to the rate of change of the return temperature of the heat transfer medium with respect to time moving from one predetermined range of rates of change of return temperature -to another range. In one embodiment of the invention the energy output of the refrigeration circuit is varied in response to the rate of change of the return temperature of the heat transfer medium with respect to time reaching a predetermined value. Preferably, the method comprises the step of controlling the compressor means for varying the energy output of the refrigeration circuit. In another embodiment of the invention the method comprises the step of controlling the mark/space ratio of a power supply being delivered to the compressor means. Advantageously, the method comprises the step of varying the mark/space ratio of the power supply to the compressor means inversely to the rate of change of the return temperature with respect to time. Advantageously, the method comprises the step of varying the energy output of the refrigeration circuit in response to a change in the return temperature of the heat transfer medium. Preferably, the method comprises the step of varying the energy output of the refrigeration circuit in response to the return temperature of the heat transfer medium moving from one predetermined range of return temperature to another range. In one embodiment of the invention the method comprises the step of varying the mark/space ratio of the power supply to the compressor means proportionately to the temperature difference between the return temperature of the heat transfer medium and the flow temperature of the heat transfer medium flowing from the main heat exchanger. In a further embodiment of the invention the method further comprises the steps of varying the rate of circulation of the heat transfer medium through the main heat exchanger in response to the rate of change of the return temperature of the heat transfer medium to the main heat exchanger with respect to time. Advantages of the invention The advantages of the invention are many. A particularly important advantage of the multi-zone temperature control apparatus is that it permits independent control of the temperature of different zones. Furthermore, it permits heating of one or more zones while at the same time another or others of the zones are being cooled. Another advantage of the invention is that it permits the energy output of the central unit to be substantially matched to the demand of the remote or remote units. Furthermore, where a control unit is provided in temperature control apparatus with only one remote unit, the energy output of the central unit can be substantially matched to the demand of the remote unit. Further, the invention provides a multi-zone temperature control apparatus which is relatively efficient to manufacture, to install and to use. The apparatus is also relatively inexpensive and robust. Where the heat transfer medium is provided by water, a particularly environmentally friendly apparatus is provided, and furthermore, the apparatus does not present a health hazard and additionally, the apparatus can be readily easily installed in a building or other location. The central unit, the remote unit and the temperature control apparatus are also relatively efficient to manufacture, install and use, and can be provided at a relatively low cost. Installation of the multi-zone temperature control apparatus and the temperature control apparatus as well as the central unit and remote unit can be carried out with minimum inconvenience. The method according to the invention for controlling the energy output of the central is a particularly effective and efficient method for controlling such a central unit. The invention will be more clearly understood from the following description of some preferred non-limiting embodiments thereof given by way of example only with reference to the accompanying drawings. Brief description of the drawings Fig. 1 is a schematic diagram of temperature control apparatus according to the invention for space heating and/or cooling of a building for controlling the temperature of a zone of the building, Fig. 2 is a schematic diagram of portion of the temperature control apparatus of Fig. 1 illustrated in a different mode of operation, Fig. 3 (a) and (b) is a flow chart of a computer programme for controlling a remote unit of the apparatus of Fig. 1, Fig. 4 is a flow chart of a computer programme for controlling a central unit of the apparatus of Fig. 1, Fig. 5 is a flow chart of a sub-routine of the computer programme of Fig.4, Fig. 6 is a flow chart of another sub-routine of the computer programme of Fig. 4, Fig. 7 is a schematic diagram of multi-zone temperature control apparatus according to the invention for space heating and/or cooling of a plurality of zones in a building, and Fig.8 is a perspective schematic diagram of the apparatus of Fig.7 installed in a building. Detailed description of the invention Referring to the drawings and initially to Figs. 1 to 6 there is illustrated temperature control apparatus according to the invention indicated generally by the reference numeral 1 for space heating and/or cooling a single zone in a building. The heat control apparatus 1 comprises a central unit 2 also according to the invention for supplying a heat transfer medium, namely, water to a remote unit 3, also according to the invention, for mounting in the zone of the building for heating and/or cooling the zone. A circulating circuit 4 connects the central unit 2 and the remote unit 3 for circulating the heat transfer medium between the units 2 and 3 as will be described below. The central unit 2 comprises a reversible refrigeration circuit 8 which is operable in a chilling mode for supplying cooling energy and in a heat pump mode for supplying heating energy from the central 2 to the remote unit 3. A refrigerant medium, namely, freon gas is provided in the refrigeration circuit 8. The refrigeration circuit 8 comprises a master heat exchanger 10 which in this case is provided by a fan assisted coil heat exchanger for exchanging heat between the refrigerant medium and the ambient air adjacent the central unit 2. A main heat exchanger 11 in the refrigeration circuit 8 exchanges heat between the refrigerant medium and the heat transfer medium in the circulating circuit 4. The main heat exchanger 11 is provided by a plate heat exchanger. A compressor means, namely, a compressor 12, in this case a scroll compressor compresses the refrigerant medium. An expansion means, namely, a pair of expansion valves 14 and 15 are connected between the master heat exchanger 10 and the main heat exchanger 11 for expanding the refrigerant medium. A receiver 16 between the expansion valve 14 and 15 receives and buffers the expanded refrigerant medium. Bypass valves 5 and 6 connected in parallel with the expansion valves 14 and 15 are alternately opened so that one of the expansion valves 14 and 15 is bypassed and the other is operational depending on the mode of operation of the refrigeration circuit 8. In a chilling mode the refrigerant medium is expanded through the expansion valve 14, while in a heat pump mode the refrigerant medium is expanded through the expansion valve 15. Reversing means for reversing the refrigeration circuit 8 to operate in a chilling mode and in a heat pump mode comprises a reversing valve 18 which connects the master heat exchanger 10, the main heat exchanger 11 and the compressor 12. When the refrigeration circuit 8 is to operate in the chilling mode the reversing valve 18 is configured as illustrated in Fig. 1 and the flow of refrigerant medium through the refrigeration circuit 8 is in the direction of arrows A. In this configuration the master heat exchanger 10 acts as a condenser, and the main heat exchanger 11 acts as an evaporator, thus removing heat from the heat transfer medium in the main heat exchanger 11 for delivering cooling to the remote unit 3. When the refrigeration circuit 8 is operating in the heat pump mode the reversing valve 18 is configured as illustrated in Fig.2 and flow of the refrigerant medium through the refrigeration circuit 8 is in the direction of the arrows B. In the heat pump mode configuration, the master heat exchanger 10 acts as a evaporator and the main heat exchanger 11 acts as a condenser, thus transferring heat into the heat transfer medium circulating through the main heat exchanger 11, thus delivering heating to the remote unit 3. The reversing valve 18 is operated by a solenoid 13 under the control of a first control means comprising a first control circuit 25. The first control circuit 25 comprises a microprocessor 26 which controls the solenoid 13 under the control of a computer programme. The control circuit 25 and computer programme for controlling the microprocessor 26 are described in more detail below. The reversing valve 18 is normally configured as illustrated in Fig.2 with the refrigeration circuit 8 operating in a heat pump mode. The microprocessor 26 controls the operation of the bypass valves 5 and 6 through solenoids 17 for switching the valves 5 and 6 on the operating mode of the reversing valves 18 being changed. An electrically powered motor 20 drives the compressor 12. Power from a power supply unit 28 is delivered to the compressor motor 20 through a compressor control means, namely, a compressor controller 29 for controlling the operation of the compressor 12 for enabling the heating and/or cooling energy output of the refrigeration circuit 8 to be varied to match the demand of the remote unit 3. The compressor controller 29 operates under the control of the microprocessor 26 as will be described below. The compressor controller 29 comprises means for varying the mark/space ratio of the power supply being delivered to the compressor motor 20 under the control of the microprocessor 26 for varying the energy output of the refrigeration circuit 8. In this case, the minimum mark/space cycle time is two minutes. The minimum mark time is one minute and the minimum space time is one minute. In other words, where the mark/space ratio is one the power supply is delivered to the compressor motor 20 for one minute and is off for one minute. Needless to say, a cycle may be any