WO2005103586A2

WO2005103586A2 - Heat pump

Info

Publication number: WO2005103586A2
Application number: PCT/GB2005/001568
Authority: WO
Inventors: Richard Freeborn
Original assignee: Thermal Energy Ventures Limited
Priority date: 2004-04-24
Filing date: 2005-04-22
Publication date: 2005-11-03
Also published as: WO2005103586A3; GB0409147D0

Abstract

A heat pump that uses the air as its heat source comprising a closed heat pump circuit having a working fluid circulating therein, the circuit having an evaporator (9) coil heat exchanger, a compressor (11), a condenser (19, 20) heat exchanger, an expansion valve (17) and control means (27). The control means (27) being adapted to control the heat imparted to the heat transfer fluid in the condenser heat exchanger (20) as a function of the ambient air temperature. Further, the heat pump includes a fan (10) for drawing ambient air over the evaporator coil, wherein the control means incorporates ambient air temperature sensing means (23) and varies the fan (10) speed as a function of this temperature. The control means also incorporates evaporator (9) coil temperature sensing means (25) and is adapted to initiate a defrost cycle when the evaporator coil (9) reaches a preset start-defrost temperature.

Description

HEAT PUMP

This invention relates to a heat pump that operates using the air as its heat source, and in particular, to a heat pump that is optimised to provide heat and hot water for a domestic building.

Heat pumps are devices that recover and transfer heat from one place to another and can be configured to operate efficiently and with minimal environmentally harmful output. Thus, as environmental regulations are tightened, there will be demand for a more efficient and environmentally friendly means for heating dwellings. Further, mean energy consumption is increasing and conventional fuels, such as oil and gas, are diminishing, thus a system such as a heat pump is desirable.

Typically, heat pumps have a compressor, an expansion valve and two heat exchangers connected in a closed circuit, which has a volatile working fluid circulated therethrough. Figure 1 shows a simplified diagrammatic heat pump system. In this system volatile working fluid, initially as a cool liquid, is passed through an evaporating heat exchanger. It is ensured that the working fluid is at a lower temperature than the surroundings and thus heat will be absorbed by the working fluid. Typically the volatile working fluid leaves the evaporator heat exchanger as a gas. The gaseous working fluid is then compressed by the compressor, thereby increasing the pressure and temperature of the working fluid, which is then passed through the second heat exchanger. The second, or condensing, heat exchanger also has a heat transfer fluid passing through it, at a lower temperature than the hot gaseous working fluid, to remove the heat therefrom. The gaseous working fluid obviously loses energy to the heat transfer fluid at this stage and typically condenses and leaves the second heat exchanger as a warm pressurized liquid. The cooler liquid is then passed through the expansion valve that allows the volatile fluid to expand in order to begin the cycle once again. This thermodynamic cycle is known in the art as a vapour compression cycle. The compressor and evaporator are powered by an external energy source.

There are two main types of heat pump, air source heat pumps and ground source heat pumps. Most commonly, especially for residential heating requirements, ground source heat pumps are used. Ground source heat pumps comprise a substantial length of piping buried in the ground. A low temperature volatile working fluid in the piping absorbs heat from the ground (provided that the ground is at a higher temperature than the working fluid) . The heat absorbed by the working fluid is used in the building where the pump is located. Ground source heat pumps have been found to operate successfully as the ground temperature remains relatively constant, even during cold weather. However, it is much easier to install these devices while the building is being constructed. Installing such a heat pump in an existing building is more difficult and costly, as the surrounding ground has to be excavated to bury the piping.

Air source heat pumps do not require substantial lengths of piping. These devices operate using a fan that draws ambient air from outside the building over the evaporative heat exchanger, which has the volatile working fluid passing through it. This type of heat pump also operates in reverse, thus being able to cool by absorbing heat from the building and transferring it to the surroundings by the fan and heat exchanger. Devices that operate on this principle are known, and are typically used in fuel station shops or kiosks. These devices provide heat in cold weather and cooling in hot weather. Although air source heat pumps are sufficient for providing ambient heating and cooling for fuel station shops and the like, they have been commercially unsuccessful in providing effective, efficient and useful space and domestic water heating for residential requirements.

