Device and method for controlling an orifice of a valve in an HVAC system

Abstract

For controlling an orifice of a valve (10) in an HVAC system (100) to regulate the flow (Φ) of a fluid through a thermal energy exchanger (2) of the HVAC system (100) and adjust the energy transfer rate ({dot over (Q)}) of the thermal energy exchanger (2) in response to a demand value (d), the orifice of the valve (10) is controlled in a first mode of operation where the flow (Φ) of the fluid through the thermal energy exchanger (2) is regulated within efficiency constraints on the energy transfer rate ({dot over (Q)}) with respect to an efficiency threshold value. Upon receiving an override signal (OS), the orifice of the valve (10) is controlled in a second mode of operation where the flow (Φ) of the fluid through the thermal energy exchanger (2) is not regulated with respect to the first efficiency threshold value.

Claims

1. A method of controlling an orifice of a valve in an HVAC system to regulate the flow of a fluid through a thermal energy exchanger of the HVAC system and adjust an energy transfer rate of the thermal energy exchanger in response to a demand value, the method comprising: controlling the orifice of the valve in a first mode of operation where the flow of the fluid through the thermal energy exchanger is regulated within efficiency constraints on the energy transfer rate with respect to a first efficiency threshold value, the efficiency constraints being determined by comparing a temperature difference, between a supply temperature of the fluid entering the thermal energy exchanger and a return temperature of the fluid exiting the thermal energy exchanger, to the first efficiency threshold value, adjusting the orifice of the valve in order to attain the demand value, if the temperature difference is at or above the first efficiency threshold value, and adjusting the orifice of the valve in order to maintain a current flow of the fluid through the thermal energy exchanger, if the temperature difference is below the first efficiency threshold value; generating an override signal when the first mode of operation does not obtain the demand value within a first duration of time; and upon receiving the override signal, controlling the orifice of the valve in a second mode of operation where the flow of the fluid through the thermal energy exchanger is regulated within the efficiency constraints on the energy transfer rate with respect to a second efficiency threshold value, the second efficiency threshold value being lower than the first efficiency threshold value, adjusting the orifice of the valve in order to attain the demand value, if the temperature difference is at or above the second efficiency threshold value, and adjusting the orifice of the valve in order to maintain a current flow of the fluid through the thermal energy exchanger, if the temperature difference is below the second efficiency threshold value.

2. The method of claim 1, wherein the method further comprises adjusting efficiency threshold values for the efficiency constraint, using at least one of: a flow rate of a fluid flowing on a primary side of the thermal energy exchanger, a flow speed of the fluid on the primary side of the thermal energy exchanger, a flow rate of a fluid flowing on a secondary side of the thermal energy exchanger, a flow speed of the fluid on the secondary side of the thermal energy exchanger, a temperature of the fluid on the secondary side of the thermal energy exchanger, a humidity of the fluid on the secondary side of the thermal energy exchanger, and an entropy of the fluid on the secondary side of the thermal energy exchanger.

3. The method of claim 1, wherein the method comprises cancelling the override signal, if controlling the orifice of the valve in the second mode of operation has attained the demand value for a determined second duration of time.

4. The method of claim 1, wherein the override signal is generated by an electronic circuit of a thermostat unit comparing a current temperature to a target temperature defined by the demand value.

5. The method of claim 1, wherein the override signal is indicative of a current flow of the fluid through the thermal energy exchanger below a minimum flow rate threshold, said minimum flow rate threshold being dependent on a temperature of the fluid; and controlling the orifice of the valve in the second mode of operation comprises adjusting the orifice of the valve to increase the flow of the fluid through the thermal energy exchanger.

6. The method of claim 1, wherein the demand value comprises at least one of: a target temperature, a target orifice of the valve (10), a control value for the valve, a target flow rate, a target energy transfer rate, and a control value for an actuator actuating the valve.