length of time, for example, in the case of a mark/space ratio of 1:4 the cycle time would be five minutes, the power supply being delivered to the motor for one minute and off for four minutes. The compressor controller 29 also permits continuous delivery of power to the compressor controller 20. A variable speed electrically powered motor 19 drives a fan 31 of the master heat exchanger 10. Power from the power supply 28 is delivered to the motor 19 through a motor controller 27 also under the control of the microprocessor 26. The fan 31 is operated at full speed when the refrigeration circuit 8 is operating in a heat pump mode for maximising the delivery of air through the master heat exchanger 10 and in turn maximising heat transfer from the air into the refrigerant medium. When the refrigeration circuit 8 is operating in a chilling mode, the fan is operated to maintain the temperature of the liquid refrigerant medium leaving the master heat exchanger 10 at approximately 49°C. Suitable temperature sensors (not shown) connected to the microprocessor 26 are provided for monitoring the temperature of the liquid refrigerant medium, and a suitable computer programme (not shown or described) is provided for controlling the motor controller 27. The control of such fans when a refrigeration circuit is operating in a chilling mode will be well known to those skilled in the art. The circulating circuit 4 comprises a flow line 21 and a return line 22. Circulating means, namely a pump means comprising a variable output circulating pump 23 in the flow line 21 circulates the heat transfer medium through the circulating circuit 4 and in turn through the main heat exchanger 11. A variable speed electrically powered motor 24 drives the pump 23. Power from the power supply unit 28 is delivered to the motor 24 through a pump control means, namely, a pump controller 30 for controlling the operation of the motor 24 and in turn the circulating pump 23 for varying the delivery rate at which the circulating pump 23 delivers the heat transfer medium through the circulating circuit 4. In this way the rate of delivery of heating and/or cooling energy from the central unit 2 to the remote unit 3 is varied to match the demand of the remote unit 3. The pump controller 30 operates under the control of the microprocessor 26 and controls the motor 24 to operate at four different speeds, namely, speed one to speed four for operating the pump 23 at four different delivery rates. Speed one is the fastest speed while speed four is the slowest speed. Speeds two and three are intermediate speeds, speed two being faster than speed three. Accordingly, when the motor 24 is operating at speed one the pump 23 is circulating the heat transfer medium at the highest delivery rate, while at speed four the pump 23 is circulating at the lowest delivery rate. A return temperature monitoring means provided by a return temperature sensor 32 in the return line 22 adjacent the main heat exchanger 11 monitors the return temperature T R of the heat transfer medium returning to the main heat exchanger 11. A flow temperature monitoring means provided by a flow temperature sensor 33 in the flow line 21 adjacent the main heat exchanger 11 monitors the flow temperature of the heat transfer medium flowing from the main heat exchanger 11. The return temperature sensor 32 and flow temperature sensor 33 are connected to the microprocessor 26 so that the microprocessor 26 can read the return and flow temperatures monitored by the sensors 32 and 33, respectively. Differentiating means comprising a differentiating circuit 34 is connected to the return temperature sensor 32 for determining the rate of change of the return temperature of the heat transfer medium with respect to time, namely, the ^Z^. The differentiating circuit 34 is connected to the microprocessor 26 for enabling the microprocessor 26 to read the rate of change of the return temperature with respect to time. The microprocessor 26 controls the compressor 12 through the compressor controller 29 for varying the energy output of the refrigeration circuit 8 in response to the return temperature of the heat transfer medium monitored by the return temperature sensor 32 and the rate of change of the return temperature determined by the differentiating circuit 34. The energy output of the refrigeration circuit 8 is varied inversely to the rate of change of the return temperature with respect to time, and proportionately to the temperature difference between the return temperature monitored by the sensor 32 and the flow temperature of the heat transfer medium monitored by the flow temperature sensor 33. In other words, as the temperature difference between the return and flow temperatures reduces, that is the return temperature is moving towards the flow temperature, and the rate of change of the return temperature is increasing, the supply of heating or cooling energy from the central unit exceeds the demand, and accordingly, the microprocessor 26 reduces the energy output of the refrigeration circuit 8. This enables the energy output of the refrigeration circuit 8 to be varied to substantially match the demand for heating or cooling energy required by the remote unit 3, thereby minimizing wastage of heating or cooling energy. In this embodiment of the invention as will be described below, the energy output of the refrigeration circuit 8 is varied as the temperature difference between the return and flow temperatures of the heat transfer medium moves from one range of temperatures to another, and as the rate of change of the return temperature reaches a predetermined value. As the rate of change of the return temperature exceeds the predetermined value the energy output of the refrigeration circuit 8 is reduced, and where the rate of change of the return temperature falls below the predetermined value the energy output of the refrigeration circuit 8 is increased. The microprocessor 26 also controls the circulating pump 23 through the pump controller 30 in response to the return temperature of the heat transfer medium monitored by the return temperature sensor 32 and the rate of change of the return temperature determined by the differentiating circuit 34. The delivery rate of the pump 23 is varied inversely to the rate of change of the return temperature with respect to time, and proportionately to the temperature difference between the return temperature monitored by the sensor 32 and the flow temperature monitored by the flow sensor 33 of the heat transfer medium. In other words, as the temperature difference between the return and flow temperatures reduces, that is the return temperature is moving towards the flow temperature, and the rate of change of the return temperature is increasing, the supply of heating or cooling energy of the central unit 2 exceeds the demand of the remote unit 3, and accordingly, the microprocessor 26 reduces the delivery rate of the pump 23, thereby reducing the energy output being delivered from the refrigeration circuit 8. This further enables the energy output of the central unit 2 to be substantially matched to the demand for heating or cooling energy required by the remote unit 3. Accordingly, wastage of heating or cooling energy from the control unit 2 is further minimised. In this embodiment of the invention as will be described in more detail below, the delivery rate of the circulating pump 23 is varied as the return temperature of the heat transfer medium moves from one range of return temperatures to another, and as the rate of change of the return temperature reaches a predetermined value, which in this embodiment of the invention is different to the predetermined value at which the energy output of the refrigeration circuit 8 is varied. Needless to say, in many cases, it is envisaged that the predetermined value of the rate of change of the return temperature to which the circulating pump 23 is responsive and the refrigeration circuit 8 is responsive may be the same. When the central unit 2 is supplying cooling to the remote unit 3, in other words, the refrigeration circuit 8 is operating in a chilling mode, the flow temperature of the heat transfer medium from the main heat exchanger 11 is approximately 4°C. When the central unit 2 is supplying heating to the remote unit 3, in other words, the refrigeration circuit 8 is operating in a heat pump mode, the flow temperature of the heat transfer medium from the main heat exchanger 11 is approximately 45°C. The return temperature of the heat transfer medium to the main heat exchanger 11 depends on the demand for cooling or heating by the remote unit 3. In the case of a high demand, the temperature difference between the flow and return temperatures is relatively high, while in the case of a relatively low demand the temperature difference between the flow and return temperatures of the heat transfer medium is relatively low. In other words, the return temperature approaches the flow temperature. Additionally, where the rate of change of the return temperature is high as it is moving towards the value of the flow temperature, the supply of energy from the central unit 2 is exceeding demand from the remote unit 3 and may be reduced. Where the central unit 2 is delivering cooling, and the return temperature of the heat transfer medium is greater than 10°C, in other words, 6°C above the flow temperature of 4°C, the demand for cooling is high, and the compressor controller 29 sets the mark/space ratio so that the compressor 12 runs continuously. Where the return temperature of the heat transfer medium lies in the range between 7°C and 10°C, and the rate of change of the return temperature is less than the predetermined value of 2°C per minute, the supply of cooling energy considerably exceeds demand, and the compressor controller 29 sets the mark/space ratio so that the compressor 12 runs continuously. On the other hand, where the return temperature of the heat transfer medium lies between 7°C and 10°C and the rate of change of the return temperature is greater than or equal to the predetermined value of 2°C per minute, the demand for cooling is not quite so high, and the compressor controller 29 sets the mark/space ratio at 1:2 so that the compressor runs for