FIELD OF THE INVENTION

[0001 ] The present invention relates to geothermal energy transfer systems.

SUMMARY OF THE INVENTION

f0002| is well known to use a heat pump to transfer energy between a consumer of energy, such as a building, and a source of energy such as the surrounding environment. The heat pump uses a closed cycle that passes a refrigerant through an expansion phase, that requires the absorption of external energy, and a compression phase, which rejects energy to the building. In order to supply energy to a particular location, the rejected heat is transferred in to the heating system of that location and the energy required to effect the expansion of the refrigerant is absorbed from an external source. Similarly, when heat is to be extracted from the location, the location acts as a source and supplies the energy for the expansion of the refrigerant and the heat generated during compression is rejected to the surrounding environment thai acts as a consumer.

[0003] ^' [ ^' he environment may be the air itself, as is the case w ith traditional air conditioning units or heat pumps. However, such an arrangement has a poor efficiency due lo fluctuations in the air temperature.

[0004] A preferred external source has a substantially constant temperature and the ground or large body of water are typically used, [t is therefore known to provide a heat exchange loop between the heat pump and such a source so that heat may be absorbed in to the loop to supply energy to the heat pump or may be rejected from the loop to remove energy from the heat pump. The loops are typically an extensive run of pipe containing a saline, glycol or ethyl alcohol based heat exchange lluid. The pipe is buried in a trench between one or two meters below ⁷ the normal surface, At that depth, the earth is at a substantially constant temperature and provides an energy source to either provide energy to or absorb energy from the heat transfer iluid because of the temperature differential between the heat exchange fluid and the surrounding.

[0005] Where available, a large body of w ater may be used as the energy source. The heat transfer loop is placed in the water and heat transfer iluid circulated through the loop. [0006] The heat exchange loop is typically closed to isolate the heat transfer fluid from the environment. To compensate for l osses of fluid and changes in the condition of fluid, a flow centre is placed in the heat exchange loop to subdivide the heat exchange loop i n to a heat transfer loop and a heat absorption loop.

(0007) In a non-pressuri zed system, the flow centre acts as a reservoir for heat transfer fluid, !n a pressurized system, a dedicated reservoi r i s not provided as the system is typically charged with air after fil l ing. In both applications, the flow center i s usually placed between the loop that passes fluid through the heat pump (the heat transfer loop) and the loop that passes fl uid through the ground or water loop (the heat absorption loop). A pump circulates the heat transfer fl uid through the heat transfer loop and returns it to a manifol d from which the heat absorption loop is supplied.

|00081 These arrangements typically size the circulating pump to maintain a turbulent flow through the heat transfer loop. However, such an arrangement has been found to introduce a loss of efficiency in the overall performance of the energy transfer system.

|0009] Tt is therefore an object of the present invention to obviate or mitigate the above disadvantages.

|0010j In general terms, an energy transfer system includes a first loop to circulate fluid through a heat pump and a second loop to circulate fluid through a geothermal energy source. Each of the loops is connected to a flow center to provi de a reservoir of fluid for circulation. A respective pump is connected in each of said loops to establish respective flow rates of fluid in each of said loops, with balance flow being provided by the flow center.

BRIEF DESCRIPTION OF THE DRAWINGS

[001 1 ] Embodiments of the invention w ill now be described by way example only with reference to the accompanying drawings in which:

[0012 | Figure 1 is a schematic representation of an energy transfer system;

[00131 Figure 2 is a perspecti ve view of a flow center;

[0014] Fi gure 3 is a schematic representation of flo through the flo center of Figure 2; |0015[ Fi gure 4 is a view, similar to Figure 2 of an alternative flow center:

[0016] Figure 5 is a schematic representation of flow through the flo center of Figure 4:

[0017] Figure 6 is a further embodiment of the energy transfer system; [0018J Figure 7 i s a side elevati on of a further embodiment of flo centre:

[0019] Figure S is a section on the line VIII - VIII of Figure 7:

|0020 j Figure 9 is a section on the line IX - IX o f Figure 8.

