Method for operating a refrigerant circuit with oil flushing function

Abstract

The present invention relates to a method for operating a refrigerant circuit (10) with a compressor (11), a heating heat exchanger (12) arranged downstream therefrom and a low-pressure section arranged downstream therefrom, in which, in a first heat pump operating mode, an external heat exchanger (13) with an upstream first expansion valve (16A) and a chiller (14) with an upstream second expansion valve (16B) are arranged in parallel, wherein the chiller (14) is in heat-exchanging contact with a coolant line (20; 21A). The method according to the invention is characterized in that the refrigerant circuit (10) is temporarily switched to a second heat pump operating mode for simultaneously performing an oil flushing operation upon detection of an oil flushing condition, wherein the temperature of the coolant in the coolant line (20; 21A) upstream of the chiller (14) is lowered by a predetermined temperature amount (ΔT).

Description

METHOD FOR OPERATING A REFRIGERANT CIRCUIT WITH OIL FLUSHING FUNCTIONCross Reference

[0001] This application claims the benefit of German Patent Application No. 102024130095.7 filed on October 16, 2024. The entire disclosure of the above application is incorporated herein by reference.Technical Filed

[0002] The present invention relates to a method for operating a refrigerant circuit with an oil flushing function. The present invention is particularly suitable for automotive use because it is particularly advantageous for systems with high cost pressure, limited available installation space and dynamically changing system load.

[0003] Generic refrigerant circuits known from the prior art comprise a compressor, at least one heating heat exchanger arranged downstream therefrom and a low-pressure section arranged downstream therefrom. In the low-pressure section, in a first heat pump operating mode, an external heat exchanger with an upstream first expansion valve and a chiller with an upstream second expansion valve are arranged in parallel, wherein the chiller is in heat-exchanging contact with a first coolant line. In the first heat pump operating mode, control is typically carried out via the temperature of the heating heat exchanger. Such a control typically involves determining a temporal sequence of temperature values associated with the heating heat exchanger and adjusting the operation of the compressor so that the determined temperature values substantially correspond to a fixed target temperature value. A target temperature value is therefore set, which is controlled in such a way that the deviation of the determined temperature values from the set target temperature value does not exceed a set limit value. Alternatively, other regulations may be provided which, for example, do not follow or do not only follow a control maxim relating to the heating heat exchanger.

[0004] Parallel operation in the low-pressure range is characterized by the fact that downstream of a common branch point, the mass flow of the refrigerant is divided into two (or more) parallel paths and the paths are merged downstream in the low-pressure range, typically at a node or, for example, in an accumulator.

[0005] In refrigerant circuits, a refrigerant-lubricating oil mixture is circulated by means of the compressor, with the lubricating oil as an additive to the refrigerant, in particular for the lubrication of machine parts. At low loads, adequate oil circulation is not always possible due to the lower mass flows. In particular, lubricating oil can then accumulate in individual components, in particular in the heat exchangers. When two heat exchangers are operated in parallel, the distribution of the mass flow across the parallel paths can be very uneven depending on the operating state. This occurs, for example, when an external heat exchanger and chiller are connected in parallel in heat pump operation.

[0006] In order to avoid unwanted oil deposits in the system, devices are generally known which collect oil via tanks, pipes and pumps and return it to those regions of the refrigerant circuit where it is primarily needed. In addition to the complexity of the equipment, such devices have the particular disadvantage that they require additional installation space and are associated with costs.

[0007] There is a method in which an electrically controlled expansion valve upstream of the heat exchanger in question is suddenly operated with a higher degree of opening, so that the mass flow through the downstream region with the heat exchanger in question is increased for a short time and thus any lubricating oil stored in the heat exchanger is flushed out.