7. A control system for controlling an orifice of a valve in an HVAC system to regulate the flow of a fluid through a thermal energy exchanger of the HVAC system and adjust an energy transfer rate of the thermal energy exchanger in response to a demand value, the control system comprising an electronic circuit configured to: control the orifice of the valve in a first mode of operation where the flow of the fluid through the thermal energy exchanger is regulated within efficiency constraints on the energy transfer rate with respect to a first efficiency threshold value, the efficiency constraints being determined by comparing a temperature difference, between a supply temperature of the fluid entering the thermal energy exchanger and a return temperature of the fluid exiting the thermal energy exchanger, to the first efficiency threshold value, adjusting the orifice of the valve in order to attain the demand value, if the temperature difference is at or above the first efficiency threshold value, and adjusting the orifice of the valve in order to maintain a current flow of the fluid through the thermal energy exchanger, if the temperature difference is below the first efficiency threshold value; generate an override signal when the first mode of operation does not obtain the demand value within a first duration of time; and upon receiving the override signal, control the orifice of the valve in a second mode of operation where the flow of the fluid through the thermal energy exchanger is regulated within the efficiency constraints on the energy transfer rate with respect to a second efficiency threshold value, the second efficiency threshold value being lower than the first efficiency threshold value, adjusting the orifice of the valve in order to attain the demand value, if the temperature difference is at or above the second efficiency threshold value, and adjusting the orifice of the valve in order to maintain a current flow of the fluid through the thermal energy exchanger, if the temperature difference is below the second efficiency threshold value.

8. The control system of claim 7, wherein the electronic circuit is configured to adjust efficiency threshold values for the efficiency constraint, using at least one of: a flow rate of a fluid flowing on a primary side of the thermal energy exchanger, a flow speed of the fluid on the primary side of the thermal energy exchanger, a flow rate of a fluid flowing on a secondary side of the thermal energy exchanger, a flow speed of the fluid on the secondary side of the thermal energy exchanger, a temperature of the fluid on the secondary side of the thermal energy exchanger, a humidity of the fluid on the secondary side of the thermal energy exchanger, and an entropy of the fluid on the secondary side of the thermal energy exchanger.

9. The control system of claim 7, wherein the electronic circuit is configured to cancel the override signal, if controlling the orifice of the valve in the second mode of operation has attained the demand value for a determined second duration of time.

10. The control system of claim 7, wherein the electronic circuit is implemented in a thermostat unit and configured to generate the override signal by comparing a current temperature to a target temperature defined by the demand value.

11. The control system of claim 7, wherein the electronic circuit is configured to generate the override signal, if a current flow of the fluid through the thermal energy exchanger is below a minimum flow rate threshold, said minimum flow rate threshold being dependent on a temperature of the fluid; and control the orifice of the valve in the second mode of operation by adjusting the orifice of the valve to increase the flow of the fluid through the thermal energy exchanger.

12. The control system of claim 7, wherein the electronic circuit is configured to process a demand value comprising at least one of: a target temperature, a target orifice of the valve, a control value for the valve, a target flow rate, a target energy transfer rate, and a control value for an actuator actuating the valve.

13. A non-transitory computer readable-medium having stored thereon computer program code configured to control a processor of a control system, for controlling an orifice of a valve in an HVAC system to regulate the flow of a fluid through a thermal energy exchanger of the HVAC system and adjust an energy transfer rate of the thermal energy exchanger, in response to a demand value, such that the processor executes the steps of: controlling the orifice of the valve in a first mode of operation where the flow of the fluid through the thermal energy exchanger is regulated within efficiency constraints on the energy transfer rate with respect to a first efficiency threshold value, the efficiency constraints being determined by comparing a temperature difference, between a supply temperature of the fluid entering the thermal energy exchanger and a return temperature of the fluid exiting the thermal energy exchanger, to the first efficiency threshold value, adjusting the orifice of the valve in order to attain the demand value, if the temperature difference is at or above the first efficiency threshold value, and adjusting the orifice of the valve in order to maintain a current flow of the fluid through the thermal energy exchanger, if the temperature difference is below the first efficiency threshold value; generating an override signal when the first mode of operation does not obtain the demand value within a first duration of time; and upon receiving the override signal, controlling the orifice of the valve in a second mode of operation where the flow of the fluid through the thermal energy exchanger is regulated within the efficiency constraints on the energy transfer rate with respect to a second efficiency threshold value, the second efficiency threshold value being lower than the first efficiency threshold value, adjusting the orifice of the valve in order to attain the demand value, if the temperature difference is at or above the second efficiency threshold value, and adjusting the orifice of the valve in order to maintain a current flow of the fluid through the thermal energy exchanger, if the temperature difference is below the second efficiency threshold value.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The present invention will be explained in more detail, by way of example, with reference to the drawings in which:

(2) FIG. 1: shows a block diagram illustrating schematically an HVAC system with a fluid circuit comprising a fluid driver, a valve, and a thermal energy exchanger, and a control system for controlling the orifice (opening) of the valve to regulate the flow of a fluid through the thermal energy exchanger and adjust the energy transfer rate of the thermal energy exchanger.