one minute and is off for two minutes for each three minute cycle. This thus providing a lower cooling output from the central unit 2 to match the lower demand for cooling from the remote unit 3. Where the return temperature of the heat transfer medium lies in the range between 5°C and 7°C and the rate of change of the return temperature is less than 2°C per minute, the compressor controller 29 sets the mark/space ratio at 1:2 thus the compressor 12 runs for one minute and is off for two minutes. On the other hand, where the return temperature of the heat transfer medium lies in the range between 5°C and 7°C and the rate of change of the return temperature is greater than or equal to 2°C per minute, thus indicating a lower demand for cooling, the compressor controller sets the mark/space ratio at 1:3. In other words, the compressor is on for one minute out of every four minutes. Where the return temperature of the heat transfer medium lies in the range between 4°C and 5°C and the rate of change of the return temperature is less than 2 Q C per minute, thus indicating a reasonable demand for cooling, the compressor controller 29 sets the mark/space ratio 1:2. On the other hand, where the return temperature of the heat transfer medium lies in the range between 4°C and 5°C and the rate of change of the return temperature is greater than or equal to 2°C per minute, thus indicating a relatively low demand for heat, the compressor controller sets the mark/space ratio at 1:4. Thus, the compressor 12 is operated for one minute in every five minutes. Where the return temperature of the heat transfer medium is less than or equal to 4°C the compressor controller 29 ceases to deliver power to the compressor motor 20 thereby switching off the compressor 12. Additionally, as mentioned above the delivery rate of the circulating pump 23 is varied to meet the demand for heating or cooling of the remote unit 3. For example, where the central unit 2 is supplying cooling and the return temperature of the heat transfer medium is greater than 10°C, thus indicating a high demand for cooling, the pump controller 30 operates the pump 23 at speed one, namely, maximum speed. Where the return temperature of the heat transfer medium lies in the range between 7°C and 10°C and the rate of change of the return temperature is less than the predetermined value of 3°C per minute, thus indicating a high demand for cooling, the pump controller 30 operates the pump 23 at speed one. Where the return temperature of the heat transfer medium lies in the range between 7°C and 10°C and the rate of change of the return temperature is greater than or equal to the predetermined value of 3°C per minute, thus indicating a lower demand for cooling, the pump controller 30 operates the pump 23 at speed two. Where the return temperature of the heat transfer medium lies in the range between 5°C and 7°C and the rate of change of the return temperature is less than 3°C per minute the pump controller operates the pump 23 at speed two. Where the return temperature of the heat transfer medium lies in the range between 5°C and 7°C and the rate of change of the return temperature is greater than or equal to 3°C per minute thus indicating a still lower demand for cooling by the remote unit 3, the pump controller 30 operates the pump 23 at speed 3. Where the return temperature of the heat transfer medium lies in the range between 4°C and 5°C thus indicating a relatively low demand for cooling from the,remote unit 3, the pump controller 30 operates the pump 23 at speed four, namely, the minimum speed. Where the return temperature of the heat transfer medium is less than or equal to 4°C the pump controller 30 switches off the pump 23. When the central unit 2 is operating to supply heating to the remote unit 3, in other words, when the refrigeration circuit 8 is operating in a heat pump mode, similar control of the compressor 12 and pump 23 is exercised. The operation of the compressor 12 and pump 23 when the return temperature of the heat transfer medium is less than 37°C, in other words, 8°C below the flow temperature, the compressor 12 and pump 23 are controlled in similar fashion as when the return temperature is 10°C when the central unit 2 is supplying cooling. When the return temperature of the heat transfer medium lies in the range between 40°C and 37°C the control of the compressor 12 and pump 23 is similar to that when the return temperature lies in the range between 7°C and 10°C when the central unit 2 is supplying cooling. The return temperature of the heat transfer medium lying in the range between 43°C and 40°C corresponds to the range of 5°C and 7°C when the central unit is supplying cooling. The return temperature of the heat transfer medium lying in the range between 45°C and 43°C corresponds to the range of 4°C and 5°C when the central unit is supplying cooling. When the return temperature of heat transfer medium is greater than or equal to 45°C the compressor 12 and pump 23 are shut off. The operation of the microprocessor 26 under the control of the computer programme controlling the compressor 12 and circulating pump 23 is described in more detail below with reference to the flow charts of Figs. 4, 5 and 6. The refrigeration circuit 8 and the first control circuit 25 as well as the circulating pump 23 and pump motor 24 are housed in a single housing (not shown), but indicated by the broken line 48 of Fig. 1. Returning now to the remote unit 3, the remote unit 3 comprises a secondary heat exchanger 36, in this case a coil heat exchanger which is connected to the circulating circuit 14 for receiving the heat transfer medium, and for exchanging heat between the heat transfer medium and the ambient air in the zone. A booster heat delivery means comprising an electrically powered resistance wire heater 37 in the remote unit 3 delivers heat to the zone in the event that the central unit 2 may be supplying cooling, or the secondary heat exchanger 36 cannot cope with the demand for heat from the zone. A heat transfer means comprising a variable speed electrically powered fan 38 mounted in the remote unit 3 transfers heat between the secondary heat exchanger 36 and the zone, and the heater 37 and the zone. A second control means, namely, a second control circuit 39 comprising a microprocessor 40 controls the operation of the remote unit 3 in response to the temperature of the ambient air being returned to the remote unit 3, and activates the central unit 2 through a communicating means, namely, a cable 35 connected between the microprocessors 26 and 40 to supply heating or cooling whichever is required. The microprocessor 40 operates under the control of a computer programme which is described below with reference to the flow chart of Fig. 3. An ambient air temperature monitoring means comprising an air temperature sensor 41 is mounted in the remote unit 3 adjacent the fan 38 for monitoring the return air temperature of ambient air being returned to the remote unit 3. The air temperature sensor 41 is connected to the microprocessor 40. A power supply unit 42 in the remote unit 3 delivers electrical power to the fan 38 and the heater 37 through a fan controller 43 and a heater controller 44 which operates under the control of the microprocessor 40. The fan controller 43 under the control of the microprocessor 40 operates the fan 38 at three speeds for varying the output of heating or cooling from the remote unit 3 to the zone. The heater controller 48 under the control of the microprocessor 40 varies the mark/space ratio of power being supplied to the heater 37 from the power supply unit 42 for varying the heat output of the heater 37. A keypad 45 having a visual display 46 is connected to the microprocessor 40 for enabling a set point temperature about which the temperature of the zone is to be controlled to be inputted into the microprocessor 40. The keypad 45 may be mounted on the remote unit 3 or may be provided for mounting in the zone at a convenient location. On the temperature of the ambient air being monitored by the sensor 41 exceeding the set point temperature by 1°C or dropping below the set point temperature by 1°C, the microprocessor 40 operates the remote unit 3 and delivers a signal to the microprocessor 26 in the central unit to activate the central unit 2 to deliver heating or cooling, whichever is required. The secondary heat exchanger 36, the heater 37 and fan 38, as well as the control circuit 39 and the air temperature sensor 41 are mounted in a housing which is not shown but is illustrated by the broken line 47. Referring now to Fig. 3(a) and 3(b) there is illustrated a flow chart of a computer programme under which the microprocessor 40 operates for controlling the operation of the remote unit 3. Block 300 in Fig. 3(a) of the flow chart commences operation of the computer programme. Block 301 reads the set point temperature which is stored in the microprocessor 40 after being entered through the keypad 45. Block 302 reads the ambient temperature from the air temperature sensor 41. Block 303 compares the ambient temperature read by block 302 with the set point temperature read by block 301. If the ambient temperature is greater than or equal to 1°C above the set point temperature, cooling is required in the zone, and the computer programme moves to block 304 which will be described shortly. If the ambient temperature is not greater than or equal to l β C above the set point temperature, the computer programme moves to block 305 which checks if the ambient temperature is greater than or equal to 1°C below the set point temperature. Should block 305 determine that the ambient temperature is greater than or equal to 1°C below the set point temperature heating of the zone is required, and the computer programme moves to block 306 which in turn moves the computer programme to block 307 which is described below. On the other hand, should block 305 determine that the ambient temperature is not greater than or equal to 1°C below the set point temperature the computer programme is returned to block 301. Returning now to block 304, block 304 transmits a request from the microprocessor 40 to the microprocessor 26 of the central unit 2 requesting cooling. The computer programme then