[0008] The disadvantage of the method is that a sudden opening in the range of smaller opening degrees is particularly dependent on the most precise possible knowledge of the actual geometry of the installed expansion valve and the circuit-dependent mass flows to be expected. On the one hand, the newly selected degree of opening must enable a sufficient mass flow, and on the other hand, it must be avoided that a jump in the mass flow that is too large does not lead to a reduction in comfort or even hardware damage to the components of the refrigerant circuit, e.g. due to liquid refrigerant reaching the compressor. Due to minor manufacturing tolerances and signs of aging on the valve seat and / or the valve mechanism, such knowledge is usually insufficient. Expansion valves optimized for large opening degrees can have relatively large tolerances at smaller opening degrees, which makes their use for small mass flows (low load) impractical.

[0009] The present disclosure enables the operation of a refrigerant circuit with an alternative oil flushing function, and it is particularly cost-effective to implement and can be used without problems under low load conditions and partial load conditions.

[0010] A method for operating a refrigerant circuit according to the present disclosure includes starting from an operation in a first heat pump mode, and the refrigerant circuit is temporarily switched to a second heat pump operating mode for simultaneously carrying out an oil flushing operation upon detection of an oil flushing condition, wherein the temperature of the coolant in the coolant line upstream of the chiller is reduced by a predetermined temperature amount. Lowering the coolant temperature indirectly reduces the refrigerant mass flow through the chiller and thus increases the refrigerant mass flow through the parallel operated external heat exchanger, which now leads to a flushing effect where the mass flow was previously low.

[0011] The inventors have recognized that when using typical basic controls of the refrigerant circuit, by lowering the coolant temperature, the suction pressure of the refrigerant at the compressor inlet initially decreases and thus the temperature level in the low-pressure range is also generally lowered. Since the temperature of the ambient air remains constant over short time scales, the temperature difference between the refrigerant and the ambient air at the external heat exchanger increases. The external heat exchanger can thus evaporate more refrigerant, which enables a higher mass flow.

[0012] To avoid undesirable noise levels, the compressor is typically operated at a maximum permissible continuous speed. By lowering the temperature of the coolant, not only does the temperature level in the refrigerant circuit drop spontaneously, but also the heating output of the heating heat exchanger. According to an advantageous embodiment, it is provided that a maximum permissible first limit speed of the compressor is set for the first heat pump operating mode, which typically corresponds to the maximum permissible continuous speed. In the second heat pump operating mode, the compressor speed is increased beyond the first limit speed when the temperature values associated with the heating heat exchanger fall below the set target temperature value. In other words, exceeding the continuous speed is temporarily permitted, wherein a second limit speed, which is greater than the first limit speed, can be set, which must then also not be exceeded in the second heat pump operating mode. This allows the heating output level to be stabilized more quickly in the second heat pump operating mode. While the first limit speed is usually set for acoustic reasons, operation at a higher speed is temporarily possible without technical problems regarding operational safety.

[0013] The reduction of the temperature of the coolant can be achieved by various measures. The temperature of the coolant can be reduced by switching off a heat source in the coolant line, in particular an electric auxiliary heater or a vehicle component, and / or by switching on a heat sink in the coolant line, for example another external heat exchanger which is integrated in the coolant circuit. The heat sources / heat sinks can also be partially switched on / off, e.g. by changing the power supply or ventilation. Alternatively or additionally, coolant may be introduced from a cold reservoir and / or the coolant line may be short-circuited or disconnected from an associated coolant circuit.

[0014] Because the oil flushing only needs to be activated temporarily, it is typically designed to automatically switch back to the first heat pump operating mode after a set period of time, increasing the temperature of the coolant in the coolant line upstream of the chiller again. It is possible to optionally check whether the load conditions have changed. The temperature of the coolant is then increased to the value that existed when switching to the second heat pump operating mode if no load change was detected. Alternatively, the coolant temperature can be increased to another value if a load change is detected. This other value typically corresponds to the target value that would have existed without switching to the second heat pump operating mode.

