This invention relates to a modified heat pump and to a heat pump installation comprising such a modified heat pump. In particular, it relates to a heat pump and heat pump installation modified to enhance performance at ambient temperatures of below 5°C.

A heat pump in its simplest form comprises a closed circuit around which a refrigerant fluid is circulated. The circuit includes an electrically operated compressor which pressurises the fluid, in its gaseous form, thus causing the refrigerant gas to heat up. The hot pressurised gas is then circulated through a condenser, within which it condenses to a liquid, though still under high pressure. This causes the condenser itself to generate heat, which may be recovered to drive domestic heating and hot water systems. From the condenser, the high pressure liquid refrigerant is circulated to an expansion valve, which has the effect of lowering the pressure of the liquid refrigerant so as to promote evaporation. The low pressure liquid refrigerant is then circulated to an evaporator, where it evaporates into a gas, absorbing heat from the evaporator's surroundings as it does so. The gaseous refrigerant then returns to the compressor, and the cycle repeats.

The use of heat pumps for heating buildings has increased significantly in recent years, due to environmental concerns over more conventional energy sources such as fossil fuels. Energy efficiency and long term running costs are other factors which make heat pump technology attractive. Nevertheless, the use of heat pump installations for domestic purposes has thus far failed to achieve the full potential which this technology offers, for a number of reasons.

Firstly, attempts are often made to connect heat pumps to existing domestic water systems and heating systems using conventional radiators. Such systems are generally inefficient when run at the lower operating temperatures necessitated by the use of a heat pump. The heat pump therefore tends to be relegated to the role of auxiliary heat supply, and is used to 'top-up' the heat supply from a conventional boiler, rather than fully replacing it. An energy-efficient heating installation which addresses this problem is disclosed in the applicant's copending UK Patent Application No. 09 19636.1 . The heat pump and heat pump installation of the present invention are particularly suitable for use in such a heating installation.

Secondly, heat pumps lose efficiency when the ambient temperature of the heat source medium falls below about 5°C - around the temperature of an average winter's day in the UK. Thus, the heat pump performs inefficiently in conditions when it is needed most, again leading to heat pumps being used to 'top-up' rather than replace conventional heat sources. In addition to the loss in efficiency at ambient temperatures, it is also necessary for anti-freeze additives such as glycol to be added to the refrigerant fluid in the closed circuit. These anti- freeze additives can cause corrosion of components and thus reduce the working life of the heat pump.

The present invention seeks to address the above identified problems associated with operating heat pumps at low ambient temperatures by providing a modified heat pump and heat pump installation which exhibits decreased losses in efficiency at low ambient temperatures and which does not require the use of antifreeze additives.

According to a first aspect of the present invention there is provided a heat pump comprising:

- a closed circuit around which a refrigerant fluid is circulated, said circuit having a compressor, a condenser, an expansion valve and an evaporator;

- a plenum chamber having said evaporator located therewithin, an inlet adapted to receive a fluid heat source medium and direct it over the evaporator, thereby to extract heat from said medium, and an outlet adapted to receive cooled medium expelled from the plenum chamber; and

- an auxiliary heat exchanger located between the inlet and the evaporator, and adapted to warm said medium prior to it passing over the evaporator.

The arrangement of the auxiliary heat exchanger between the inlet and the evaporator enables warming of the incoming heat source medium. Where the ambient temperature of the heat source medium is low, this enables the temperature of the heat source medium to be raised before the medium passes over the evaporator, thus avoiding, or at least reducing, the loss in efficiency associated with operating heat pumps at low ambient temperatures. The heat pump is preferably adapted to provide heated water for a domestic heating system, and/or heated water for a domestic hot water system, or alternatively may be adapted to provide warmed air for a domestic heating system. In each case, heat is recovered from the heat pump condenser in the conventional manner.

The auxiliary heat exchanger is preferably also driven by heat recovered from the condenser. The auxiliary heat exchanger thus effectively operates as a feedback loop, using heat previously recovered from the condenser to warm new incoming heat source medium. The diversion of some of the heat recovered from the condenser to the auxiliary heat exchanger, rather than to a domestic water or heating system, will necessarily result in decreased output to the domestic water or heating system when the auxiliary heat exchanger is activated. However, it is believed that this loss in performance will be demonstrably smaller than the loss in efficiency associated with operating heat pumps at low ambient temperatures.