According to a first aspect of the invention we provide an air source heat pump for heating a building comprising a closed heat pump circuit having a working fluid circulating therein, the circuit having an evaporator coil heat exchanger, a compressor, a condenser heat exchanger and an expansion valve, the heat pump also including a fan for drawing ambient air over the evaporator coil, and means for transferring heat from the heat pump to the building comprising a heat transfer fluid that is circulated through the condenser heat exchanger and a control means incorporating output temperature sensing means to measure the temperature of the heat transfer fluid returning to the heat exchanger from the building, and air temperature sensing means to measure the temperature of the ambient air, wherein the control means controls the heat imparted to the heat transfer fluid in the condenser heat exchanger as a function of the ambient air temperature.

The control means therefore functions as a weather compensation device. The control means allows the heat pump to operate more efficiently by varying how much heat the apparatus imparts to the heat transfer fluid so that it is of a suitable temperature in comparison to the ambient air temperature.

Preferably, the heat transferred to the heat transfer fluid is reduced as the ambient temperature increases. Thus, this makes the heat pump more efficient as less heating is required when the ambient air temperature of the surroundings is higher, and therefore the control means reduces the temperature of the heat transfer fluid accordingly. Preferably, the control means alters the temperature of the heat transfer fluid by control of the compressor. Preferably this is achieved by controlling the compressor's operating cycle.

Preferably, the control means varies the temperature of the heat transfer fluid when an ambient air temperature set point is reached. Thus, the control means can be set to increase the temperature of the heat transfer fluid when the ambient air temperature falls below a set point. This accounts for different heating loads required due to changes in the ambient air temperature. Further, it is preferable that the temperature of the heat transfer fluid is kept between upper and lower limits. This ensures that the temperature of the heat transfer fluid is not too low when the ambient temperature is very high and not too high when the ambient temperature is very low.

Preferably the control means uses a linearly proportional function between the ambient air temperature and the heat transfer fluid return temperature.

Preferably the upper and lower limits and the ambient air temperature set point can be set by an operator according to the climate that the heat pump is intended to operate in. These settings may be pre-set during manufacture of the control means, but may remain adjustable to meet differing demands.

Preferably the upper limit temperature of the heat transfer fluid is 50°C and the lower limit is 35°C. Preferably the ambient air temperature set point is 10°C. However, it will be appreciated that the upper limit may be any of 40°C, 45°C, 55°C, 60°C, 65°C or 70°C. However, advances in compressor technology may allow the upper limit to be 75°C, 80°C or 85°C. Further, the lower limit may be 15°C, 20°C, 25°C, 30°C, 40°C, 45°C or 50°C. The ambient air temperature set point may be any of -10°C, -8°C, -6°C, -4°C, -2°C, 0°C, 2°C, 4°C, 6°C, 8°C, 12°C, 14°C or 16°C.

Preferably, the upper temperature limit can be temporarily increased in the event the user requires more heat or wants the heat transfer fluid to be able to heat domestic hot water to an increased temperature, for example.

The control means may have learning means that monitors the performance of the heat pump and alters the upper and lower temperature limits and the air temperature set point to improve efficiency.

According to a second aspect of the invention we provide an air source heat pump for heating a building comprising a closed heat pump circuit having a working fluid circulating therein, the circuit having an evaporator coil, a compressor, a condenser heat exchanger and an expansion valve, the heat pump also including a fan for drawing ambient air over the evaporator coil, and means for transferring heat from the heat pump to the building comprising a heat transfer fluid that is circulated through the condenser heat exchanger and a control means incorporating ambient air temperature sensing means, wherein the control means can vary the fan speed as a function of the ambient air temperature.

The control means thus operates as a fan speed compensation device that ensures that the fan does not draw more air than necessary over the evaporator coil and therefore waste energy. As the ambient air temperature decreases, the temperature differential between the air and the working fluid also falls resulting in reduced heat transfer therebetween. Therefore, in cold weather the fan needs to draw more air over the evaporator coil to achieve a heat transfer comparable to that in warmer weather. Thus, even in cold weather where the fan would typically be at full speed, if the control means determines that the heat pump is providing adequate heating, the fan speed can be reduced to improve efficiency.