[0015] For automotive applications in typical configurations, there are advantageously specific time periods for operation in the second heat pump operating mode. It can typically be provided that after a period of time of 5 to 120 seconds, preferably 30 to 90 seconds, the system automatically switches back to the first heat pump operating mode, the beginning of the period of time being calculated from the time at which the reduction by the predetermined temperature amount is reached. Alternatively or additionally, a repetitive switching pattern can be provided, i.e. switching back and forth several times. The system inertia of the coolant circuit should be kept as small as possible, e.g. by forming a small short-circuit loop, and the time periods mentioned above may need to be selected differently.

[0016] For practical implementation, it can further be provided that a temporal sequence of mass flow indication values is recorded, from which mass flow values of the refrigerant through the external heat exchanger can be derived, and the detection of an oil flushing condition takes place depending on the temporal sequence of the mass flow indication values. The mass flow indication values can in themselves be any data that can be detected and from which information on the mass flow in the parallel path with the external heat exchanger can be provided. The detection does not have to be carried out from the parallel path with the external heat exchanger itself. Since it is technically difficult to measure the mass flow with sufficient accuracy, in particular temperature measurements, the opening degrees of the expansion valves and / or their ratios can be used. Through system tests, the corresponding switching criteria can be calibrated in advance and stored in the control system.

[0017] According to one embodiment, a counter is incremented when a mass flow indication value greater than zero and less than a set mass flow limit is detected. Conversely, the counter is decremented if a mass flow indication value greater than the set mass flow limit is detected. An oil flush condition is detected when the counter has reached a set counter threshold. Furthermore, the counter is reset to zero when switching back to the first heat pump operating mode. Alternatively, it can also be set to a different value if, for example, the second heat pump operating mode has been interrupted for other reasons not described here.

[0018] The predetermined temperature amount for lowering the temperature of the coolant is typically in a range of 4K to 15K, preferably in a range of 5K to 10K. These values have proven in tests to be a good compromise between, on the one hand, rapid oil flushing and, on the other hand, an acceptable influence on heating performance.

[0019] Different basic types of control strategy of the first heat pump operating mode are possible. The selected basic type of control strategy can also be retained in the second heat pump mode. From the prior art, depending on the specific hardware configuration of the refrigerant circuit, for example, so-called superheating or subcooling regulations are known, in which a temperature signal at the output of a heat exchanger is used to control an expansion element associated with the heat exchanger in such a way that it is regulated to a corresponding target value of this temperature signal. Alternatively, some other control strategies are also known, among which regulations on a target pressure in the high pressure range (e.g. when using the supercritical refrigerant R744) or combinations of the previously mentioned control strategies are particularly noteworthy.

[0020] The invention will be explained in greater detail below in the context of exemplary embodiments, with reference to the figures.Fig. 1 schematically shows a refrigerant circuit to which a method according to the present disclosure is applicable,Fig. 2 schematically shows a coolant circuit corresponding to Fig. 1,Fig. 3 schematically shows an embodiment variant of refrigerant circuits according to Fig. 1 in log-pH representation,Fig. 4 schematically shows an embodiment variant of refrigerant circuits according to Fig. 1 in log-pH representation,Fig. 5 schematically shows an embodiment variant of refrigerant circuits according to Fig. 1 in log-pH representation,Fig. 6 schematically shows a flow chart of a method for operating a refrigerant circuit with an oil flushing function according to an exemplary embodiment of the present disclosure, andFig. 7 schematically shows the time course of some operating parameters relevant for the regulation when carrying out an oil flush according to the exemplary embodiment of the present disclosure.

[0021] Fig. 1 shows a refrigerant circuit 10 to which the operating method described in more detail below with reference to Figs. 6 and 7 can be generically applied. The method is designed in particular for refrigerant circuits 10 in motor vehicles, without being limited thereto.

[0022] The refrigerant circuit 10 comprises, in a manner known per se, a compressor 11 and a heating heat exchanger 12 arranged downstream therefrom in the high-pressure region of the refrigerant circuit 10. The heating heat exchanger 12 is generally any heat sink which transfers heat from the refrigerant circuit 10 to a heat-exchanging medium, e.g. air or a coolant, in order to heat it.