In view of the aforementioned loss in performance associated with operation of the auxiliary heat exchanger, it is desirable that the auxiliary heat exchanger should only be activated, and/or the heat source medium only directed over the auxiliary heat exchanger, when the ambient temperature is low. The heat pump may therefore comprise a damper adapted to direct the fluid heat source medium over the auxiliary heat exchanger only when the auxiliary heat exchanger has been activated. Alternatively, or additionally, the heat pump may comprise a damper (which may be the same as, or different from, the aforementioned damper) adapted to direct the fluid heat source medium over the auxiliary heat exchanger only upon the ambient temperature of said medium falling below a predetermined value. The or each damper may be adapted to be activated by thermostatic control means upon the ambient temperature of the heat source medium falling below a predetermined value, and/or upon the temperature of the auxiliary heat exchanger reaching a predetermined value.

In an alternative embodiment of the present invention, the heat pump comprises thermostatic control means adapted to activate the auxiliary heat exchanger upon the ambient temperature of the heat source medium falling below a predetermined value. Said thermostatic control means may be utilised in addition to, or instead of, the or each damper as hereinbefore described. The thermostatic control means preferably includes a motorised valve adapted to deliver heated water to the auxiliary heat exchanger upon activation.

As noted above, heat pumps generally lose efficiency below ambient temperatures of about 5°C. Therefore, the aforementioned predetermined value for the ambient temperature of the heat source medium is substantially 5°C.

Although it is envisaged that the technology of the present invention may be applied to substantially all types of heat pump, it is preferred that the heat pump is an air source heat pump.

As such, in a preferred embodiment of the present invention:

- the heat pump is an air source heat pump;

- the aforementioned fluid heat source medium is air;

- the inlet of the plenum chamber is an air inlet adapted to receive indrawn air and direct said air over the evaporator, thereby to extract heat from said air;

- the outlet of the plenum chamber is an air outlet adapted to receive cooled air expelled from the plenum chamber; and

- the auxiliary heat exchanger is located between the air inlet and the evaporator, and is adapted to warm said indrawn air prior to it passing over the evaporator.

The air source heat pump preferably further comprises a fan adapted to drive or draw air into the plenum chamber through the air inlet, and out of the plenum chamber through the air outlet.

In a preferred sub-embodiment of the first aspect of the present invention, the air source heat pump further comprises a secondary air inlet adapted to receive re-circulated air from the interior of a building in which the heat pump is installed. In this embodiment, the air inlet and the secondary air inlet are preferably arranged such that the indrawn air and re-circulated air are combined prior to passing over the evaporator. The combination of indrawn air with recirculated air also assists to warm the indrawn air and thus will serve to increase the efficiency of the heat pump and reduce the amount of time for which it is necessary to operate the auxiliary heat exchanger. So as further to increase efficiency, the air inlet and the secondary air inlet may preferably be arranged such that the indrawn air and re-circulated air are combined prior to passing over the auxiliary heat exchanger. A further feature of the present invention which serves to address the problems associated with operating heat pumps at low ambient temperatures, is that the air source heat pump according to the aforementioned preferred embodiment of the present invention is adapted to be housed within a building to be heated, rather than externally thereof as is conventional. Since the interior of the building will generally be at a higher temperature than the exterior when heating is required, housing the heat pump within the building provides further protection against the operating temperature of the heat pump falling below 5°C. The air source heat pump will preferably be further provided with sound dampening means to enable it to be used conveniently within a domestic environment.

Therefore, according to a second aspect of the present invention there is provided a heat pump installation comprising an air source heat pump as hereinbefore described, installed within a room, loft, cellar or other defined space of a building to be heated, said heat pump installation further comprising:

- an inlet vent adapted to permit the flow of air therethrough from the exterior of the building, into said room, loft, cellar or other defined space;

- an outlet vent adapted to permit the flow of cooled air from the heat pump to exit the building; and

- an outlet air duct connecting the air outlet of the heat pump with the outlet vent.