[00211 Figure 1 0 is a How chart showing a first control strategy for operation of the heating system of figure 1 ,

[0022] Figure 1 1 is flow chart showing a second control strategy for operation of the heating system of figure 1 in a heating mode, and

|0023 | Figure 1 is a flow chart showing the second control strategy for operation of the heating system of figure 1 in a cooling mode. DETAILED DESC RIPTION O F THE IN VENTION

[0024 ] Referri ng therefore to F i gure 1 . a bui lding 1 0 has a heating and cooling system 12 to distribute heat through the building or to remove heat from the building. The heat distribution system may be an air circulating system, or a water ci rculating system that transfers heat between di fferent areas of the building and a heat source. The heating and cool ing system 1 2 includes a heat exchanger 1 4 that cooperates with a heat exchanger 16 to transfer iieat between a heat pump 1 8 and the bui lding 1 0.

[002 1 The heat pump 1 8 is of conventional construction and includes a heat exchanger 20 connected in a refrigerant loop 1 to the heat exchanger 16 through a throttle valve 22 and a compressor 24. Expansion of a refrigerant through the throttle valve 22 causes heat to be absorbed in to the refrigerant and compression of the refrigerant through the pump 24 causes heat to be rejected .

[0026 j The heat exchangers 1 6 and 20 absorb or reject the heat depending upon the mode of the operation of the refrigerant cycle. A reversing valve 23 reverses the How direction to allow the heat pump 18 to function in a heating mode to supply heat to the bui lding, or a cooling mode in wh ich heat i s extracted from the building 1 0. A thermostat 27 and controller 25 is i ncorporated in to the system 12 to control operation and mai ntain the required temperature in the building 1 0,

[0027] The heat exchanger 20 cooperates with a further heat exchanger 26 to transfer heat between the refrigerant loop 19 and a heat transfer loop indicated at 28. The heat transfer loop 28 includes a pump 30 that circulates a heat transfer fl uid, typically a saline, glycol or ethyl alcohol based mixture, through a return pipe 32 and a supply pipe 34. |0028{ The pi pes 32. 34 arc connected in series with a pair of header pipes 36, 38, one of which, 36 acts as a supply and the other. 38 acts as a return. The header pipes 36, 38 that are connected to opposite sides of a heat transfer unit 40 to provide a heat absorption loop 41 . The heat transfer unit 40 may be a loop or multiple loops connected in paral lel, to the header pipes 36. 38. The loop is buried in the ground or under water, or. preferably, is a self contained heat transfer unit of the type more fully described in United States Patent

Application 61/367, 166, and the contents of which are incorporated herein by reference. The loops may also include loops to auxiliary heat consumers, such as a poo] or spa, if required and as shown in figure 6, with a suffix "b" for clarity. A pump 42 is connected in the header pipe 36 to circulate fluid through the heat absorption loop 41 defined by the pipes 36,38 and the heat transfer unit 40.

{0029] A How center 44 is connected in parallel with the pipes 36, 38 and 32. 34 through stub pipes 46. The flow center 44 is seen more fully in Figure 2 and, in its simplest form, comprises a cy l indrical housing 50 sealed at its lower end. A cap 52 with a vent valve 54 is fitted to the housing 50 to provide venting to accommodate expansion and contraction of fluid i n the fluid circulation loops 28, 4 ] , The stub pi pes 46 are connected on diametrically opposite sides of the housing 50. In the case of the pressurized configuration, the vent valve 54 is replaced with an air valve allowing the system to be pressurized. The Cap 52 is instal led as to seal the system.