moves to block 308 which causes the microprocessor 40 to control the fan controller 43 to operate the fan 38 at its low speed. The computer programme then moves to block 309 which checks if the ambient temperature monitored by the air temperature sensor 41 is less than or equal to 2°C above the set point temperature. If the ambient temperature is less than or equal to 2°C above the set point temperature the computer programme moves to block 310 which causes the microprocessor 40 to operate the fan controller 43 to run the fan 38 at the medium speed and the computer programme is moved to block 311 which is described below. On the other hand, should block 309 determine that the ambient temperature is greater than 2°C above the set point temperature, the computer programme is moved to block 312 which causes the microprocessor 4 to operate the fan controller 43 to run the fan 38 at its high speed. The computer programme then moves to block 311. Block 311 again reads the ambient temperature and moves to block 313 which checks if the ambient temperature is less than or equal to 1°C above the set point temperature. If block 313 determines that the ambient temperature is less than or equal to 1°C above the set point temperature, the computer programme moves to block 314 which causes the microprocessor 40 to operate the fan controller 43 to run the. fan 38 at its low speed and the computer programme moves to block 315. Block 315 checks if the ambient temperature read by block 311 is less than or equal to the set point temperature, and if so, the computer programme moves to block 316 which causes the microprocessor 40 to transmit a request to the microprocessor 26 of the central unit 2 cancelling the request for cooling. The computer programme then returns to block 301. On the other hand should block 313 have determined that the ambient temperature is not less than or equal to 1°C above the set point temperature, the computer programme is returned to block 309. If block 315 determines that the ambient temperature is greater than the set point temperature, the computer programme moves to block 311. Referring now to Fig. 3(b) the part of the computer programme of the microprocessor 40 which controls the remote unit 3 in the event of a requirement for heating of the zone will now be described. Block 307 transmits a request from the microprocessor 40 to the microprocessor 26 of the central unit 2 for heating. The computer programme then moves to block 317 which causes the microprocessor 40 to operate the fan controller 43 to run the fan 38 at its low speed. The computer programme then moves to block 318 which checks if the ambient temperature read by block 302 is less than or equal to 2°C below the set point temperature. If the ambient temperature is less than or equal to 2°C below the set point temperature the computer programme moves to block 219 which causes the microprocessor 40 to operate the fan controller 43 to run the fan 38 at the medium speed. On the other hand, if the ambient temperature is determined by block 318 to be greater than 2°C below the set point temperature the computer programme moves to block 320 which causes the microprocessor 40 to operate the fan controller 43 to run the fan 38 at high speed. After passing to block 319 or block 320 the computer programme then moves to block 321 which reads the ambient temperature from the air temperature sensor 41 and the computer programme moves to block 322. Block 322 checks if the ambient temperature is greater than 2°C below the set point temperature, and if so the computer programme moves to block 323. If block 322 determines that the ambient temperature is less than or equal to 2°C below the set point temperature the computer programme moves to block 324 which will be described below. Block 323 checks if the ambient temperature is less than or equal to 2.5°C below the set point temperature. If so, the computer programme moves to block 325 which causes the microprocessor 40 to control the heater controller 44 to run the electrically powered heater 37 at a mark/space ratio of 40%. Should block 323 determine that the ambient temperature is greater than 2.5°C below the set point temperature the computer programme moves to block 326 which checks if the ambient temperature is greater than or equal to 5°C below the set point temperature. If so, the computer programme moves to block 327 which causes the microprocessor 40 to operate the heater controller 44 to run the heater 37 continuously. Should block 326 determine that the ambient temperature is less than 5°C below the set point temperature, the computer programme is moved to block 328 which causes the microprocessor 40 to control the heater controller 44 at a mark/space ratio between 40% and continuous running which is proportional to the amount by which the ambient temperature is below the set point temperature between to 2.5°C and 5°C. The computer programme after passing through blocks 325, 327 or 328 then returns to block 321. Returning now to block 324, should block 324 determine that the ambient temperature is less than or equal to 1°C below the set point temperature, the computer programme is moved to block 329 which causes the fan controller 43 to run the fan 38 at its low speed. The computer programme then moves to block 330 which checks if the ambient temperature is greater than or equal to the set point temperature. If so, the computer programme moves to block 331 which causes the microprocessor 40 to transmit a request to the microprocessor 26 of the central unit to cancel the request for heating and the computer programme then moves to block 332 which returns the programme to block 301. In the event that block 324 determines that the ambient temperature is greater than 1°C below the set point temperature the computer programme moves to block 333, which returns the computer programme to block 303. In the event that block 330 determines that the ambient temperature is less than the set point temperature, the computer programme is returned to block 321. Referring now to Fig. 4 a flow chart of the main computer programme which controls the operation of the microprocessor 26 of the central unit 2 for controlling the central unit 2 is illustrated. Block 400 of the flow chart starts the computer programme. The computer programme then moves to block 401 which checks if there is a request from the microprocessor 40 of the remote unit 3 for heating or cooling. If no request has been received the computer programme moves to block 402 which puts the microprocessor-26 to sleep to await an interrupt which returns the computer programme to block 401. On block 401 determining that there has been a request for heating or cooling from the remote unit 3 the computer programme moves the block 403 which causes the microprocessor 26 to operate the pump controller 30 to operate the circulating pump 23 at speed number one, namely, its highest speed for circulating heat transfer medium through the circulating circuit 4 to the remote unit 3. The computer programme then moves to block 404 which checks if the request from the remote unit 3 is for cooling. If so the computer programme moves to block 405. While on the other hand, if the request is for heating the computer programme moves to block 406. Block 406 will be dealt with below. Returning to block 405, block 405 times a time delay of one minute and then the computer programme moves to block 407 which activates the reversing valve 18 for operating the refrigeration circuit 8 in a chilling mode and the computer programme moves to block 408. Block 408 times a further delay of thirty seconds and moves the computer programme onto block 409 which causes the microprocessor 26 to operate the compressor controller 29 to switch on the compressor 12 to run continuously. The computer programme then moves to block 410 which checks if the request for cooling from the remote unit 3 has been cancelled. If so the computer programme moves to block 411 which switches off the compressor 12 and in turn moves to block 412 which returns the computer programme to the start block 400. Should the block 410 determine that the request for cooling from the remote unit has not been cancelled the computer programme moves to block 413 which reads the return temperature of the heat transfer medium in the return line 22 from the return temperature sensor 32 and the computer programme moves to block 430 which reads the numerical value of dτ / dt from the differentiating circuit 34. The computer programme moves to block 414 which calls up sub-routine 1 which will be described below with reference to Fig. 5. Sub-routine 1 controls the operation of the compressor 12 and the pump 23 in response to the return temperature of the heat transfer medium to the main heat exchanger 11 and the rate of change of the return temperature with respects to time as will be described below. Returning now to block 406. Block 406 times a time delay of one minute and moves the computer programme to block 415. Block 415 operates the reversing valve 18 so that the refrigeration circuit 8 operates in a heat pump mode for delivering heat to the remote unit 3. The computer programme then moves to block 416 which times a further thirty second delay and moves the computer programme onto block 417 which switches on the compressor in the same fashion as block 409. The computer programme then moves to block 418 which checks if the request for heating by the remote unit 3 has been cancelled. If so, the computer programme moves to block 411 and in turn to block 412 both of which have already been described. In the event that the request for heating in the remote unit 3 has not been cancelled the computer programme moves to block 419 which reads the return temperature of the heat transfer medium from the return temperature sensor 32 and then proceeds to block 431. Block 431 reads the numerical value of dT / dt from the differentiating circuit 34. The computer programme then moves to block 420 which calls up sub-routine number 2 which is illustrated in Fig. 6. -Sub-routine number 2 controls the operation of the compressor 12 and the pump 23 in response to the return temperature of the heat transfer medium and the rate of change of the return temperature with respect to