Preferably, the air source heat pump is installed in a loft, and the inlet and outlet vents are standard roof tile vents. The installation of the heat pump in a loft enables air drawn in through the inlet vent to be mixed with air in the loft space - which will generally be at a higher temperature when heating is required - thus further raising the temperature of the indrawn air before it enters the heat pump. Alternatively, or additionally, the heat pump installation may further comprise an inlet air duct connecting the air inlet of the heat pump with the inlet vent, to ensure constant delivery of air to the heat pump.

Where the heat pump installation according to the second aspect of the present invention comprises an air source heat pump having a secondary air inlet according to the preferred sub-embodiment of the first aspect of present invention, the heat pump installation preferably further comprises a secondary inlet vent adapted to permit the flow of re-circulated air therethrough from the interior of the building, into the loft space in which the heat pump is installed. Where no inlet air duct is utilised, this enables indrawn air to be mixed with the re-circulated air prior to entering the heat pump. Alternatively, or additionally, the heat pump installation may further comprise a secondary inlet air duct connecting the secondary air inlet of the heat pump with the secondary inlet vent, to ensure constant delivery of air to the heat pump.

In order that the present invention may be more clearly understood, preferred embodiments thereof will now be described in detail, though only by way of example, with reference to the accompanying drawing in which:

Figure 1 shows a first embodiment of heat pump installation according to the present invention; and

Figure 2 shows a second embodiment of heat pump installation according to the present invention.

Referring first to Figure 1 , there is shown a first embodiment of heat pump installation, generally indicated 10, installed in a building 1 1 to be heated.

The heat pump installation comprises an air source heat pump 12 having a plenum chamber 13, an air inlet 14 and an air outlet 15. In addition to the standard heat pump components (compressor, condenser, expansion valve and evaporator - not individually shown) the air source heat pump 12 is provided with an auxiliary heat exchanger 16 located between the air inlet 14 and the plenum chamber 13 in which the evaporator is housed. The auxiliary heat exchanger 16 is fed with heated water by connectors 17, driven by heat recovered from the condenser. Further connectors 18 feed heater water to a domestic water system and/or a domestic heating system (hot shown), again driven by heat recovered from the condenser.

The air inlet 14 is connected via an inlet air duct 19 to an inlet vent 21 located in an external wall of the building 1 1 . Similarly, the air outlet 15 is connected via an outlet air duct 22 to an outlet vent 23 located in an external wall of the building 1 1 . The air source heat pump 12 is further provided with a secondary air inlet 24, which is connected via a secondary inlet air duct 25 to a secondary inlet vent 26 located in an internal partition in the building 1 1 . The inlet air duct 19 delivers air from the exterior of the building 1 1 , drawn in through the inlet vent 21 , to the air inlet 14. The secondary inlet air duct 25 delivers recirculated air from the interior of the building 1 1 , drawn in through the secondary inlet vent 26, to the secondary air inlet 24. The air inlet 14 and secondary air inlet 24 are arranged adjacent to one another so that the two air streams can combine in a mixing chamber 27 before entering the plenum chamber 13.

Referring now to Figure 2, there is shown a second embodiment of heat pump installation, generally indicated 30, installed in a loft space 31 of a building 1 1 to be heated. The second embodiment 30 is similar in many respects to the first embodiment 10 described above with reference to Figure 1 , and where applicable like reference numerals have been used to denote like components.

The second embodiment 30 does however differ from the first embodiment in a number of important respects. Firstly, a plurality of inlet air ducts 19 is provided, connecting a plurality of inlet vents 21 with a plurality of air inlets 14. Similarly, a plurality of outlet air ducts 22 are provided, connecting a plurality of outlet vents 23 with a plurality of air outlets 15. This arrangement allows for smooth and constant flow of air. It can also be seen from Figure 2 that the outlet air ducts 22 take the form of roof tile vents 32.

Secondly, the second embodiment 30 as shown here does not feature the secondary re-circulated air feature, though the plurality of air inlets 14 and inlet air ducts 19 mean that it can easily be adapted to incorporate such a feature if required.

And thirdly, the auxiliary heat exchanger 16 is provided with a motorised valve 33 in communication with thermostatic control means (not shown) adapted to activate the auxiliary heat exchanger 16 upon the ambient temperature falling below 5°C.