[0030] In operation, the heat transfer loop 28 and the heat absorption loop 41 are filled with fluid through filling the housing 50. The vent 54 allows for venti ng of air from the system and a cap 52 for adding/replenishing fluid duri ng/after initial installation. The pumps 30 and 42 operate to circulate fluid through the heat exchanger 26 and through the heat exchanger 40. The pump 30 is sized to provide a turbulent flow through the heat transfer loop 28 at a rate that maximizes heal transfer between the heat exchangers 26 and 20. The rate required to attain optimum heat transfer wil l vary in different design conditions but for a supply of fluid at a particular temperature an optimum rate can be determined, from operating characteristics of the heat pump 18.

[0031 ] For a given heat transfer rate into the building 1 0. and with a design temperature of the heat transfer fluid and the known characteristics of the heat exchanger 26, an appropriate flo rate of the fluid passing through the heat exchanger 26 can be determined. Frequently, the design temperature and flow rates are specified by the manufacturer of the heat pump. For example, with a Gcostar Model GT064, a nominal heat transfer of 27100 Btu/hr is specified with a flow rate of 16 US gpm and an assumed entering water temperature of 20 . Correction tables are provided to compensate lor different entry water temperatures (BWT).

[0032] Similarly, the pump 42 is sized to provide a circulation through the heat absorption loop at a rate that optimizes the transfer of energy between the heat exchanger 40 and the surroundings. Again this will depend upon the particular design conditions but an optimum flow rate can be attained, taking into account the temperature of the heat source, the thermodynamic properties of the fluid and the heat transfer characteristics of the heat transfer unit 40.

[0033] For the same thermal load, the heat absorption rate from the surroundings through the heat exchanger 40 may require a different How rate through the heat absorption loop 41 to that in the heat transfer loop, 28.

10f>34| The pumps 30, 42 can then be sized to provide those respective flow rates.

Preferably, each of the pumps 30, 42 arc variable flow rate pumps that can be adjusted to increase or decrease the flo rate to suit particular control strategies. Alternatively, one of the pumps 30, 42 may be a fixed capacity and the other variable to permit adj stment of the respective How rates. If a steady condition is anticipated then both pumps may be of fixed How rating for the anticipated conditions in the respective loop. However, as will be explained more fully below; the ability to adj ust the How rates may be used advantageously in the operation and control of the heating and cooling system 12.

10035 J As illustrated in Figure 3, the flow center 44 operates as a reservoir to recei e excess fluid from the heat absorption loop 41 and supply a balancing fluid back into that loop through respective ones of the stub pipes 46. Typically, it is found that the flow rate through the heat absorption loop 41 is greater than that required in the heat transfer loop and so the flow center 44 receives fluid from, and delivers fluid to. the heat absorption loop 41. In Figure 3. the Ho required through the heal transfer loop 28 is denoted by Y and the flow rate required in the heat absorption loop 4 ] is X + Y. The flow center 44 thus receives X gallons per minute from the heat absorption loop 41 through one of the stub pipes 46 acting as an inlet and similarly delivers X gallons per minute to that loop 41 from the other stub pipes 56 acting as an outlet to supply the pump 42, In one installation with an eighteen kilowatt heat load, it has been found that a flow rate through the heat absorption loop 41 in the order of 23 gallons per minute is optimum with a flow rate through the heat transfer loop 28 of 16 gallons per minute.

[0036] Those flow rates wi ll of course depend upon the nature of the heat exchanger 40 and the temperature of the environment T in which the heat exchanger 40 operates.

[0037] Fluid circulation in the heat absorption loop 41 may also enable a selective precooling or preheating of the fluid in the flow center 44. For example, w hen heating a dwelling, the fluid can be preheated in the flow center 44 from fluid circul tion in the heat absorption loop 41 and when cooling a dwelling, the fluid can be prccooled in the flow center from fluid circulation in the heat absorption loop 41 ,

10038] Λ further embodiment of ilow center is shown in Figures 4 and 5 i which like components will be denoted w ith like reference numerals with the suffix a added for clarity. Referring therefore to Figure 4. the How center 44a includes a pair of cylindrical housings 50a I 50a? Each of the housings has a cap 52a and vent valve 54a. A balancing tube 60 interconnects the upper end of the housings 50a to allow for fluid to flow between the housing.