Preferably, the fan speed is decreased as the ambient air temperature increases. Preferably, the control means sets the fan speed to maximum once the ambient temperature is below a low threshold temperature. Further, the control means may be set to prevent the fan speed dropping below a lower fan speed threshold. The control means may also be set to ensure that the fan is reduced to its lower threshold speed when the ambient air is at a high threshold temperature.

Preferably, the control means varies the fan speed as a linear function of the ambient air temperature.

Preferably the minimum and maximum fan speeds and the function that controls the fan speed are preset but they may be adjustable by the user in the field. Most preferably, the control means may have learning means that monitors the performance of the heat pump and alters the minimum and maximum fan speeds and the function that controls the fan speed to achieve maximum efficiency as well as reducing noise levels from the fan.

Preferably, the control means varies the fan speed as a function of the relationship between the ambient air temperature and the evaporator coil temperature.

The control means may also include further sensing means that measures the pressure in the evaporator coil, whereby the control means uses both the pressure and the ambient air temperature to determine the fan speed. According to a third aspect of the invention we provide an air source heat pump for heating a building comprising a closed heat pump circuit having a working fluid circulating therein, the circuit having an evaporator coil, a compressor, a condenser heat exchanger and an expansion valve, the heat pump also including a fan for drawing ambient air over the evaporator coil, and means for transferring heat from the heat pump to the building comprising a heat transfer fluid that is circulated through the condenser heat exchanger and a control means incorporating sensing means to measure the temperature of the evaporator coil, and wherein the control means initiates a defrost cycle when the evaporator coil reaches a preset start-defrost temperature.

Thus, the control means operates as a defrost system, which ensures that the flow of air over the evaporator coil, and thus transfer of energy from the ambient air to the working fluid, is unhindered.

Preferably, the control means only initiates the defrost cycle once the evaporator coil temperature has reached or been below the preset start- defrost temperature for a preset frosted time interval.

Preferably, the defrost cycle continues for a minimum preset time interval. Preferably the control means prevents the defrost cycle from continuing for more than a preset maximum time interval.

Preferably, the control means ends the defrost cycle if a preset end- defrost temperature is reached in the time period between the minimum and maximum preset time intervals. It is preferable that the control means initiates successive defrost cycles until the preset end-defrost temperature is reached, the successive defrost cycles being separated by a preset time interval.

Preferably, during the defrost cycle the compressor is stopped and then the heat pump is operated in reverse whereby the working fluid is circulated in a reverse direction to its normal "heating" direction. Thus, the heat pump operates in a similar way to an air-conditioning system in which energy from the warmer air in the building is absorbed by the working fluid, which is then passed through the evaporator coil, thereby heating the coil and melting any ice or frost thereon. The control system therefore initiates a reverse cycle defrost as known in the art.

Preferably, a reversing valve is used to circulate the working fluid in the opposite direction thereby operating the heat pump in reverse.

Preferably, there is a preset delay time period at the start of the defrost period between when the compressor is stopped and when its operation is reversed. Preferably, there is a preset delay time period at the end of the defrost cycle between when the compressor is stopped and when it reverts to "normal" operation.

There now follows by way of example only a detailed description of the present invention with reference to the accompanying drawings in which;

Figure 1 shows a simplified diagram of a typical heat pump;

Figure 2 shows a diagrammatical plan of the apparatus illustrating all aspects of the present invention; Figure 3 is a graph that shows typical operation of the weather compensation system according to the first aspect of the invention;

Figure 4 is a graph showing typical operation of the fan speed compensation system according to the second aspect of the invention; and

Figure 5 is a graph showing operation of the defrost system according to the third aspect of the invention.

A simplified heat pump 1, as shown in Figure 1, has four primary components comprising an evaporating heat exchanger 2, a compressor 3, a condensing heat exchanger 4 and an expansion valve 5. The four components are connected together in a circuit and a working fluid (not shown) is circulated therethrough in a clockwise direction. In the evaporating heat exchanger 2 the working fluid absorbs heat from the surroundings and in the condensing heat exchanger 4 the working fluid transfers the heat to another fluid, such as water for a domestic central heating system, for example. The principles of its operation are discussed in the introduction.