(3) FIG. 2: shows a state diagram illustrating two modes of operation for controlling the orifice of a valve to regulate the flow of a fluid through the thermal energy exchanger and adjust the energy transfer rate of the thermal energy exchanger.

(4) FIG. 3: shows a flow diagram illustrating an exemplary sequence of steps for adjusting the orifice of a valve in a first mode of operation for regulating the flow of a fluid through a thermal energy exchanger.

(5) FIG. 4: shows a flow diagram illustrating an exemplary sequence of steps for adjusting the orifice of a valve in a second mode of operation for regulating the flow of a fluid through a thermal energy exchanger.

(6) FIG. 5: shows a flow diagram illustrating an exemplary sequence of steps for controlling the orifice (opening) of a valve to regulate the flow of a fluid through a thermal energy exchanger and adjust the energy transfer rate of the thermal energy exchanger, in response to a demand value.

(7) FIG. 6: shows a flow diagram illustrating an exemplary sequence of steps for controlling the orifice (opening) of a valve, in response to a demand value and depending on a temperature of a fluid, for regulating the flow of the fluid through a thermal energy exchanger.

(8) FIG. 7: shows a graph illustrating an example of a temperature-difference-per-flow curve with different efficiency threshold values used in controlling the orifice (opening or position) of a valve to regulate the flow of a fluid through a thermal energy exchanger and adjust the energy transfer rate of the thermal energy exchanger.

(9) FIG. 8: shows a flow diagram illustrating an exemplary sequence of steps for determining fluid and/or flow characteristics on a primary and/or secondary side of a thermal energy exchanger, and adjusting efficiency threshold values used in controlling the orifice (opening) of a valve to regulate the flow of a fluid through the thermal energy exchanger and adjust the energy transfer rate of the thermal energy exchanger.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

(10) In FIG. 1, reference numeral 100 refers to an HVAC system with a fluid circuit 101 comprising a fluid driver 3, e.g. a pump, a valve 10, a thermal energy exchanger 2, e.g. a heat exchanger for heating or cooling a room, which are interconnected by way of pipes. Depending on the embodiment, the valve is rotary valve, e.g. a ball valve, or a linear valve, e.g. a piston or stem valve. The valve 10 is provided with an actuator 11, e.g. an electrical motor, for opening and closing the valve 10 and thus controlling the orifice of the valve 10 and the flow through the fluid circuit 101, using different positions (or sizes of orifice) of the valve 10. Further, the fluid driver 3 may further vary the flow through the fluid circuit 101. As illustrated schematically, the HVAC system 100 further comprises an optional building control system 4 and/or a thermostat 5 unit with an electronic circuit (processor) 51. One skilled in the art will understand that the depiction of the HVAC system 100 is very simplified and that the HVAC system 100 may include a plurality of fluid circuits 101 (zones), having in each case one or more valves 10, actuators 11, and thermal energy exchangers 2. For example, the fluid is a liquid heat transportation medium such as water and the HVAC system 100 further includes a heater for heating the liquid or a chiller for cooling the liquid.

(11) As illustrated schematically in FIG. 1, the thermal energy exchanger 2 is provided with two temperature sensors 21, 22 arranged at the inlet of the thermal energy exchanger 2, for measuring the supply temperature T.sub.in of the fluid entering the thermal energy exchanger 2, and at the exit of the thermal energy exchanger 2, for measuring the return temperature T.sub.out of the fluid exiting the thermal energy exchanger 2. One skilled in the art will understand that the temperature sensors 21, 22 may be arranged in different locations, for example, the temperature sensor 21 for measuring the supply temperature T.sub.in may be arranged at the valve 10 or at the exit of a heater.