[0023] In the following, it is assumed for the exemplary embodiments that the heating heat exchanger 12 operates as a condenser or gas cooler for a heat pump operating mode in order to heat vehicle components and / or a passenger cabin, wherein the heat transfer can take place directly to the air of the air conditioning system and / or indirectly via a fluid circuit (not shown). According to one embodiment variant, in a first heat pump operating mode it is provided in the regulation that the control principle is aligned to maintain a target temperature in the passenger cabin. This typically involves determining a temporal sequence of temperature values associated with the heating heat exchanger 12 and adjusting the operation of the compressor 11 so that the determined temperature values substantially correspond to a fixed target temperature value. The temperature values can be measured at any suitable location or otherwise estimated. They are conveniently measured, for example, at the air outlets in the passenger cabin, but are not limited to this. Alternatively, a different control principle may be provided in other design variants.

[0024] In the refrigerant circuit 10, downstream of the heating heat exchanger 12, there is a low-pressure section in which, in the first heat pump operating mode, an external heat exchanger 13 with an upstream first expansion valve 16A and a chiller 14 with an upstream second expansion valve 16B are arranged in parallel. The chiller 14 is in heat-exchanging contact with a first coolant line 21A of a coolant circuit 20. As is generally known to those skilled in the art, the external heat exchanger 13 can also be switchable to the high pressure side in cooling operating modes. In this case, an evaporator 15 with an upstream expansion valve 16C is typically operated in the low-pressure section parallel to the chiller 14 in order to provide cool air for the passenger cabin. At least in the first (and further below also in the second) heat pump operating mode, the evaporator 15 is not required and is deactivated in one or another known manner.

[0025] An accumulator 18 is provided for temporarily storing and mixing refrigerant. Typically, a number of other components such as valves and sensors are provided, as is familiar to those skilled in the art. The method for operating the refrigerant circuit 10 is regulated by a control unit 30 depending on various parameters relevant to the operation. Alternative configurations may have different circuits and components. Instead of the accumulator 18, for example, a receiver 17 can be provided, as shown with reference to Fig. 3.

[0026] Fig. 2 schematically shows an example of a coolant circuit 20 which is suitable for heat exchange with the chiller 14 in connection with the present disclosure. The coolant circuit 20 has several coolant lines 21A to 21D. In a first circuit, the chiller 14 is connected in a loop to a controllable electrical auxiliary heater, also often referred to as PTC element 23, and the pump 22 in a first coolant line 21A and at least one electrical component, e.g. a battery 24, in a second coolant line 21B. A third coolant line 21C is provided to bypass the battery 24 in order to be able to operate the chiller in a short circuit if necessary. Alternatively or additionally, a fourth coolant line 21D is provided, which is connected to a cold reservoir 25 and / or a radiator 26 as a low-temperature external heat exchanger. Furthermore, additional coolant lines, valves and pumps may be provided (not shown). The features of the coolant circuit 20 are designed to temporarily lower the temperature of the coolant in the first coolant line 21A upstream of the chiller 14, as will be explained in more detail below with reference to the method according to the present disclosure.

[0027] Figs. 3 to 5 schematically show embodiment variants of refrigerant circuits 10, 10A, 10B according to Fig. 1 in log-pH representation. For this purpose, possible basic control strategies for the first heat pump operating mode are described as examples, as they can be used in connection with the present invention. Alternatively, different control strategies can be used.

[0028] Fig. 3 shows a log-pH diagram for a receiver-based refrigerant circuit 10 in which a receiver 17 is arranged downstream of the heating heat exchanger 12 in the high pressure region. The typical control strategy is based on controlling the expansion valves 16A, 16B to a fixed superheat value SH1, SH2 at the outlet of the external heat exchanger 13 and the chiller 14. In other words, the superheat values SH1, SH2 are determined (e.g. via a calculation from measured pressure and temperature values), transmitted to the control unit 30, the respective opening degrees of the expansion valves 16A, 16B are calculated in the control unit 30 by means of a control characteristic and the corresponding control signals are sent to the expansion valves 16A, 16B.