An embodiment of the heat pump apparatus 8 for a residential building according to the invention is shown in Figure 2 and principally comprises the four components shown in Figure 1. Similarly, the heat pump apparatus 8 has working fluid (not shown) circulating therethrough in the direction of the arrows in normal "heating operation" . The heat pump apparatus 8 comprises an evaporator coil heat exchanger 9, which is equivalent to the heat exchanger 2 of Figure 1. The heat exchanger 9 has a fan 10 mounted adjacent thereto for drawing ambient air from the surroundings over the heat exchanger 2. Thus, heat exchanger 9 and fan 10 are typically mounted on the outside wall of a building. The heat exchanger 9 is connected to a vapour compressor 11 via a reversing valve 12. The reversing valve has four fluid flow paths 31, 32, 34 and 35. In normal operation fluid is only allowed to flow along paths 31, 32. The fluid output from the heat exchanger 9 flows along paths 31 to the vapour compressor 11. The output 13 of pressurized gas from the vapour compressor also passes through the reversing valve 12 along path 32 and is connected to a heat exchanger 14, which is equivalent to the heat exchanger 4 of Figure 1. The working fluid (not shown) then passes through check valve 15, which only allows fluid flow in the direction of the arrows. Thus, the working fluid passes through liquid filter drier 16 and expansion valve 17. The expansion valve 17 is connected to the evaporator coil heat exchanger 9, thus completing a working circuit 18. The vapour compressor 11, the heat exchanger 14, the filter drier 16 and expansion valve 17 are typically located within the heat pump in the building.

The heat exchanger 14 is of known construction, being divided into sections 19, 20, wherein section 19 has working fluid passing therethrough, while section 20 receives a heat transfer fluid (not shown) . The sections 19, 20 are arranged to maximise the surface area over which they are in contact, thereby improving heat transfer. Heat transfer fluid leaves the heat exchanger 14 through exit pipe 21 and enters through return pipe 22. The heat transfer fluid may then be used to heat a residential building by a heat delivery system (not shown) such as an under-floor heating system, for example. The heat delivery system would typically be in the form of a circuit with the exit pipe 21, return pipe 22 and heat exchanger 14 completing the circuit.

The heat pump apparatus 8 also has several sensing means 23, 24, 25, 26 comprising temperature sensors. Sensing means 23 measures the ambient air temperature outside the building and sensing means 24 measures^' the temperature of the air once it has passed through the heat exchanger 9. Sensing means 25 measures the temperature of the evaporator coil heat exchanger 9 itself. The temperature of the returning heat transfer fluid is measured by sensing means 26, which is located adjacent the heat transfer fluid return pipe 22.

The sensing means 23, 24, 25, 26 are connected (by electrical connections, not shown) to a control means 27, which controls the heat pump apparatus 8 including the speed at which the fan 10 operates and the state of the reversing valve 12.

In normal operation, the heat pump 8 is optimised to absorb heat from the ambient air through the heat exchanger 9 and pump it to the heat exchanger 14 so that the heat transfer fluid can distribute the heat through the building in a heat delivery system, such as under-floor heating. On a typical working cycle starting at the evaporator coil heat exchanger 9, working fluid enters the heat exchanger typically as a cold gas. Provided that the working fluid is at a lower temperature than the ambient air temperature, it will absorb heat from the air that is drawn over the heat exchanger by the fan 10. Thus, the working fluid will be warmed and may leave the heat exchanger 9 at approximately -2°C, for example. The working fluid then passes through reversing valve 12 along flow path 31. The vapour compressor 11 rapidly compresses the working fluid resulting in it typically leaving the compressor 11 as a high pressure, superheated gas at approximately 100°C. The superheated working fluid then passes through reversing valve 12 along path 32 toward the plate heat exchanger 14. The superheated working fluid is passed through section 19 so that it can transfer its energy to the heat transfer fluid in section 20. The working fluid would typically leave the heat exchanger 14 as a cooler high pressure liquid at approximately 50°C, for example. The working fluid passes through check valve 15 and liquid filter drier 16. The working fluid is then allowed to expand rapidly through expansion valve 17, which causes the pressure and temperature of the working fluid to decrease. The working fluid would, for example, leave the expansion valve at -8°C. The working fluid would then complete the circuit by entering the evaporator coil heat exchanger 9.

The graph shown in Figure 3 represents operation of the weather compensation feature of the control means 27. The control means 27 uses sensing means 25 to determine the evaporator coil temperature. The temperature of the heat transfer fluid returning from the heat delivery system is measured by sensing means 26. The control means 27 then uses a pre-programmed function to calculate what the return temperature set point of the heat transfer fluid should be.