(12) The fluid circuit 101 further comprises a flow sensor 12 for measuring the flow Φ, i.e. the rate of fluid flow, through the valve 10 or fluid circuit 101, respectively. Depending on the embodiment, the flow sensor 12 is arranged in or at the valve 10, or in or at a pipe section connected to the valve 10. For example, the flow sensor 12 is an ultrasonic sensor or a heat transport sensor.

(13) In FIG. 1, reference numeral 1 refers to a control system for controlling the valve 10 or the actuator 11, respectively, to adjust the orifice (opening or position) of the valve 10. Accordingly, the control system 1 regulates the flow Φ, i.e. the rate of fluid flow, through the valve 10 and, thus, through the thermal energy exchanger 2. Consequently, the control system 1 regulates the amount of thermal energy exchanged by the thermal energy exchanger 2 with its environment and thus the energy transfer rate {dot over (Q)} of the thermal energy exchanger 2. Depending on the embodiment, the control system 1 is arranged at the valve 10, e.g. as an integral part of the valve 10 or attached to the valve 10, or the control system 1 is arranged at a pipe section 12 connected to the valve 10. One skilled in the art will understand that the control system 1 may also be arranged remotely from the valve 10 and actuator 11, communicatively coupled by way of a wired or wireless communication link, e.g. through a wired communication bus, a Local Area Network (LAN), or a Wireless Local Area Network (WLAN).

(14) The control system 1 comprises an electronic circuit 14, e.g. an operational processor (microprocessor) with program and data memory or an application specific integrated circuit (ASIC). The control system 1 comprises computer program code configured to direct the processor 14 or another electronic circuit of the control system 1 to perform various functions, as will be explained later in more detail. The computer program code is stored on a non-transient computer-readable medium which is connected to the control system 1 in a fixed or removable fashion. One skilled in the art will understand, however, that in alternative embodiments, functional modules configured to perform said functions can be implemented partly or fully by way of hardware components. Moreover, in alternative embodiments, the processor 14 is arranged in different components of the HVAC system 100, e.g. in the actuator 11, the flow sensor 12, the thermostat 5, or the building control system 4.

(15) As is illustrated in FIG. 1, the flow sensor 12 is connected to the control system 1 for providing timely or current-time measurement values of the flow (rate) Φ to the control system 1. Furthermore, the control system 1 is connected to the actuator 11 for supplying control signals z to the actuator 11 for controlling the actuator 11 to open and/or close the valve 10, i.e. to control the electric motor of the actuator 11 to actuate a regulating member of the valve 10 to adjust the orifice (opening or position) of the valve 10.

(16) Moreover, the temperature sensors 21, 22 of the thermal energy exchanger 2 are connected to the control system 1 for providing to the control system 1 timely or current-time measurement values of the supply temperature T.sub.in and the return temperature T.sub.out of the fluid entering or exiting the thermal energy exchanger 2, respectively.

(17) Depending on the embodiment, the control system 1 is further connected to the building control system 4 and/or the thermostat 5 unit for receiving from the building control system 4 or thermostat 5 control signals and/or demand values, e.g. user or system settings for a target temperature, a target orifice of the valve, a control value for the valve, a target flow rate, a target energy transfer rate, and/or a control value for an actuator or its electric motor actuating the valve.

(18) In an embodiment, the HVAC system 100 further comprises sensors which are arranged in the space where the thermal energy exchanger 2 is located, in the primary fluid circuit 101 (primary side) of the thermal energy exchanger 2, and/or in a secondary circuit 23 (secondary side) of the thermal energy exchanger 2, for example a sensor for measuring the speed of flow of the fluid on the primary side 101 of the thermal energy exchanger 2, a sensor for measuring the speed of flow of a fluid on the secondary side 23 of the thermal energy exchanger 2, a flow sensor for measuring the flow rate of a fluid flowing on the secondary side of the thermal energy exchanger 2, e.g. an air flow sensor, for measuring the air flow across the thermal energy exchanger 2, a temperature sensor for measuring the temperature of the fluid on the secondary side 23 of the thermal energy exchanger 2, e.g. a temperature sensor for measuring the air temperature in the space around the heat exchanger 2, a sensor for measuring the humidity of the fluid on the secondary side 23 of the thermal energy exchanger 2, e.g. a humidity sensor, for measuring the humidity of the air in the room where the thermal energy exchanger 2 is arranged, and/or a sensor for measuring the entropy of the fluid on the secondary side 23 of the thermal energy exchanger 2.