100391 Fach of the housings 50a receives the return from one loop and the supply to another of the loops. Thus, the housing 5 a| receives fluid returned from the heat absorption loop 41 a through the pi e 38a and supplies fluid to the heat transfer loop 28a. Similarly, the housing 50a? receives the return through pipe 38a from the heat transfer loop 28a and supplies fluid through the pipe 34, 36a to the heat absorption loop 41 a.

[0040] The interconnection of the units is shown in Figure 5, from which it will be appreciated that the differential fluid returned from the heat absorption loop 41 through the pipe 38a may flo from the housing 50a through the bridge 60 to the housing 50a? to supplement supply to the pump 42a. Again, the pumps 30a and 42a will be sized to accommodate the optimum flow rates through the respective transfer loops.

|0041 J A further embodiment of flow centre is shown in Figures 7 through 9 in which like reference numerals will be used for like components w ith a suffix "c" added for clarity. Flow centre 44c can be used interchangeably with the How centres 44. 44a, 44b shown in the previous embodiments. The flow centre 44c has a cylindrical housing 50c which is encompassed in an insulating foam 70 and encased in an outer casing 72. A cap 52c is secured to the housing 50c and has an upstanding square boss 76. A retaining bracket 78 is fitted over the cap and has a square hole 80 that fits around the boss 76. The bracket 78 is 1 secured to the casing 72 by bolts 82 and thereby tamper proofs the cap by preventing

2 unauthorized removal. The bracket 78 may also be used, alter release of the bolts 82 and

3 inversion of the bracket 78. as a wrench to remove the cap 52c.

4 [0042] A pair of cross lubes 90. 92 extend diametrically through the housing 50c and are

5 sealed at the intersection of the tubes with the housing 50c. Each end of the tubes 90. 92 is

6 threaded to provide a connectio with respective ones of the pipes 32c. 34c. 36c. 38c, as will

7 be described in more detail below. Hach of the cross tubes 90, 92 has an array of holes 94 at

8 its midpoint. The holes 94 are evenly distributed around the circumference of the tube 90. 92

9 and in the embodiment shown there are four holes 94 equally spaced about the

0 circumference. A greater or lesser number of holes 94 may be provided depending upon the I particular circumstances, The aggregated cross section of the holes 94 is the same as or 2 slightly greater than the cross section of the corresponding tube 90. 92.

1004 1 As can be seen in Figure 7, a sight glass 96 is provided on the exterior of the flow 4 centre 44c to provide an indication of the level of fluid contained within the flow centre 44c. 5 Conveniently, a spectrum indicating different colors of fluid corresponding to the

6 approximate composition of the solution being circulated through the How centre is provided 7 alongside the sight level for easy reference and routine maintenance.

S [0044J The tube 90 is connected between the return pipe 38c of the heat absorption loop 41 c and the supply pipe 34c of the heat transfer loop 28c so that one end acts as an inlet from0 loop 41 c and the other as an outlet to loop 28. The tube 92 is similarly connected between 1 the return pipe 32c of heat transfer loop 28c and the supply pipe 36c of the heat absorption2 loop 41 c to provide respective inlets and outlets.

3 [0045] In operation, fluid from the heat absorption loop 41 c is delivered by the pump 42c4 to the tube 90 w here it flow s from the return pipe 38c to the supply pipe 34c. Similarly, How5 in the heat transfer loop 28c from the pump 30c is delivered to the tube 92 from the return6 pipe 32c to the supply pipe 36c of the heat absorption loop 41 c. The pumps 30c. 42c have a7 differential How rate so that typically the How delivered to (he tube 90 from the absorption8 loop 41 c is greater than the flow rate extracted from the tube 90 by the transfer loop 28c. The9 balance of the How is discharged through the holes 94 in to the reservoir provided by the0 interior of the housing 50c.