Thus, when the ambient air temperature decreases below an ambient air set point of 10°C, the control means 27 controls the compressor operating cycle such that the temperature of the returning heat transfer fluid is caused to increase. In Figure 3 the control means 27 is set such that the returning heat transfer fluid temperature is programmed to vary between an upper limit of 50°C and a lower limit of 35° C as the ambient air temperature varies between 0°C and the set point of 10°C. These values may be pre-programmed in the control means 27 or may be user-defined. Thus, the control means has an interface 28 to allow a user to input the required values. Alternatively, the interface 28 may allow an additional device (not shown) to communicate with the control means 27.

As it is the temperature of the returning heat transfer fluid that is measured, the control means 27 ensures that the heat transfer fluid carries sufficient heat to satisfy the heating demand of the building. Thus, the heat transfer fluid leaves the exit pipe 21 with sufficient heat to heat the building and thus prevent it cooling below a useful temperature before it returns to the heat exchanger 14 for re-heating by the heat pump 8.

Figure 4 shows how the control means 27 operates as a fan speed compensation device by varying the fan speed in relation to the evaporator coil temperature. The control means 27 uses evaporator coil temperature readings from sensing means 25 to determine the fan speed. The graph shows that the control means 27 has been set to reduce the fan speed from its maximum speed, represented by 100%, to a preset threshold fan speed of 22%. The control means 27 is such that the speed of the fan 10 is linearly reduced as the ambient air temperature increases from 5°C to 14°C. At ambient temperatures below 5°C (a low threshold temperature) , the control means 27 sets the fan 10 to operate at full speed (100%) and at ambient temperatures of above 14°C (a high threshold temperature) , the fan operates at its minimum threshold value of 22%.

The low and high threshold temperatures and the minimum threshold fan speed may be preset, or the user, using the interface 28, may define them. Alternatively, the control means may have learning means 29 that operates algorithmically to select the most efficient values for the low and high threshold temperatures and the minimum fan speed, by monitoring the performance of the heat pump 8 and/or the climate.

As the evaporator heat exchanger 9 is located outside the building in which the heat pump 8 is installed, it is prone to the build up of frost and ice in cold weather. The formation of ice and frost can affect the performance of the heat pump 8, as the effective surface area of the heat exchanger 9 is reduced thereby limiting the amount of heat that can be transferred from the air to the working fluid circulating therethrough. Further, the continued build-up of ice may cause damage to the heat exchanger 9. The control means 27 is able to control operation of the heat pump 8 such that the frost and ice can by efficiently removed from heat exchanger 9. Thus, the control means 27 uses a series of defrost cycles 30, one of which is illustrated in the graph of Figure 5.

Figure 5 shows the variation of the temperature of the heat exchanger 9, as measured by sensing means 25, over time. In order for a defrost cycle 30 to be initiated the heat exchanger 9 must be at or below a preset start-defrost temperature for a preset "frosted" time interval. The control means 27 stores these values and in this example, the start-defrost temperature is set at -10°C and the frosted time interval is set at 60 seconds. The control means 27 also stores an end-defrost temperature at which the defrost cycle is terminated, which, in this example, is 20°C. Further, the control means 27 stores values corresponding to the minimum and maximum durations of a defrost cycle. These values are set to 60 seconds and 180 seconds respectively. Further parameters include a time interval between successive defrost cycles (typically set at 20 minutes) and a "start reverse operation" delay time period (set at 60 seconds) and an "end reverse operation" delay time period (set at 60 seconds) .

Referring to the graph of Figure 5, at t = 0 the temperature of heat exchanger 9 is approximately -5°C as measured by sensing means 25. The temperature falls below the start-defrost temperature at t = 20 seconds, and remains below this temperature for 30 seconds. During this period the heat exchanger 9 may be frosted, however as this time period is less than the frosted time interval, the control means 27 does not initiate the defrost cycle. Between t = 50 seconds and t= 60 seconds the temperature of the heat exchanger 9 increases above the start-defrost temperature. Between t = 60 seconds and t= 120 seconds the temperature of the heat exchanger 9 is continually below the start-defrost temperature for a period equal to the frosted time interval, thus the control means 27 initiates the defrost cycle 30.