(19) In the following paragraphs, described with reference to FIGS. 2-8 are possible sequences of steps performed by the electronic circuit 14 (processor) of the control system 1 for controlling the orifice (opening or position) of the valve 10 to regulate the flow Φ through the thermal energy exchanger 2.

(20) As illustrated in the state diagram of FIG. 2, the control system 1 or its electronic circuit 14 (processor), respectively, has two modes of operation M1, M2 for controlling the orifice of the valve 10 to regulate the flow Φ of fluid through the thermal energy exchanger 2 and thereby adjust the energy transfer rate {dot over (Q)} of the thermal energy exchanger 2. The electronic circuit 14 switches from the first mode of operation M1 to the second mode of operation M2 when it receives or generates an override signal OS in step S100. The electronic circuit 14 switches from the second mode of operation M2 to the first mode of operation M1 when the override signal OS is cancelled in step S200. One skilled in the art will understand, that depending on the embodiment the override signal OS is implemented as an actual control signal transmitted through a wired or wireless connection or as a control value in a memory, for example.

(21) In the first mode of operation M1, the electronic circuit 14 regulates the flow Φ of the fluid through the thermal energy exchanger 2 within efficiency constraints on the energy transfer rate {dot over (Q)} with respect to a first efficiency threshold FT.sub.1 (see FIG. 8).

(22) Accordingly, as illustrated in FIG. 3, in step S1, the electronic circuit 14, respectively, adjusts the orifice of the valve 10 in the first mode of operation M1.

(23) In step S11, the electronic circuit 14 determines the temperature difference ΔT=T.sub.in−T.sub.out, between the supply temperature T.sub.in of the fluid entering the thermal energy exchanger 2, as measured by temperature sensor 21, and the return temperature T.sub.out of the fluid exiting the thermal energy exchanger 2, as measured by temperature sensor 22.

(24) In step S21, the electronic circuit 14 compares the temperature difference ΔT to the first efficiency threshold FT.sub.1.

(25) In step S13, if the temperature difference ΔT is greater than or equal to the first efficiency threshold FT.sub.1, the electronic circuit 14 directs the actuator 11 or its electric motor, respectively, to adjust the orifice of the valve 10 to meet a demand value d.

(26) In step S14, if the temperature difference ΔT is below the first efficiency threshold FT.sub.1, the electronic circuit 14 directs the actuator 11 or its electric motor, respectively, to adjust the orifice of the valve 10 to maintain the current flow Φ of fluid, as measured by the flow sensor 12.

(27) In the second mode of operation M2, the electronic circuit 14 does not regulate the flow Φ of the fluid through the thermal energy exchanger 2 with respect to the first efficiency threshold FT.sub.1. Two alternative or combined embodiments of regulating the flow Φ of fluid through the thermal energy exchanger 2 in the second mode of operation M2 will be described below with reference to FIGS. 4 and 6, respectively.

(28) In a first version of the second mode of operation M2, illustrated in FIG. 4, the electronic circuit 14 regulates the flow Φ of the fluid through the thermal energy exchanger 2 within efficiency constraints on the energy transfer rate {dot over (Q)} with respect to a second efficiency threshold FT.sub.2 (see FIG. 8). A value of zero for the second efficiency threshold FT.sub.2 corresponds to have no efficiency constraints on the energy transfer rate {dot over (Q)}.

(29) Accordingly, in step S2, the electronic circuit 14, respectively, adjusts the orifice of the valve 10 in the second mode of operation M2.

(30) In step S21, the electronic circuit 14 determines the temperature difference ΔT=T.sub.in−T.sub.out, between the supply temperature T.sub.in of the fluid entering the thermal energy exchanger 2, as measured by temperature sensor 21, and the return temperature T.sub.out of the fluid exiting the thermal energy exchanger 2, as measured by temperature sensor 22.

(31) In step S22, the electronic circuit 14 compares the temperature difference ΔT to the second efficiency threshold FT.sub.2.

(32) In step S23, if the temperature difference ΔT is greater than or equal to the second efficiency threshold FT.sub.2, the electronic circuit 14 directs the actuator 11 or its electric motor, respectively, to adjust the orifice of the valve 10 to meet a demand value d.