time, as will be described below. Referring now to Fig. 5 sub-routine 1 of the computer programme will now be described. Block 500 starts sub-routine 1 and the computer programme moves to block 501. Block 501 checks if the return temperature of the heat transfer medium read by block 413 is greater than 10°C. If so, the computer programme moves to block 502 which causes the microprocessor 26 to operate the compressor controller 29 to run the compressor motor 20 continuously thereby operating the compressor 12 continuously. The computer programme then moves to block 503 which causes the microprocessor 26 to operate the pump controller 30 to run the pump motor 24 at speed one, namely, its maximum speed thereby running the pump 23 at its maximum speed for maximum delivery of the heat transfer medium through the circulating circuit 4. The computer programme then moves to block 504 which returns control of the microprocessor 26 to block 413 of the main computer programme of Fig.3. Should block 501 determine that the return temperature read by the return temperature sensor 32 is greater than 7°C but less than or equal to 10°C the computer programme moves to block 506. Block 506 checks if the numerical value of '"/ d t rea£ I by block 430 is greater than or equal to 2°C per minute. If so, the computer programme moves to block 507 which causes the microprocessor 26 to control the compressor controller 29 to deliver power to the compressor motor 20 with a mark/space ratio 1:2. The computer programme then moves to block 508 which checks if the numerical value of ^/^ is greater than or equal to 3°C per minute. If so, the computer programme moves to block 509 which causes the microprocessor 26 to operate the pump controller 30 for operating the pump motor 23 and in turn the pump 23 at speed number two. The computer programme then moves to block 504. In the event that block 506 determines that the numerical value of Λ / άt is less than 2°C per minute, the computer programme moves to block 510 which operates the compressor controller 30 to run the compressor motor 20 continuously, and in turn the compressor 12 continuously. The computer programme moves to block 511 which causes the microprocessor 26 to operate the pump controller 30 to run the pump motor 24 at speed one and in turn the pump 23 is operated at the maximum delivery rate. The computer programme then moves to block 504 which has already been described. Should block 508 determine that the numerical value of '"/a is less than 3°C per minute, the computer programme moves to block 511 which has just been described. In the event that block 505 determines that the return temperature does not lie between 7°C and 10°C the computer programme moves to block 512. Block 512 checks if the return temperature is greater than 5°C and less than or equal to 7°C. If so, the computer programme moves to block 513 which checks if the numerical value of dτ / dt is greater than or equal to 2°C per minute. If so, the computer programme moves to block 514 which causes the microprocessor 26 to control the compressor controller 29 to deliver power to the compressor motor 20 at a mark/space ratio 1:3. The computer programme then moves to block 515 which checks if the numerical value of dT / dt is greater than or equal to 3°C per minute. If so, the computer programme moves to block 516 which causes the microprocessor 26 to operate the pump controller 30 to run the pump motor 24 at speed three and in turn the circulating pump 23 is operated at speed three. The computer programme then moves to block 504 which has already been described. Should block 513 determine that the value of dτ / is less than 2°C per minute, the computer programme moves to block 517 which causes the microprocessor 26 to control the compressor controller 29 for delivering power to the compressor motor 20 with a mark/space ratio of 1:2. The computer programme then moves to block 518 which causes the microprocessor 26 to operate the pump controller 30 for running the pump motor 24 at speed two and in turn the circulating pump 23 at speed two. The computer programme then moves to block 504. Should block 515 determine that the numerical value of dT / dt i less than 3°C per minute, the computer programme moves to block 518 which has just been described. In the event that block 512 determines that the return temperature does not lie between 5°C and 7°C the computer programme moves to block 519 which checks if the return temperature is greater than 4°C and less than or equal to 5°C. If so, the computer programme moves to block 520 which checks if the numerical value of dτ / dt is greater than or equal to 2°C per minute. If so the computer programme moves to block 521 which causes the microprocessor 26 to operate the compressor controller 29 to deliver power to the compressor motor 20 with a mark/space ratio 1:4. The computer programme then moves to block 522 which causes the microprocessor 26 to operate the pump controller 30 for running the pump motor 24 at speed four, namely, the minimum speed and in turn run the circulating pump 23 at its minimum delivery rate. The computer programme then moves to block 504 which has already been described. Should block 520 determine that the numerical value of dτ / t s less than 2°C per minute, the computer programme moves to block 523 which causes the microprocessor 26 to operate the compressor controller 29 for delivering power to the compressor motor 20 with a mark/space ratio of 1:2 thereby operating the compressor 12 with a mark/space ratio of 1:2. The computer programme then moves to block 522 which has just been described. In the event that block 519 determines that the return temperature of the heat transfer medium does not lie between 4°C and 5°C, the computer programme moves to block 524. Block 524 checks if the temperature is less than or equal to 4°C. If block 524 determines that the return temperature is less than or equal to 4°C the computer programme moves to block 525 which causes the microprocessor 26 to operate the compressor controller 29 to switch off the compressor motor 20 and in turn the compressor 12. The computer programme then moves to block 526 which causes the microprocessor 26 to operate the pump controller 30 for running the pump motor 24 at speed 4. The computer programme then moves to block 527 which returns the control of the microprocessor 26 to the main computer programme of Fig.4 by returning to the start block 400. If block 524 determines that the return temperature is greater than 4°C the computer programme moves to block 530. Block 530 causes the microprocessor 26 to operate the compressor controller 29 to deliver power to the compressor motor 20 with a mark/space ratio of 1:4. Block 530 also causes the microprocessor 26 to operate the pump controller 30 to run the pump motor 24 at speed four. The computer programme then moves to block 531 which reads the return temperature of the heat transfer medium and returns the computer programme to block 524. Referring now to Fig. 6 the flow chart of sub-routine number 2 of the main computer programme is illustrated. The flow chart of the sub-routine number two is substantially similar to the flow chart of the sub-routine number 1 and similar blocks are identified by the same reference numerals. Sub-routine number 2 is called up by block 420 of the main flow chart of Fig. 1 when the remote unit 3 is calling for heating from the central unit 2. Thus, the only blocks which are different in sub-routine number 2 to those in sub-routine number 1 are blocks 501, 504, 505, 502, 519 and 524. Accordingly, only the equivalent to these blocks in sub-routine number 2 will be described. Block 600 starts sub¬ routine number 2. Block 601 checks if the return temperature of the heat transfer medium read by block 419 from the return temperature sensor 32 is less than 37°C. If the return temperature is less than 37°C the computer programme moves to block 502. On the other hand, the computer programme moves to block 605 which checks if the return temperature is less than 40°C and greater than or equal to 37°C. If so, the computer programme moves to block 506. On the other hand, the computer programme moves to block 612 which checks if the return temperature is less than 42°C and greater than 40°C. If so, the computer programme moves to . block 513. On the other hand, the computer programme moves to block 619 which checks if the return temperature is less than 45°C and greater than or equal to 43°C. If so, the computer programme moves to block 520. If not, the computer programme moves to block 624 which checks if the return temperature is greater than or equal to 45°C. If so, the computer programme moves to block 525. If not, the computer programme moves to block 530. On the sub-routine moving to block 604 which is equivalent to block 504 of sub-routine 1, the sub¬ routine 2 is returned to block 419 of Fig. 4. In the case of blocks 506, 513, 520, 508, 515 and 522 these blocks check the value of dτ / dt read by block 431. In use, the apparatus 1 is mounted in a building, in general, with the central unit 2 mounted exteriorly of the building, generally, in a covered location, but with sufficient ventilation to permit the passage of air efficiently over the master heat exchanger 10 for efficient running of the refrigeration circuit 8 whether running in a chilling mode or in a heat pump mode. The remote unit 3 is mounted in a suitable location in the zone for heating or cooling the zone. The remote unit 3 may be mounted on a wall, ceiling, or the like or may be free standing on a floor. The keypad 45 may be mounted on the remote unit 3 or may be mounted in any other suitable or desirable location in the zone for easy access by an occupant. The power supply units 28 and 42 are connected to a suitable mains electricity power supply. An occupant of the zone enters the desired set point temperature through the keypad 45 at which the ambient temperature of the zone is to be maintained. The entered set point is displayed on the visual display 46 for verification. The microprocessor 40 of the remote unit 3 under the control of the computer programme described with reference to Fig. 3 monitors the ambient temperature by reading the air temperature sensor 41. On the air