1 [00461 Similarly, the flow required from the tube 92 to supp!y the absorption loop 41 c is2 greater than that delivered by the return pipe 32c of the transfer loop 28c and therefore makeup fluid is provided through the holes 94 in the tube 92 from the housing 50c, The holes 94 therefore provide for a cross flow between the heat transfer loop and absorption loop to maintain the desired flow rates as determined by the respective pumps.

J0047] The effect of the delivery of the fluid in the return pipe 38c to the tube 90 is to supply it directly to the inlet to the pump 30c, effectively supercharging the inlet to pump 30c to a positive pressure, to ensure that it is operating under optimum conditions. The pump 30c is not required to operate at a reduced inlet suction pressure, but at the same time ensures that the required flow rates between the two loops is maintained to provide optimum efficiencies. (0048] The controller 25 is used to control operation of the heating system 10 and may be a simple thermostat interacting with the heat pump 18 to switch pumps 30, 42 on or off. However, as explained in greater detail below, the controller 25 may also be used to modulate operation of the pumps 30, 42. The pumps 30, 42 may be fixed flow rate pumps, or one pump may be variable and the other fixed. In the preferred implementation, each of the pumps 30, 42 is a variable flow pump to provide differing flow rates in the respective loops 28, 41 . An example of such a pump and a suitable controller is a Dan oss VLT micro drive - FC51. The controller 25 provides a variable reference frequency to the motor of the pump which adjusts the rotational speed of the motor to match the reference frequency. Variable flow rates may also be provided by using a pair of pumps connected in series and selectively switching one of (he pumps on or off.

|0049] The controller 25 in a preferred embodiment, is a programmable controller having outputs, namely Y| , Y ₂ . and O. Outputs Y| , Y ₂ control operation of the compressor 24 with Y| calling for a first intermediate load, typical ly 67% of compressor capacity, and Y? calling for a full, 100% load. The output (> controls reversing valve 23 to switch between heating mode and cooling mode.

[0050] In general terms, the output Y| is used to provide a reference frequency that sets the pump 30 at an intermediate flow rate, to match the required flow rate through the loop 28 when the compressor 24 has an intermediate load, and to maintain the pump 42 at a corresponding predetermined flow rate in excess of pump 32. Upon an output Yi being received, when the compressor is conditioned to full load, the output of each pump 30. 42 is correspondingly increased to match the ilow rates to the full load operating condition of the system. |0051 j The ilow centre 44c of Figure 9 facilitates initial setup of the relative flow rates in the heating and cooling system 12, which, in turn, enhances control of the system 12 after the initial setup.

[00521 During initial setup, assuming a single, variable flow pump 30c is used in the heat transfer loop 28c, the pump 30c is set to an initial intermediate ilow rate, typically that specified by the manufacturer of the heat pump 18, The flow rate is determined by measuring the pressure drop across the heat exchanger 26c, after applying a correction factor to accommodate for varying temperatures of the fluid in the loop 28c. A first set point i of the reference frequency is established for the required flow rate of pump 30c, With the flow rate in loop 28c established, the flow rate of the pump 42c is adjusted to match that of the pump 30c. This is facilitated in the ilow centre 44c by reducing the level of fluid through the drain port provided on the sight glass, so that the fluid is level with the upper cross tube 90. At this level, the relative flow rates in the loops 28c, 41c, can be observed from the flow through the cross ports 94. When the flows are equal, there is no net flow across the ports 94 and the flow rates are balanced.

[0053] Upon attaining a balanced Ilow, the pump 42c is adjusted to increase the flow in the loop 41 c to achieve a nominally increased flow rate. It has been found that an increased flow rate of 5% - 10% is satisfactory for typical installations. A first set point zi of the reference frequency is established for the pump 42c.