[0029] Fig. 4 shows a log-pH diagram for an accumulator-based refrigerant circuit 10A in which an accumulator 18 is arranged downstream of the external heat exchanger 13 and the chiller 14 in the low-pressure region. The typical control strategy is based on controlling the expansion valve 16A to a fixed subcooling value SC and the expansion valve 16B to a superheating value SH3 at the output of the accumulator 18, the individual steps being controlled via the control unit 30 in a manner analogous to that described with reference to Fig. 3.

[0030] Fig. 5 shows a log-pH diagram for an accumulator-based supercritical refrigerant circuit 10B. Since no phase transition takes place in the high pressure range during supercritical operation, instead of the subcooling control described with reference to Fig. 4, the expansion valve 16A is controlled via a fixed pressure value P in the high pressure region downstream of the heating heat exchanger 12. The control of the expansion valve 16B can be carried out according to the embodiment shown in Fig. 4.

[0031] The method for operating a refrigerant circuit with an oil flushing function will now be explained in more detail using an exemplary embodiment with reference to Fig. 6. Fig. 6 shows a schematic flow chart of method steps. The execution is supported in particular by the control unit 30.

[0032] When the system starts in step 100, the refrigerant circuit 10 is started normally in a manner known per se and is switched to a first heat pump operating mode in step 110 based on the detected ambient conditions. Heat is absorbed into the refrigerant circuit 10 via the external heat exchanger 13 from the ambient air and via the chiller 14 arranged in a parallel line from the coolant circuit20. The chiller 14, in contact with the coolant circuit20, can absorb heat into the refrigerant circuit 10 either via electrically generated heat from the PTC element 23 or via waste heat from electrical components such as the battery 24. Notwithstanding the oil flushing function described below, further measures can be temporarily provided in this first heat pump operating mode, such as a short-term start-up with a so-called "boost function", defrosting of the external heat exchanger 13 or other situation-specific operating adjustments.

[0033] In step 120, in order to detect possible oil settling in the external heat exchanger 13, a mass flow indication value is first recorded, from which the mass flow value of the refrigerant through the external heat exchanger 13 can be derived. The mass flow indication value can be any physical-technical parameter relating to the refrigerant circuit 10 which provides a sufficiently accurate and reliable indication of the actual mass flow of the refrigerant through the external heat exchanger 13. The mass flow indication value can, for example, have been determined taking into account the compressor speed, the ambient temperature, the opening degrees of the expansion valves 16A, 16B and other ascertainable system parameters such as the pressure and the temperature in the refrigerant circuit 10 in previous system tests, simulations, calculations or a combination thereof and is stored accordingly in the control unit 30.

[0034] For quantitative estimation of oil settling, a counter C is provided, which starts at zero during start-up and is increased whenever the recorded mass flow indication value is smaller than a fixed mass flow limit value MG. In the trivial case that no mass flow through the external heat exchanger 13 is to be assumed (if, for example, the electronically switchable expansion valve 16A is completely closed), the counter C remains unchanged. If the mass flow indication value is greater than the set mass flow limit value MG, the counter is decremented.

[0035] Typically, a maximum permissible first limit speed D1 of the compressor 11 is set for the first heat pump operating mode. When switching to the second heat pump operating mode in step 120, this limit speed D1 is temporarily replaced by a higher limit speed D2 in order to compensate for the reduction in temperature of the coolant at the chiller 14 due to a higher output at the compressor 11. In other words, the speed D of the compressor 11 can then be increased beyond the first limit speed D1 if the temperature values associated with the heating heat exchanger 12 (e.g. measured at the air outlets) fall below the set target temperature value in the cabin. The second limit speed D2 > D1 is selected so that safe operation is ensured for the temporary period of oil flushing.