temperature exceeding the set point temperature by 1°C or falling below the set point temperature by 1°C the microprocessor 40 under the control of the computer programme of Fig. 3 operates the remote unit 3 as already described and transmits a signal to the central unit 2 requesting heating or cooling. The central unit 2 on receiving the request for heating or cooling as the case may be operates under the control of the computer programme and sub-routines 1 and 2 of Figs.4 to 6 for delivering cooling or heating to the remote unit 3. Referring now to Figs. 7 and 8 there is illustrated multi-zone heat control apparatus according to the invention indicated generally by the reference numeral 50 for controlling the temperature in a plurality of zones 51 in a building 52. In Fig. 8 four zones 51 are illustrated. The apparatus 50 comprises one central unit 2 substantially of the type described with reference to Figs. 1 to 6 and a plurality of remote units 3, namely, four remote units 3a to 3d, one in each zone 51. The remote units 3a to 3d are similar to those described with reference to Figs. 1 to 6, and may be either wall mounted, ceiling mounted or otherwise. The secondary heat exchangers 36 of the respective remote units 3a to 3d are independently connected to the main heat exchanger 11 of the central unit 2 by four independent circulating circuits 4. Circulating pumps 23a to 23d driven by a pump motor 24a to 24d are provided in the flow lines 21a to 21d of the respective circulating circuit 4 adjacent the main heat exchanger 11 for independently circulating the heat transfer medium to the remote units 3a to 3d. The pump motors 24a to 24d are controlled by the microprocessor 26 through pump controllers 30a to 30d, respectively, for delivering heat transfer medium through the circulating circuits 4 to the remote units 3a to 3d independently of each other. A flow manifold 56 and a return manifold 57 connect the circulating circuits 4 directly to the main heat exchanger 11 of the central unit 2. Return temperature sensors 32a to 32d and flow temperature sensors 33a to 33d for monitoring the return and flow temperatures of the heat transfer medium are provided in the return lines 22a to 22d and the flow lines 21a to 21d, respectively. Flow meters 58a to 58d are provided in the respective circulating circuits 4 for determining the quantity of heat transfer medium flowing in each circulating circuit 4 for determining in combination with the return and flow temperature sensors 32a to 32d and 33a to 33d, the quantity of heat delivered to the secondary heat exchanger 36 of each remote unit 3. The microprocessor 26 of the central unit 2 operates under the control of a computer programme and sub-routines substantially similar to those described with reference Figs. 4 to 6. The microprocessors 40 of the remote units 3 operate under respective computer programmes substantially similar to that described with reference to Fig. 3. Should the microprocessor 40 of any of the remote units 3 determine that the temperature sensed by the air temperature sensor 41 of that remote unit 3 is greater than or equal to 1°C above the set point temperature of the remote unit 3 or greater than or equal to 1°C below the set point temperature of the remote unit 3, the microprocessor 40 under the control of the computer programme operates the remote unit 3 as described with reference to Fig.3. A request for heating or cooling as the case may be is delivered to the central unit 2 with the identity of the remote unit 3. Should the request be for cooling and the central unit 2 is inactive, then the microprocessor 26 under the control of the computer programme of Figs.4 to 6 operates the central unit 2 as already described with reference to Figs.4 to 6. The refrigeration circuit 8 is operated in a chilling mode. The circulating pump 23 of the circulating circuit 4 corresponding to the remote unit 3 requesting cooling is operated by the microprocessor 26 under the control of the computer programme and the sub-routine 1, and delivers cooling to the remote unit 3. The microprocessor 26 reads the return and flow temperature sensors 32 and 33, respectively, corresponding to the remote unit 3 requesting cooling and the corresponding differentiating circuit 34, and controls the central unit 2 and the cooling energy output of the refrigeration circuit 8 and the delivery rate of the circulating pump 23 corresponding to the remote unit 3 in response to the return temperature and the rate of change of return temperature of the heat transfer medium returning from that remote unit 3. Where a remote unit 3 requests heating from the central unit, and the central unit 2 is inactive, the central unit 2 operates under the control of the computer programme of Figs. 4 and 6 and delivers heating to the remote unit in response to the return temperature and the rate of change of the return temperature of the heat transfer medium returning from that remote unit 3. Where two or more remote units 3 are being supplied with cooling or heating from the central unit 2, the cooling or heating energy output of the refrigeration circuit 8 is matched to the sum of the demands of the remote units 3. This is achieved by operating the compressor 12 of the refrigeration circuit 8 in response to the return temperature and the rate of change of the return temperature read from the return temperature sensor 32 and the differentiating circuit 34 which indicates the greatest demand for energy. The circulating pumps corresponding to the remote units 3 are controlled in response to the return temperature and the rate of change of the return temperature of the heat transfer medium being returned from the corresponding remote unit 3. Should the central unit 2 on receiving a request for heating from the remote unit 3 be in the process of satisfying a request for cooling from another remote unit 3, the central unit 2 continues to supply the cooling request of that remote unit 3 until the request for cooling has been satisfied. The central unit 2 then reverses the refrigeration circuit 8 to operate in a heat pump mode and supplies heating to the remote unit 3 requesting heating. However, in the intervening period before the central unit 2 commences to supply heating to the remote unit 3 requesting heating if the return temperature monitored by the air temperature sensor 41 is determined to fall within the comparisons of blocks 323 and 326 of the flow chart of Fig. 2, the electrically powered heater 37 is operated in accordance with block 325 and 327. In the event that the central unit 2 is delivering heating to the remote unit 3 and the comparisons of blocks 323 and 326 are found to apply the heater 37 of the remote unit 3 is also operated under the control of block 325 and 327. If the central unit 2 is operating in a heat pump mode delivering heat to a remote unit 3 and another remote unit 3 demands cooling, the computer programme controlling the microprocessor 26 of the central unit 2 goes to block 405 and in turn reverses the refrigeration circuit 8 to operate in a chilling mode and commences to proceed to block 408 onwards. In which case, if the demand for heating by the remote unit 3 which had been receiving heating from the central unit 2 has not been satisfied, and the comparisons of blocks 323 and 326 of the computer programme of Fig.3 apply, then the electrically powered heater 37 of that remote unit 3 is operated under the control of blocks 325 or 327. In use, the occupants of the respective zones 51 enter the desired set point temperature at which the ambient air in the zones is to be maintained into the microprocessors 40 of the respective remote units 3 through the appropriate keypads 45. This operation is similar to that described with reference to the apparatus of Figs. 1 to 6. The remote units 3 then operate under the control of the computer programmes described with reference to Fig. 3, and the central unit 2 operates under the control of the computer programme and sub-routines described with reference to Figs.4 to 6. Where a request for cooling by a remote unit 3 is made to the central unit 2, the central unit 2 is operated to supply cooling through the heat transfer medium to the remote unit 3 as already described. Where a request for heating is made by remote unit 3, the demand for heating is supplied by the central unit 2 provided that the central unit is not already supplying cooling to another remote unit 3. In which case, the central unit continues to supply cooling to that remote unit 3 until its demand has been satisfied. The central unit 2 then, should the demand still remain from the remote unit 3 for heating, reverses to operate in a heat pump mode and supplies heating to the remote unit 3 requiring heating. Where heating is not being supplied to a remote unit 3 demanding heating by the central unit 2, the microprocessor 40 under the control of the computer programme operates the electrically powered heater 37 of that remote unit 3 until the demand for heating has been satisfied, or the central unit 2 can supply sufficient heating that the electrically powered heater 37 is no longer required. At which stage the heater 37 is deactivated by the microprocessor 40 of the remote unit 3 under the control of the computer programme. It is envisaged that various other controls may be incorporated in the computer programmes of the microprocessors 26 and 40. For example, it is envisaged that a sub-routine may be provided for permitting disabling of some or more of the remote units and in certain cases the central unit during predetermined periods of a twenty-four cycle, particularly, for example, at night from midnight to six a.m. It is also envisaged that maximum values of set point temperatures which may be selected by occupiers in remote units may be controlled from the central unit, for example, it is envisaged that the maximum set point temperature which may be selected during the morning might be set a maximum limit over which an occupier could not exceed and such maximum limit may be lower during the morning of a twenty-four hour period than in the evening, when, in general, a higher ambient temperature would be required, particularly, in a residential zone. It is also envisaged that a number of remote units