(33) In step S24, if the temperature difference ΔT is below the secondefficiency threshold FT.sub.2, the electronic circuit 14 directs the actuator 11 or its electric motor, respectively, to adjust the orifice of the valve 10 to maintain the current flow Φ of fluid, as measured by the flow sensor 12.

(34) In an alternative or additional (combined) second version of the second mode of operation M2, illustrated in FIG. 6, the electronic circuit 14 adjusts the valve 10 or its orifice, respectively, to increase the flow Φ of the fluid through the thermal energy exchanger 2 in step S2*. This second version of the second mode of operation M2 will be initiated by an override signal OS triggered by different conditions, as will be described later in more detail with reference to FIG. 6.

(35) FIG. 5 illustrates an exemplary sequence of steps for controlling the orifice (opening) of the valve 10 to regulate the flow Φ of fluid through the thermal energy exchanger 2 in response to a demand value d, for adjusting the energy transfer rate {dot over (Q)} of the thermal energy exchanger 2.

(36) In step S0, the electronic circuit 14 receives a demand value d, e.g. from the building control system 4 or thermostat 5 or another external unit.

(37) If the electronic circuit 14 is in the first mode of operation M1, as indicated schematically by step S10, the electronic circuit 14 adjusts the orifice of the valve 10 in step S1 in the first mode of operation M1, as described above with reference to FIG. 3.

(38) In step S15, it is checked whether the demand value d was met within a defined duration of time, depending on the demand value d, this may be a duration of a few minutes, e.g. ten to fifteen minutes, or a longer duration of more than fifteen minutes, e.g. twenty to thirty minutes. If the demand value d was met within the expected time, the demanded value d is maintained and processing continues in step S0.

(39) If the demand value d was not met within the expected time, an override signal OS is generated in step S100. Generating the override signal OS may further be conditioned on the setting of an override mode, e.g. per configuration or user activation, e.g. through a user interface of the thermostat 5 unit. Consequently, the electronic circuit 14 of the control system 1 continues processing in the second mode of operation M2 by adjusting the orifice of the valve 10 in step S2 in the second mode of operation M2, as described above with reference to FIG. 4.

(40) Depending on the embodiment and/or configuration, the checking of step S15 and/or the generating of the override signal OS in step S100 are executed by the electronic circuit 14 of the control system 1 or by the electronic circuit 51 (processor) of the thermostat 5 or of another external unit that initiated the respective demand. For example, the electronic circuit 51 of the thermostat 5 compares a current temperature, e.g. a current room temperature or a current temperature of supply air, to a target temperature defined by the demand value d for deciding whether or not to generate the override signal OS.

(41) If the electronic circuit 14 is in the second mode of operation M2, upon receiving the demand value d, as indicated schematically by step S20, the electronic circuit 14 adjusts the orifice of the valve 10 in step S2 in the second mode of operation M2, as described above with reference to FIG. 4.

(42) In step S25, the electronic circuit 14 checks whether the demand value d was met during a defined duration of time, depending on the demand value d, this may be a duration of one or more hours, e.g. one to two hours, or a longer duration of more than two hours, e.g. three to four hours. If the demand value d was not met for the defined duration of time, the electronic circuit 14 continues processing in step S2.

(43) If the demand value d was met for the defined duration of time or a defined maximum override time is exceeded, the electronic circuit 14 cancels the override signal OS in step S200. Consequently, the electronic circuit 14 of the control system 1 continues processing in the first mode of operation M1 by adjusting the orifice of the valve 10 in step S1 in the first mode of operation M1, as described above with reference to FIG. 3.

(44) FIG. 6 illustrates an exemplary sequence of steps for controlling the orifice (opening) of the valve 10 to regulate the flow Φ of fluid through the thermal energy exchanger 2 in response to a demand value d and depending on the temperature of the fluid, as measured by a temperature sensor 21, 22 arranged in the primary fluid circuit 101.

(45) In step S0, the electronic circuit 14 receives a demand value d, e.g. from the building control system 4 or thermostat 5 or another external unit.

(46) In step S3, the electronic circuit 14 determines the temperature of the fluid in the primary fluid circuit 101.