[0036] In an iteration loop with step 130, it is checked whether the counter C is still below the counter threshold C1 or has already reached it. If the counter C is still below the counter threshold C1, step 120 is repeated and thus a temporal sequence of mass flow indication values is recorded, wherein the counter C is always counted up or down accordingly or kept constant. If the counter threshold C1 is reached during the check in step 130, the oil flushing condition is detected and the refrigerant circuit 10 is temporarily switched to the second heat pump operating mode for simultaneously performing an oil flushing operation.

[0037] In a step 140, measures are initiated to reduce the temperature of the coolant in the coolant line 21A upstream of the chiller 14 to a predetermined temperature amount ΔT, e.g. 8K. Depending on the design variant of the coolant circuit20, the reduction of the temperature of the coolant may include one or more of the following actions. The PTC element 23 can be switched off or its heat output can be reduced. The coolant line 21A can be separated from the coolant line 21B and operated, for example, in a short-circuit loop with the coolant line 21C or together with the coolant line 21D in order to tap into a comparatively cooler coolant source via an optionally installed cold reservoir 25 and / or via an optionally installed radiator 26 or to release additional heat to the environment.

[0038] In a step 150 it is then checked whether the temperature has been reduced by the desired predetermined temperature amount ΔT. If this is not yet the case, the measures in step 140 are continued and checked again at regular intervals in step 150.

[0039] Once the desired temperature reduction has been achieved, a timer is started in step 160 and after a predefined period of time, e.g. 60 seconds, the system automatically switches back to the first heat pump operating mode. After the timer has expired, the measures for lowering the temperature in the coolant line 21A are terminated in a step 170, wherein the temperature of the coolant is increased again. This step also checks whether the load conditions have changed in the meantime, wherein the temperature of the coolant is increased to the value that existed when switching to the second heat pump operating mode if no load change was detected. Alternatively, the coolant temperature can be increased to another value if a load change was detected.

[0040] When switching back to the first heat pump operating mode (step 110), the counter C is reset to zero and the second limit speed D2 of the compressor 11 is reduced to the first limit speed D1.

[0041] With reference to Fig. 7, the temporal course of some operating parameters relevant for the regulation during the implementation of an oil flush is shown schematically and by way of example. The three graphs shown one above the other show, from top to bottom, i) the temperature of the coolant at the inlet of the chiller 14, ii) the speed D of the compressor 11 and iii) the course of the counter C during the implementation of the method according to the present disclosure from steps 110 to 170.

[0042] The counter C is continuously increased due to low mass flows of the refrigerant through the external heat exchanger 13, while the temperature T at the inlet of the chiller 14 and the speed D remain virtually unchanged. When the counter threshold C1 is reached, the oil flushing function is started and the temperature at the chiller 14 is initially reduced. During a first time interval Δt1, the temperature drops continuously, while the speed D of the compressor 11 reacts with a small time offset to the now cooler low-pressure range and thus the reduced heating output. The compressor speed increases. When the desired predetermined temperature amount ΔT is reached, a timer is started and after a second time interval Δt2 (here, for example, 60s) has elapsed, the counter C is set to zero, the refrigerant at the inlet of the chiller 14 is increased and the compressor speed D is reduced because the limit speed D2 of the compressor 11 has been reduced to D1.

[0043] The method can be modified in many ways and adapted to the specific connections of the refrigerant circuit. Thus, instead of one mass flow limit value MG, several mass flow limit values MG1 to MGn can be provided, wherein the counter C is modified to varying degrees depending on the corridor in which the mass flow indication value lies. For example, it is incremented by two in a first corridor 0 < M1 < M < M2 and by one in a second corridor M2 < M < M3. In a third corridor M3 < M < M4, the counter C is initially not changed and only counts down again when M > M4. If necessary, two or more temperature values ΔTx can also be provided for reducing the coolant temperature, which form temperature corridors within which the temperature of the coolant can move in the second heat pump operation. When the first temperature limit value is reached, the timer can be triggered while the temperature reduction continues.