may be connected to one remote unit. In such cases, it is envisaged that one of the remote units would act as a master remote unit and the others would act as slave remote units under the control of the master remote unit. In which case, the master and its corresponding slave remote units would be connected to the central unit through a single circulating circuit which would include a single circulating pump. Signals requesting heating or cooling from the central unit would be transmitted from the master remote unit to the central unit. While the muHi-zone heat control apparatus of Figs. 7 and 8 has been described for controlling the temperature of four zones of a building, it will readily be apparent that the apparatus may be used for controlling the temperature of any number of zones from two upwards. In which case, a remote unit would be provided for each zone and the remote units for each zone would be connected independently of each other to the central unit 2. It is envisaged that separate fans may be provided for transferring heat from the secondary heat exchanger and the electrically powered heater of each remote unit. Needless to say, any suitable booster heat delivery means besides an electrically powered heater may be provided. While the secondary heat exchangers have been described as coil heat exchangers, any other suitable heat exchangers may be provided. Needless to say, while the main heat exchanger has been described as being a plate heat exchanger any other suitable heat exchanger may be used. It will also be appreciated that any other suitable heat exchanger may be used besides a fan assisted coil heat exchanger for the master heat exchanger. While the refrigerant medium has been described as being freon, any other suitable refrigerant medium may be used. Additionally, while it is preferable that the heat transfer medium should be water, any other suitable heat transfer mediums may be used. In practice, it is envisaged that the heat transfer medium will be a liquid medium. While the apparatus of Figs. 1 to 6 has been described for both heating and cooling, in certain cases, it is envisaged that the apparatus may be provided for space cooling only. In which case, the refrigeration circuit would not be reversible. Alternatively, it is envisaged that the refrigeration circuit of the apparatus of Figs. 1 to 5 may be constructed to act as a heat pump only, in which case, the apparatus of Figs. 1 to 6 would only provide space heating. It is also envisaged that the temperature control apparatus of Fig. 1 could include a number of remote units which would be supplied by the same central unit either in parallel or in series with each other. While the differentiating means for determining the rate of change of temperature of the heat transfer medium returning to the central unit has been described as being provided by a differentiating circuit, any other suitable differentiating means may be provided. Indeed, in many cases, it is envisaged that the differentiating means may be provided in the microprocessor 26 and could be implemented by a suitable computer programme. While the communicating means for communicating the microprocessors 40 of the remote units 3 with the microprocessor 26 of the central unit 2 have been described as being cables, any other suitable communicating means may be used, for example, radio transmission communication means or the like. While specific ranges of return temperatures of the heat transfer medium, and specific predetermined values of the rate of change of the return temperature of the heat transfer medium have been described at which the output of the refrigeration circuit and the circulating pump are changed, it will be readily apparent to those skilled in the art, that other ranges of return temperature or temperature differences between return and flow temperatures and predetermined rates of change of return temperature may be used. Indeed, in certain cases, it is envisaged that the energy output of the refrigeration circuit and the delivery rate of the circulating pump may be responsive to relatively small increments or decrements of change of return temperature or temperature difference and to relatively small increments or decrements of rate of change of return temperature.

Claims: CLAIMS: 1. A central unit (2) for supplying a heat transfer medium to at least one remote unit (3) of temperature control apparatus (1,50) for transferring heat between the central unit (1) and the remote unit (3), the central unit (1) being of the type comprising a refrigeration circuit (8) having a refrigerant medium therein, the refrigeration circuit (8) comprising a master heat exchanger (10) for exchanging heat with the refrigerant medium, and a main heat exchanger (11) for exchanging heat between the refrigerant medium and the heat transfer medium, a compressor means (12) for compressing the refrigerant medium, and an expansion means (18) for expanding the refrigerant medium, characterised in that, return temperature monitoring means (32) for monitoring the return temperature of the heat transfer medium returning to the main heat exchanger (11) is provided, differentiating means (34) for determining the rate of change of the return temperature of the heat transfer medium with respect to time is provided, and first control means (25) responsive to the differentiating means (34) is provided for controlling the energy output of the refrigeration circuit (8) in response to the rate of change of the return temperature of the heat transfer medium with respect to time. 2. A central unit as claimed in Claim 1 characterised in that the first control means (25).varies the energy output of the refrigeration circuit (8) in response to a change in the rate of change of the return temperature of the heat transfer medium with respects to time. 3. A central unit as claimed in Claim 1 or 2 characterised in that the first control means (25) is responsive to the rate of change of the return temperature of the heat transfer medium with respect to time moving from one predetermined range of rates of change of return temperature to another range. 4. A central unit as claimed in any preceding claim characterised in that the first control means (25) is responsive to the rate of change of the return temperature of the heat transfer medium with respect to time reaching a predetermined value. 5. A control unit as claimed in any preceding claim characterised in that the first control means (25) comprises compressor control means (29) for controlling the compressor means (12) for varying the energy output of the refrigeration circuit (8). 6. A central unit as claimed in Claim 5 characterised in that the compressor control means (29) comprises means for controlling the mark/space ratio of a power supply (28) being delivered to the compressor means (12). 7. A central unit as claimed in Claim 6 characterised in that the compressor control means (29) varies the mark/space ratio of the power supply to the compressor means (12) inversely to the rate of change of the return temperature with respect to time. 8. A central unit as claimed in any preceding claim characterised in that a pump means (23) for circulating the heat transfer medium through the main heat exchanger (11) is provided, the first control means (25) comprising pump control means (30) for controlling the delivery of the pump means (23), the pump control means (30) being responsive to the differentiating means for controlling the delivery of the pump means (23) in response to the rate of change of the return temperature of the heat transfer medium to the main heat exchanger (11) with respect to time. 9. A central unit as claimed in Claim 8 characterised in that the pump control means (30) varies the delivery of the pump means (23) inversely to the rate of change of the return temperature with respect to time. 10. A central unit as claimed in any preceding claim characterised in that the first control means (25) is responsive to the return temperature of the heat transfer medium. 11. A central unit as claimed in Claim 10 characterised in that the first control means (25) is responsive to the return temperature of the heat transfer medium moving from one predetermined range of return temperatures to another range. 12. A central unit as claimed in Claim 10 or 11 when dependent on Claim 6, characterised in that the compressor control means (29) varies the mark/space ratio of the power supply to the compressor means (12) proportionately to the temperature difference between the return temperature of the heat transfer medium and the flow temperature of the heat transfer medium flowing from the main heat exchanger (11). 13. A central unit as claimed in Claim 10 to 12 when dependent on.Claim 8 characterised in that the pump control means (30) varies the delivery of the pump means (23) proportionately to the difference between the return temperature of the heat transfer medium and the flow temperature of the heat transfer medium flowing from the main heat exchanger (11). 14. A central unit as claimed in any preceding claim characterised in that the refrigeration circuit (8) is reversible and is operable in a chilling mode and a heat pump mode, and means (18) for reversing operation of the refrigeration circuit between the two modes is provided. 15. A central unit as claimed in any preceding claim characterised in that the compressor means (12) is a scroll compressor. 16. A central unit as claimed in any preceding claim characterised in that the heat transfer medium is water. 17. A remote unit (3) for receiving a heat transfer medium from a central unit (2) of temperature control apparatus (1,50) for transferring heat between the central unit (2) and the remote unit (3), the remote unit comprising a secondary heat exchanger (36) for exchanging heat with the heat transfer medium, a booster heat delivery means (37), and a heat transfer means (38)for transferring heat to or from the secondary heat exchanger (36) and the booster heat delivery means (37) , air temperature monitoring means (41) for monitoring the return temperature of air to the remote unit, and second control means (39) responsive to the air temperature monitoring means (41) for controlling the heat transfer means and for delivering a signal to the first control means (2) for activating the central unit (2). 