(47) In step S4, the electronic circuit 14 sets a minimum flow threshold depending on the temperature of the fluid in the primary fluid circuit 101. The minimum flow threshold is set depending on the type of fluid, such that the fluid does not freeze at the current temperature of the fluid in the primary fluid circuit 101.

(48) In step S5, the electronic circuit 14 checks whether the current flow rate Φ of the fluid, as measured by flow sensor 12, is below the minimum flow threshold. If the current flow rate Φ is not below the minimum flow threshold, the electronic circuit 14 continues processing in the first mode of operation M1 by adjusting the orifice of the valve 10 in step S1 in the first mode of operation M1, as described above with reference to FIG. 3.

(49) Otherwise, if the current flow rate Φ is below the minimum flow threshold, the electronic circuit 14 generates an override signal OS in step S100 for triggering the second version of the second mode of operation M2.

(50) Consequently, in step S2*, the electronic circuit 14 adjusts the valve 10 or its orifice, respectively, to increase the flow Φ of the fluid through the thermal energy exchanger 2 in the second version of the second mode of operation M2, such as to prevent the fluid from freezing at the current temperature of the fluid in the primary fluid circuit 101.

(51) FIG. 7 shows an example of a temperature-difference-per-flow curve ΔT=f (Φ) indicating the energy E=ΔT.Math.Φ transfer (and corresponding energy transfer rate {dot over (Q)}) achieved depending on the flow rate Φ of the fluid through the thermal energy exchanger 2, specifically, E.sub.1=FT.sub.1.Math.Φ.sub.1 with flow rate Φ.sub.1 and a resulting temperature difference ΔT corresponding to the first efficiency threshold FT.sub.1, and E.sub.2=FT.sub.2.Math.Φ.sub.2 with flow rate Φ.sub.2 and a resulting temperature difference ΔT corresponding to the second efficiency threshold FT.sub.2. As shown in FIG. 7, the value of the first efficiency threshold FT.sub.1 is higher than the value of the second efficiency threshold FT.sub.2, such that efficiency constraints with respect to the first efficiency threshold FT.sub.1 demand a higher energy transfer rate {dot over (Q)} than efficiency constraints with respect to the second efficiency threshold FT.sub.2.

(52) FIG. 8 shows an exemplary sequence of steps for adjusting the efficiency thresholds FT.sub.1, FT.sub.2 depending on one or more fluid and/or flow characteristics on the primary and/or secondary sides 23, 101 of the thermal energy exchanger 2. The efficiency thresholds FT.sub.1, FT.sub.2 are adjusted on a periodic basis and/or per request from the building control system 4 or a user, e.g. via a user interface of the thermostat 5.

(53) In step S6, the electronic circuit 14 determines one or more fluid and/or flow characteristics on the secondary side 23 of the thermal energy exchanger 2, including the flow rate of the fluid flowing on the secondary side 23, e.g. the air flow across the thermal energy exchanger 2, the temperature of the fluid on the secondary side 23, e.g. the air temperature in the space around the heat exchanger 2, the humidity of the fluid on the secondary side 23, e.g. the humidity of the air in the room where the thermal energy exchanger 2 is arranged, and/or the entropy of the fluid on the secondary side 23.

(54) In (optional) step S7, the electronic circuit 14 determines one or more fluid and/or flow characteristics on the primary side 101 of the thermal energy exchanger 2, including the speed of flow of the fluid in the primary fluid circuit 101, and/or the flow rate of the fluid flowing in the primary fluid circuit 101.

(55) In step S8, the electronic circuit 14 adjusts the second efficiency threshold FT.sub.2 depending on the one or more fluid and/or flow characteristics on the secondary side 23 of the thermal energy exchanger 2 and/or on the primary side 101 of the thermal energy exchanger 2.

(56) In (optional) step S9, the electronic circuit 14 adjusts the first efficiency threshold FT.sub.1 depending on the one or more fluid and/or flow characteristics on the secondary side 23 of the thermal energy exchanger 2 and/or on the primary side 101 of the thermal energy exchanger 2.

(57) It should be noted that, in the description, the sequence of the steps has been presented in a specific order, one skilled in the art will understand, however, that the computer program code may be structured differently and that the order of at least some of the steps could be altered, without deviating from the scope of the invention.