Claims

1. A method for operating a refrigerant circuit (10) with a compressor (11), a heating heat exchanger (12) arranged downstream of the compressor and a low-pressure section arranged downstream of the heating heat exchanger, in which, in a first heat pump operating mode, an external heat exchanger (13) with an upstream first expansion valve (16A) and a chiller (14) with an upstream second expansion valve (16B) are arranged in parallel, wherein the chiller (14) is in heat-exchanging contact with a coolant line (20, 21A), the method comprising: detecting whether an oil flushing condition is met; and temporarily switching the refrigerant circuit (10) to a second heat pump operating mode for simultaneously performing an oil flushing operation upon detecting that the oil flushing condition is met, wherein a temperature of a coolant in the coolant line (20, 21A) upstream of the chiller (14) is reduced by a predetermined temperature amount (ΔT) in the second heat pump operating mode.

2. The method according to claim 1, further comprising: determining a temporal sequence of temperature values associated with the heating heat exchanger (12) in the first heat pump operating mode for regulation; and adapting an operation of the compressor (11) so that the temperature values substantially correspond to a fixed target temperature value.

3. The method according to claim 1 or 2, further comprising increasing a speed (D) of the compressor (11) in the second heat pump operating mode beyond a maximum admissible first limit speed (D1) of the compressor (11) when temperature values associated with the heating heat exchanger (12) fall below a target temperature value, the maximum admissible first limit speed (D1) of the compressor being set for the first heat pump operating mode.

4. The method according to any one of claims 1 to 3, wherein the temperature of the coolant is reduced by at least one of: switching off a heat source (23, 24) in the coolant line (20), in particular an electric heater (23); connecting a heat sink in the coolant line; introducing a coolant from a cold reservoir (25); or disconnecting the coolant line (21A, 21C), in particular by short-circuiting the coolant line, from an associated coolant circuit (20).

5. The method according to any one of claims 1 to 4, further comprising automatically switching the refrigerant circuit back to the first heat pump operating mode after a fixed period of time (Δt1, Δt2) has elapsed, wherein the temperature of the coolant in the coolant line (20, 21A) upstream of the chiller (14) is increased again.

6. The method according to claim 5, further comprising: determining whether load conditions have changed; and increasing the temperature of the coolant to a value that existed when switching to the second heat pump operating mode upon detecting no load change; or increasing the temperature of the coolant to a different value upon detecting a load change.

7. The method according to claim 5 or 6, further comprising switching the refrigerant circuit back to the first heat pump operating mode after a period of time (Δt2) of 5 to 120 seconds, wherein a beginning of the period of time (Δt2) is calculated from a time at which the temperature of the coolant is reduced by the predetermined temperature amount (ΔT).

8. The method according to claim 5 or 6, further comprising switching the refrigerant circuit back to the first heat pump operating mode after a period of time (Δt2) of 30 to 90 seconds, wherein a beginning of the period of time (Δt2) is calculated from a time at which the temperature of the coolant is reduced by the predetermined temperature amount (ΔT).

9. The method according to any one of claims 1 to 8, further comprising detecting a temporal sequence of mass flow indication values, the mass flow indication values being values from which mass flow values of a refrigerant through the external heat exchanger (13) is derived, wherein the detecting of the oil flushing condition takes place depending on the temporal sequence of the mass flow indication values.

10. The method according to claim 9, wherein the detecting of the oil flushing condition includes detecting that the oil flushing condition is met when a counter (C) has reached a set counter threshold, wherein the counter (C) is incremented when a mass flow indication value that is greater than zero and less than a set mass flow limit is detected, and the counter is decremented when a mass flow indication value that is greater than the set mass flow limit is detected.

11. The method according to claim 10, wherein the counter (C) is reset to zero upon switching the refrigerant circuit back to the first heat pump operating mode.

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