18. Temperature control apparatus (1,50) comprising a central unit (2) as claimed in any of Claims 1 to 16 and a remote unit (3) as claimed in Claim 17, the remote unit (3) being connected to the central unit (2) by a circulating circuit (4) for circulating a heat transfer medium between the central unit (2) and the remote unit (3). 19. Multi-zone temperature control apparatus (50) comprising a plurality of remote units (3), one remote unit (3) being provided for each zone (51), a central unit (2) for supplying a heat transfer medium to the remote units (3) for transferring heat between the central unit (2) and the respective remote units (3) for controlling temperature of the zones (51), each central unit (2) comprising a main heat exchanger (11) for exchanging heat with the heat transfer medium, and first control means (25) for controlling the central unit (2), each remote unit (3) comprising a secondary heat exchanger (36) for exchanging heat with the heat transfer medium, a heat transfer means (38) for transferring heat between the secondary heat exchanger (36) and the zone, air temperature monitoring means (41) for monitoring the temperature of air in the zone, and second control means (39) responsive to the air temperature monitoring means (41) for controlling the heat transfer means (38) and for delivering a signal to the first control means (25) for activating the central unit (2) in response to a change in temperature of the air, the apparatus (50) further comprising a plurality of circulating circuits (4) for communicating the secondary heat exchangers (36) of respective remote units (3) with the main heat exchanger (11) of the central unit (2) for circulating the heat transfer medium between the main heat exchanger (11) and the respective secondary heat exchangers (36), characterised in that, circulating means (23) are being provided in respective circulating circuits (4) for circulating the heat transfer medium. 20. Multi-zone temperature control apparatus as claimed in Claim 19 characterised in that each circulating means (23) is responsive to the first control means (25). 21. Multi-zone temperature control apparatus as claimed in Claim 19 or 20 characterised in that the heat transfer medium is water. 22. Multi-zone temperature control apparatus as claimed in any of Claims 19 to 21 characterised in that the circulating circuits (4) are connected to the main heat exchanger (11) independently of each other. 23. Multi-zone temperature control apparatus as claimed in any of Claims 19 to 22 characterised in that each remote unit comprises a booster heat delivery means (37), the heat transfer means (38) co-operating with the booster heat delivery means (37) for transferring heat between the booster heat delivery means (37) and the zone (51). 24. Multi-zone temperature control apparatus as claimed in Claim 23 characterised in that the booster heat delivery means (37) is responsive to the second control means (39). 25. Multi-zone temperature control apparatus as claimed in Claim 23 or 24 characterised in that each booster heat delivery means (37) comprises a heat source. 26. Multi-zone temperature control apparatus as claimed in Claim 25 characterised in that each booster heat delivery means (37) is provided by an electrically powered heat source. 27. Multi-zone temperature control apparatus as claimed in any of Claims 19 to 26 characterised in that each secondary heat exchanger (36) is provided by a coil heat exchanger. 28. Multi-zone temperature control apparatus as claimed in any of Claims 19 to 27 characterised in that each heat transfer means (38) comprises a fan. 29. Multi-zone temperature control apparatus as claimed in Claim 28 characterised in that the fan (38) is electrically powered. 30. Multi-zone temperature control apparatus as claimed in any of Claims 19 to 29 characterised in that each circulating means (23) comprises a circulating pump. 31. Multi-zone temperature control apparatus as claimed in Claim 30 characterised in that each circulating pump (23) is an electrically powered variable speed circulating pump. 32. Multi-zone temperature control apparatus as claimed in any of Claims 19 to 31 characterised in that the air temperature monitoring means (41) are mounted in the respective remote units (3) for monitoring the return air temperature of air returning to the respective remote units (3). 33. Multi-zone temperature control apparatus as claimed in any of Claims 19 to 32 characterised in that the central unit (2) comprises a refrigeration circuit (8) having a refrigerant medium therein and comprising the main heat exchanger (11) for exchanging heat between the refrigerant medium and the heat transfer medium, a master heat exchanger (10) for exchanging heat with the refrigerant medium, a compressor means (12) for compressing the refrigerant medium and an expansion means (14) for expanding the refrigerant medium, the refrigeration circuit (8) being responsive to the first control means (25). 34. Multi-zone temperature control apparatus as claimed in Claim 33 characterised in that the refrigeration circuit (8) is reversible, and means (18) for reversing the refrigeration circuit is provided. 35. Multi-zone temperature control apparatus as claimed in any of Claims 19 to 32 characterised in that the central unit (2) is a central unit (2) as claimed in any of Claims 1 to 15. 36. Multi-zone temperature control apparatus as claimed in any of Claims 19 to 35 characterised in that flow temperature monitoring means (33) for monitoring the flow temperature of the heat transfer medium from the main heat exchanger (11) is provided in each circulating circuit (4). 37. Multi-zone temperature control apparatus as claimed in Claim 36 characterised in that flow measuring means (58) is provided in each circulating circuit (4), the flow temperature monitoring means (33) and flow measuring means (58) being connected to the first control means (25) for enabling computation of the energy delivered to the secondary heat exchangers (36) of the respective remote units (3). 38. A method for controlling the energy output of a central unit (2) of temperature control apparatus (1,50), wherein the central unit (2) is of the type which supplies a heat transfer medium to at least one remote unit (3) of the temperature control apparatus (1,50) for transferring heat between the central unit (2) and the remote unit (3), and the central unit (2) comprises a refrigeration circuit (8) having a refrigerant medium therein, the refrigeration circuit (8) comprising a master heat exchanger (10) for exchanging heat with the refrigerant medium, and a main heat exchanger (11) for exchanging heat between the refrigerant medium and the heat transfer medium, a compressor means (12) for compressing the refrigerant medium, and an expansion means (14) for expanding the refrigerant medium, the method being characterised in that, the method comprises the steps of determining the rate of change of the return temperature of the heat transfer medium returning to the main heat exchanger (11) with respect to time, and controlling the energy output of the refrigeration circuit (8) in response to the rate of change of the return temperature of the heat transfer medium. 39. A method as claimed in Claim 38 characterised in that the method comprises the step of varying the energy output of the refrigeration circuit (8) in response to a change in the rate of change of the return temperature of the heat transfer medium wit respect to time. 40. A method as claimed in Claim 38 or 39 characterised in that the energy output of the refrigeration circuit (8) is varied in response to the rate of change of the return temperature of the heat transfer medium with respect to time moving from one predetermined range of jrates of change of return temperature to another range. 41. A method as claimed.in any of Claims 38 to 40 characterised in that the energy output of the refrigeration circuit is varied in response to the rate of change of the return temperature of the heat transfer medium with respect to time reaching a predetermined value. 42. A method as claimed in any of Claims 38 to 41 characterised in that the method comprises the step of controlling the compressor means (12) for varying the energy output of the refrigeration circuit (8). 43. A method as claimed in Claim 42 characterised in that the method comprises the step of controlling the mark/space ratio of a power supply being delivered to the compressor means (12). 44. A method as claimed in Claim 43 characterised in that the method comprises the step of varying the mark/space ratio of the power supply to the compressor means (12) inversely to the rate of change of the return temperature with respect to time. 45. A method as claimed in any of Claims 38 to 44 characterised in that the method comprises the step of varying the energy output of the refrigeration circuit (8) in response to a change in the return temperature of the heat transfer medium. 46. A method as claimed in Claim 45 characterised in that the method comprises the step of varying the energy output of the refrigeration circuit (8) in response to the return temperature of the heat transfer medium moving from one predetermined range of return temperature to another range. 47. A method as claimed in any of Claims 43 to 46 characterised in that the method comprises the step of varying the mark/space ratio of the power supply to the compressor means (12) proportionately to the temperature difference between the return temperature of the heat transfer medium and the flow temperature of the heat transfer medium flowing from the main heat exchanger (11). 48. A method as claimed in any of Claims 38 to 47 characterised in that the method further comprises the steps of varying the rate of circulation of the heat transfer medium through the main heat exchanger (11) in response to the rate of change of the return temperature of the heat transfer medium to the main heat exchanger (11) with respect to time.

Inventors: TANGNEY JAMES GERARD, TANGNEY, JAMES, GERARD
Applicants: CASSOWARY LTD, CASSOWARY LIMITED
Application year: 1992
Main classification: F24F11
IPC codes: F24F3, F24F11, F25B13, F25B49