At the start of the defrost cycle 30 the control means 27 stops the compressor 11 and then waits for the start delay time period of 60 seconds. At the end of this period. t= 180 seconds and the control means 27 operates the heat pump in reverse. Further, at t= 180 seconds the minimum time interval for a defrost cycle has been exceeded and thus the defrost cycle can now be terminated as soon as the heat exchanger temperature reaches the preset end-defrost temperature of 20°C.

The heat pump is operated in reverse by actuation of the reversing valve 12. During a defrost cycle 30, the reversing valve 12 prevents fluid flow along flow paths 31, 32 and allows flow along the flow paths 34, 35 (represented by dashed lines). Thus, after the start delay period has expired the control means 27 initiates the following flow circuit to defrost the evaporator coil heat exchanger 9. The compressor 11 compresses the working fluid thereby heating it. The working fluid leaves the compressor along output 13 and passes through the reversing valve along path 34. Thus, the heated working fluid is directed through the frosted heat exchanger 9. The heated working fluid will warm the heat exchanger 9 causing the frost and ice to melt and drain away through condensate drain 36. The condensate drain 36 is also used to drain away any condensation that forms on the heat exchanger 9 in normal operation. Upon leaving the heat exchanger 9 the working fluid passes through a check valve 33, which only allows flow in this direction, and towards a second expansion valve 37. The working fluid then completes the circuit by passing through heat exchanger 14, reversing valve 12 along path 35 and back to the compressor 11. In addition to temperature sensors the sensing means may comprise humidity sensors that are connected to the control means 27 for measuring the humidity of the ambient air, for example. The information from the humidity sensors may be used by the control means to more efficiently operate the defrost cycle, the speed of the fan or the output of the heat pump in response to changes in the temperature of the ambient air.

Claims

1. An air source heat pump for heating a building comprising a closed heat pump circuit having a working fluid circulating therein, the circuit having an evaporator coil heat exchanger, a compressor, a condenser heat exchanger and an expansion valve, the heat pump also including a fan for drawing ambient air over the evaporator coil, and means for transferring heat from the heat pump to the building comprising a heat transfer fluid that is circulated through the condenser heat exchanger and a control means incorporating output temperature sensing means to measure the temperature of the heat transfer fluid returning to the heat exchanger from the building, and air temperature sensing means to measure the temperature of the ambient air, wherein the control means controls the heat imparted to the heat transfer fluid in the condenser heat exchanger as a function of the ambient air temperature.

2. An air source heat pump according to claim 1, in which the heat transferred to the heat transfer fluid is reduced as the ambient temperature increases.

3. An air source heat pump according to claim 1 or claim 2, in which the control means alters the temperature of the heat transfer fluid by control of the compressor.

4. An air source heat pump according to claim 3, in which the compressor's operating cycle is controlled.

5. An air source heat pump according to any preceding claim, in which the control means varies the temperature of the heat transfer fluid when an ambient air temperature set point is reached.

6. An air source heat pump according to any preceding claim, in which the control means is set to increase the temperature of the heat transfer fluid when the ambient air temperature falls below a set point.

7. An air source heat pump according to any preceding claim, in which the temperature of the heat transfer fluid is kept between upper and lower limits.

8. An air source heat pump according to any preceding claim, in which the control means uses a linearly proportional function between the ambient air temperature and the heat transfer fluid return temperature.

9. An air source heat pump according to claim 6 and claim 7, in which the upper and lower limits and the ambient air temperature set point are set by an operator according to the climate that the heat pump is intended to operate in.

10. An air source heat pump according to claim 6 and claim 7, in which the upper and lower limits and the ambient air temperature set point are pre-set during manufacture of the control means.

11. An air source heat pump according to claim 10, in which the upper and lower limits and the ambient air temperature set point remains adjustable to meet differing demands.

12. An air source heat pump according to any of claims 7 to 11, in which the upper limit temperature of the heat transfer fluid is 50°C.

13. An air source heat pump according to any of claims 7 to 12, in which the lower limit temperature of the heat transfer fluid is 35°C.

14. An air source heat pump according to any of claims 5 to 13, in which the ambient air temperature set point is 10°C.

15. An air source heat pump according to any of claims 7 to 14, in which the upper temperature limit can be temporarily increased.

