Patent Description:
Heat activated absorption cycles, using a wide variety of working fluids, have been utilized to provide cooling, refrigeration, and heating for many years. Absorption cycles utilize heat energy as the primary energy source, instead of mechanical work (most commonly using compressors driven by electric motors) utilized by vapor-compression heat pump cycles. The most common working fluids for absorption cycles are ammonia-water (NH<NUM>-H<NUM>O) and lithium bromide-water (LiBr-H<NUM>O), although there are many other suitable combinations. Document <CIT> discloses a method of controlling a sorption heat pump by measuring the temperature of the hydronic fluid exiting a condenser and then controlling the hydronic flow rate through the condenser using a valve or pump.

<CIT> discloses a method according to the preamble of claim <NUM>.

An absorption heat pump transfers low grade (low temperature) heat and 'pumps' it up to a higher, more useful temperature, using a higher grade energy source (combustion of fossil fuels, solar, thermal or waste heat, for example). The resulting heating cycle efficiency is greater than <NUM>% (typically <NUM> -<NUM>%) depending upon the cycle and temperatures involved (or more accurately, a coefficient of performance (COP) <NUM>-<NUM> based on the total delivered heat output divided by the higher grade heat input). In a domestic water heating application, the low grade heat energy source is typically indoor or outdoor ambient air (although other sources such as geothermal can also be used), and water is heated from typical ground temperatures (approximately <NUM>-<NUM> (<NUM>-<NUM>°F)) to <NUM>-<NUM> (<NUM> - <NUM>°F). For space heating applications, the low grade heat energy source is outdoor ambient air or geothermal, and a hydronic fluid connected to a building internal heating system, is typically heated to a temperature of <NUM>-<NUM> (<NUM> to <NUM>°F).

An absorption heat pump comprises several specialized heat exchangers, some operating at a higher pressure, others at a lower pressure, and a solution pump moving the cycle working fluids from the low pressure side to the high pressure side. A refrigerant is desorbed from an absorbent in the desorber at the high pressure using thermal energy from a suitable heat source. This refrigerant vapor is condensed to a liquid in the condenser. The saturation temperature of the refrigerant liquid exiting the condenser determines the value of the high side pressure (a lower saturation temperature corresponds to a lower high side pressure). The liquid refrigerant exiting the condenser is expanded to the low side pressure using a throttling valve or fixed restriction (orifice or capillary tube) before entering the evaporator where it is vaporized by the low-grade energy source (typically outdoor air or a geothermal sink). The temperature of the low-grade energy source determines the value of the low side pressure since the low side pressure corresponds to a boiling temperature of the refrigerant lower than the low- grade energy source. Absorbent weak in refrigerant exiting the desorber is also expanded to the low side pressure using a throttling valve or fixed restriction, and enters the absorber along with the low pressure refrigerant exiting the evaporator where the refrigerant vapor is absorbed into the absorbent. The refrigerant-absorbent liquid exiting the absorber is pumped back to the high side pressure where it enters the desorber to start the cycle again.

For the cycle to function properly, the heat pump working fluids (refrigerant and absorbent) must continuously flow back to the low side pressure portion of the system (to the pump) from the high side pressure portion of the system through the restriction devices. The differential pressure between the high and low sides provides for this flow. If the differential pressure is not high enough, the working fluid flow rates may decrease and may not allow the absorption system to work properly, or cause operational problems.

For heating applications, a fluid (typically a hydronic fluid such as water or a glycol-water mixture) may be circulated through the condenser and absorber to collect the heat of condensation and absorption, increasing its temperature. The heated hydronic fluid may then be used to heat a load, which could be air inside a building (space heating, for example) or water in a storage tank (domestic hot water heating, for example). The hydronic fluid may flow through the condenser and absorber in series, parallel, or some combination thereof. To maximize cycle efficiency, it is desirable to cool the absorber to the maximum extent possible, and for the high side pressure to be as low as possible. For absorption heating cycles, the maximum efficiency may be obtained using a parallel flow configuration of the hydronic fluid through the condenser and absorber.