[0054] The demands of the heat pump 18 with the compressor 24 operating at full load require an increased flow rate in the loop 28c. Accordingly, a second set point, \' . is established for the increased flow rate required from pump 30c, either empirically or by measuring the pressure drop across the heat exchanger 26c as specified for a lull load, and a corresponding set point z: established for the pump 42c. This may be done by observing net flows in the flo centre 44 or by extrapolation from the previous settings,

[0055 j With the initial conditions established, the fluid is replaced in the flow centre 44, The controller 25 may then be used to control the pumps 30, 42 in normal use.

|0056| In one embodiment of the control strategy, as shown in Figure 10, the outputs of controller 25 are used to adjust the flo rates from the pumps 30c. 42c. in the required ratio, to meet the demands of the system 12,

[0057] The output O determines the mode, heating or cooling, and upon the thermostat cal l ing for an increase in temperature {in the healing mode), or a reduction of temperature (in

- y - the cooling mode), an output Y | is applied to the compressor 24 and each of the pumps 30, 42.

[0058] The compressor 24 operates at the intermediate load (e.g. 67%) and the pumps 30. 42 circulate fluid at the rates determ ined by the set points xi . z _t respectively.

[0059] If after a set period. 30 minutes to 120 minutes, the thermostat has not attained its required temperature, or i f the thermostat cal ls for an immediate increase in temperature greater than 2°C. the controller 25 provides outputs Y: to the com pressor 24 and each of the pumps 30c, 42c.

[0060] The compressor 24 increases to ful l load and the output of pumps 30c, 42c, is increased to set points x ₂ . z; respectively. The system 12 operates at these conditions until the required temperature is reached, or a further time limit is reached and the auxiliary heat is switched on by output W.

|0()61 j Upon attainment of the required temperature, the controller 25 removes the outputs Y i . Yj and W, and the system returns to an at rest condition, with the compressor and the pumps 30c, 42c switched off.

|0062] By matching the flow rates of the pumps 30c, 42c, to the demands of the compressor, the operation of the overall system may be optimized with the flow rates i n the respective loops maintained in the required ratio.

[0063] It will be appreciated that the relati ve flow rates of the pumps 30c, 42c ma be adjusted to suit a particular installation and system configuration with the set points for each pump chosen to provide the opti mum flow rates.

[0064] The flexibility provided by the control ler 25 and the use of a pump in each of the loops 28c. 4 1 c, may be utilized to further optimize the operation of the system 1 2.

[0065] As shown in the schematic of Figure 1 1 and 1 2, different operating conditions are attained depending on the mode of operation.

|0066 | I n a heating mode, i .c, one in wh ich heat is transferred in to the building 1 0, the set points Z] zi are selected so that the relative flow rate of the pump 42c is i ncreased beyond that needed to balance the flows i n each loop. Typically a flow rate of 1 10% of that of the pump 30c is found satisfactory, although flows in the range 1 05% to 1 25% may be used. The increased flow from pump 42c is transferred through the flow centre 44c between the cross tubes 90, 92, and is used to heat the fluid returning from the loop 28c as it enters the loop 41 . 1 | 0067 | The fluid delivered through loop 4 1 c to the loop 28c wi ll have a temperature

2 approaching that of the ground source.

3 100681 The fluid is transferred at that temperature to the inlet 34c of the loop 28c and

4 delivered to the heat exchanger 26c. Heat is extracted from the fluid for del ivery to the

5 building, resulting in a significant reduction of the temperature of the fluid. The fl uid is

6 returned at that temperature to the flow centre 44c. where it is delivered through the cross

7 tube 92 to the supply header 36c.