16. An air source heat pump according to any preceding claim, in which the control means has learning means that monitors the performance of the heat pump and alters the upper and lower temperature limits and the air temperature set point to improve efficiency.

17. An air source heat pump for heating a building comprising a closed heat pump circuit having a working fluid circulating therein, the circuit having an evaporator coil, a compressor, a condenser heat exchanger and an expansion valve, the heat pump also including a fan for drawing ambient air over the evaporator coil, and means for transferring heat from the heat pump to the building comprising a heat transfer fluid that is circulated through the condenser heat exchanger and a control means incorporating ambient air temperature sensing means, wherein the control means can vary the fan speed as a function of the ambient air temperature.

18. An air source heat pump according to claim 17, in which the fan speed is decreased as the ambient air temperature increases.

19. An air source heat pump according to claim 17 or claim 18, in which the control means sets the fan speed to maximum once the ambient temperature is below a low threshold temperature.

20. An air source heat pump according to any of claims 17 to 19, in which the control means is set to prevent the fan speed dropping below a minimum fan speed threshold.

21. An air source heat pump according to claim 20, in which the control means is set to ensure that the fan is reduced to the minimum threshold speed when the ambient air is at a high threshold temperature.

22. An air source heat pump according to any of claims 17 to 21, in which the control means varies the fan speed as a linear function of the ambient air temperature.

23. An air source heat pump according to claim 22, in which the minimum threshold fan speed and maximum fan speed and the function that controls the fan speed are preset.

24. An air source heat pump according to claim 22, in which the minimum and maximum fan speeds and the function that controls the fan speed are adjustable by the user in the field.

25. An air source heat pump according to claim 22, in which the control means has learning means that monitors the performance of the heat pump and alters the minimum and maximum fan speeds and the function that controls the fan speed to achieve maximum efficiency.

26. An air source heat pump according to any of claims 17 to 25, in which the control means varies the fan speed as a function of the relationship between the ambient air temperature and the evaporator coil temperature.

27. An air source heat pump according to any of claims 17 to 26, in which the control means includes further sensing means that measures the pressure in the evaporator coil, whereby the control means uses both the pressure and the ambient air temperature to determine the fan speed.

28. An air source heat pump for heating a building comprising a closed heat pump circuit having a working fluid circulating therein, the circuit having an evaporator coil, a compressor, a condenser heat exchanger and an expansion valve, the heat pump also including a fan for drawing ambient air over the evaporator coil, and means for transferring heat from the heat pump to the building comprising a heat transfer fluid that is circulated through the condenser heat exchanger and a control means incorporating sensing means to measure the temperature of the evaporator coil, and wherein the control means initiates a defrost cycle when the evaporator coil reaches a preset start-defrost temperature.

29. An air source heat pump according to claim 28, in which the control means only initiates the defrost cycle once the evaporator coil temperature has reached or been below the preset start-defrost temperature for a preset frosted time interval.

30. An air source heat pump according to claim 28 or claim 29, in which the defrost cycle continues for a minimum preset time interval.

31. An air source heat pump according to any of claim 28 to 30, in which the control means prevents the defrost cycle from continuing for more than a preset maximum time interval.

32. An air source heat pump according to claim 31, in which the control means ends the defrost cycle if a preset end-defrost temperature is reached in the time period between the minimum and maximum preset time intervals.

33. An air source heat pump according to claim 32, in which the control means initiates successive defrost cycles until the preset end- defrost temperature is reached, the successive defrost cycles being separated by a preset time interval.

34. An air source heat pump according to any of claim 28 to 33, in which during the defrost cycle the compressor is stopped and then the heat pump is operated in reverse whereby the working fluid is circulated in a reverse direction to its normal "heating" direction.

35. An air source heat pump according to claim 34, in which a reversing valve is used to circulate the working fluid in the opposite direction thereby operating the heat pump in reverse.

36. An air source heat pump according to claim 34 or claim 35, in which there is a preset delay time period at the start of the defrost period between when the compressor is stopped and when its operation is reversed.

37. An air source heat pump according to any of claim 34 to 36, in which there is a preset delay time period at the end of the defrost cycle between when the compressor is stopped and when it reverts to "normal" operation.