It is highly desirable for the heating or cooling capacity of an absorption heat pump to increase quickly to the maximum possible after the system is started to maximize efficiency and provide optimal customer satisfaction. When an absorption system is activated after being in an off mode for a period of time, the pressure differential between the high side and low side is often very low or zero. When the system is activated and heat applied to the desorber, the high side pressure will start to increase as refrigerant vapor is generated and enters the condenser. However, the initial flow rate of refrigerant vapor may be low, as is the temperature of the condenser heat exchanger (typically constructed of steel), so the high side pressure will often rise very slowly. Therefore the pressure difference between the high side and low side increases slowly, causing the circulation rate of the working fluids between the high side and low side to be low and the system heating or cooling capacity to increase slower than desired.

For heating applications, especially domestic hot water applications where a heat pump is used to heat cold water in a storage tank, the temperature of hydronic fluid at system start-up may be very low. The hydronic temperature at start-up for a space heating application may also be very low if the heating system has been in an off mode for an extended period. When the hydronic fluid temperature flowing through the condenser and absorber is low, the refrigerant temperature exiting the condenser will also be low, resulting in a low high side pressure. This may create a situation where the pressure difference between the high side and low side is insufficient for the heat pump to operate, or operate without problems. In extreme cases where the hydronic temperature is low and the low-grade heat source coupled to the evaporator is warm (such as domestic hot water heating on a hot day), the saturation pressure difference between the high side and low side may be very low, or potentially even negative.

Using a series configuration for the hydronic loop, where the hydronic fluid first passes through the absorber (where the hydronic is heated to a higher temperature) prior to entering the condenser, provides the benefit of faster start-ups and improved operation at low hydronic temperatures by increasing the hydronic temperature entering the condenser. However, this practice decreases the maximum efficiency of the heat pump after the start-up period by causing the high side pressure to be higher than necessary. A series configuration can also limit how high the hydronic temperature can be heated, due to the maximum working pressure of the heat pump components.

A method of controlling a sorption heat pump according to the invention is defined in claim <NUM>. The method includes measuring the temperature of hydronic fluid entering the condenser and controlling a flow rate of the hydronic fluid entering the condenser in response to the temperature measured, whereby the flow rate of hydronic fluid entering the condenser is reduced relative to a total possible flow rate when the temperature is below a pre-determined value.

It will be appreciated that the following description is intended to refer to specific examples of structure selected for illustration in the drawings and is not intended to define or limit this disclosure, other than in the appended claims.

We provide a system and method of controlling a sorption heat pump comprising a condenser and an absorber, the method comprising measuring the temperature of hydronic fluid entering or exiting the condenser and controlling a flow rate of the hydronic fluid entering the condenser in response to the measured temperature, wherein the flow rate of hydronic fluid entering the condenser is reduced relative to a total potential flow rate when the temperature is below a pre-determined value.

We also provide a system and method of controlling a sorption heat pump comprising a condenser and an absorber, the method comprising controlling a flow rate of the hydronic fluid entering the condenser for a predetermined amount of time following activation of the heat pump from an off-mode.

A simple, single effect absorption heat pump cycle <NUM> is shown in <FIG>. For the purposes of this description, the cycle is assumed to be a gas-fired NH<NUM>-H<NUM>O cycle. However, any number of cycle configurations and working fluid pairs are possible and well-known in the industry.