8 [0069] Because of the reduced temperature, there is a risk, in some operating conditions,

9 that localized freezing may occur, particularly on the surface of the header 36c and heat

1 0 exchanger 40c, that impairs heat transfer. This is mitigated by the admixture of the excess

1 1 flow from the pump 42c with the return flo from the loop 28. which elevates the

1 2 temperature of the lluid in the loop 41 c. With a How rate of the pump 42 at 1 10% of pump

1 32, and a fluid temperature at around -5 C. it has been found that the overflow and admixture

1 4 can elevate the fluid temperature by 2°C. su fficient to mitigate the surface freezing.

1 5 J007 1 When operating in a cooling mode, i .e. when heat is rejected to the ground source.

1 as shown in Figure 1 2, the temperature of returni ng fl uid is elevated. In this situation,

1 7 admixture with the excess How will reduce the temperature and reduce the rate of dissipation

1 8 across the heat exchanger, which is undesirable. Accordingly, the pump 42 is control led so

1 that the flow differential is reduced and pump 42 is set to operate at a slightly greater flow 0 rate. i.e. 2% greater than pump 32. In this case, the set points _\ z? are selected to minimize 1 the cross over How in the How centre.

2 [0071] The control strategy, therefore, operates the pump 42c to over supply the loop 28c 3 during heating mode to permit admixture, whereas in cooling mode the admixture i s

4 minimized by matching the outputs of pumps 42c and 28c.

5 |0072 ] During transient conditions, the variabil ity of the flow rates may also be used to 6 advantage and coordinated with the operation of the heat pum p 1 8. as al so shown i n the 7 schematics of Figures 1 1 and 1 2.

8 [0073 ] Assum i ng the building 1 0 is at the required temperature, the controller 25 9 maintains the heat pump 1 8 inacti ve and both pumps 32. 42 oft ^" i.e. no flow.

0 [0074] When the controller 25 cal ls for heating, i nitially the fan and heat pump

1 compressor associated with the building 10 is switched on as indicated as "C on at IJ ^" . After

- I I - a pre set delay, e.g. 5 seconds, a control signal Y] is sent to the pump 30c to initiate flow in the loop 28c at the rate determined by the set point xi . After a further delay, e.g. 20 seconds, the control signal Υτ is applied to the pump 42 to operate it at its maximum flow rate, i.e. at set point zi and the pump 30 is maintained operating at the intermediate speed xi . The increased flow rate is accommodated in the flow centre 41 and is maintained for an initial purge period, typically a period sufficient to provide a complete circulation of fluid in the loop 41 , in the order of 300 seconds. A flow ramp up period of 30 seconds is provided to avoid sudden changes. If preferred, a higher flow rate than the set point Z;> may be used for purging, but it is convenient to use the set point 22.

100751 After the initial purge period, the control signal to the pump 42 reverts to Yi and the output of the pump 42 will be ramped down over a period of 30 seconds, to set point Z] _ The pump 30 is switched on at set point X ] _, as the heating mode is selected, the set point z i provides an over capacity providing admixture with fluid returning from loop 28.

[0076] If the required temperature is attained, the control signal Yi is removed. The controller 25 asserts an output Y _: to the pump 42 for a shut down period, typically 300 seconds, to maintain circulation in the heat transfer loop 1 . Thereafter, the flow rate is ramped down and the pump 42 switched off.

[0077] When the required temperature has not been attained after a preset interval, i.e. 30 - 120 minutes, the output Y ₂ is asserted to each of the pumps 32, 42 and both operate at their maximum respective rates, as determined by set points 2 and ¾. Once the temperature is attained, the pumps 32, 42 are shut down as described above.

(0078] Λ similar sequence is implemented in the cooling mode, with the set point z? of pump 42 being the lower value that matches the maximum capacity of the pump 32.

|0079) The independent operation of the two pumps 32. 42, may therefore be used to establish optimum flow rales in each loop for steady slate and transient conditions, without impacting on the design conditions for the heat pump 1 8.

|0080] It will be appreciated that the examples above are exemplary and other combinations may be used to meet the particular design parameters of the system 12.

- ! 2 -