A high temperature heat source (not shown) provides heat energy to a Desorber (DES) <NUM> which causes refrigerant (NH<NUM>) to vaporize out of NH<NUM>-H<NUM>O solution at high pressure (typically about <NUM>-<NUM>*<NUM>^<NUM> Pa (<NUM> - <NUM> psia)). The NH<NUM> vapor exits the Desorber <NUM> and is transferred to the Rectifier (RECT) <NUM> by rectifier NH<NUM> vapor supply line <NUM>. The small amount of water vapor present in the NH<NUM> vapor stream in the Rectifier <NUM> may be removed by condensation line <NUM> and returned to the Desorber <NUM>. Heat may be removed from the purified NH<NUM> vapor by transferring the purified NH<NUM> vapor to the Condenser (COND) <NUM> through condenser supply line <NUM> and causing the purified NH<NUM> vapor to condense into a liquid in the Condenser <NUM>. The liquid NH<NUM> in Condenser <NUM> may be transferred by refrigerant heat exchanger liquid supply line <NUM> to the Refrigerant Heat Exchanger (RHX) <NUM>. The liquid NH<NUM> may be cooled further in the Refrigerant Heat Exchanger (RHX) <NUM> and then reduced to low pressure (typically <NUM>-<NUM>*<NUM>^<NUM> Pa (<NUM> - <NUM> psia)) by transferring the liquid NH<NUM> from Refrigerant Heat Exchanger (RHX) <NUM> to a first restriction device <NUM> through first restriction device input line <NUM>. The low pressure liquid NH<NUM> may then be transferred by first restriction device output line <NUM> to the Evaporator (EVAP) <NUM>. The low pressure liquid NH<NUM> may be evaporated in the Evaporator (EVAP) <NUM> using heat from a low grade energy source (not shown) in the Evaporator (EVAP) <NUM>, thereby cooling the low grade heat source. The evaporated NH<NUM> is transferred from Evaporator <NUM> to the Refrigerant Heat Exchanger <NUM> by refrigerant heat exchanger vapor supply line <NUM>. In the Refrigerant Heat Exchanger <NUM>, the evaporated NH<NUM> from refrigerant heat exchanger vapor supply line <NUM> is heated. The heated NH<NUM> vapor may then be transferred from the Refrigerant Heat Exchanger <NUM> to the Absorber (ABS) <NUM> by absorber supply line <NUM>.

Hot, high pressure NH<NUM>-H<NUM>O solution with a low concentration of NH<NUM> (often called "weak" solution) exits the Desorber <NUM> through weak solution supply line <NUM> and is transferred to the Solution Heat Exchanger (SHX) <NUM>. The weak solution may be cooled in the Solution Heat Exchanger (SHX) <NUM>. The cooled weak solution may then be reduced to a low pressure by transferring the cooled weak solution to a second restriction device <NUM> through a second restriction device input line <NUM>. The low pressure, cooled weak solution may be transferred from the second restriction device <NUM> to the Absorber <NUM> by second restriction device output line <NUM>. In the Absorber <NUM>, the NH<NUM> vapor is absorbed back into the weak NH<NUM>-H<NUM>O solution. This is an exothermic process, and the heat of absorption is preferably continually removed to keep the absorption process going.

The cooled, high NH<NUM> concentration solution (often called "strong" solution) exiting the Absorber <NUM> via absorber output line <NUM> may be pumped back to high pressure by pump <NUM> and transferred by strong solution supply line <NUM> to the coils <NUM> of Rectifier <NUM>. The strong solution passes through the Rectifier coil <NUM> to cool and purify the NH<NUM> vapor. The strong solution may then be transferred from the Rectifier <NUM> to the Solution Heat Exchanger <NUM> by solution heat exchanger vapor supply line <NUM>. The strong solution may then be pre-heated in the Solution Heat Exchanger <NUM> before entering the Desorber <NUM> through desorber vapor supply line <NUM> to start the process over.

Combustion of carbon fuels, solar, waste heat or the like can also be used to provide high grade heat to Desorber <NUM>. The Evaporator <NUM> may utilize a direct refrigerant to air fin-tube coil heat exchanger, or an indirect refrigerant to hydronic working fluid heat exchanger. One advantage of the indirect method is a possible reduction in total refrigeration charge.

<FIG> depicts a simple, single effect absorption heat pump cycle <NUM>, configured for space or water heating having a controller. The Desorber <NUM>, Rectifier <NUM>, Condenser <NUM>, Refrigerant Heat Exchanger <NUM>, Evaporator <NUM>, Absorber <NUM>, and Solution Heat Exchanger <NUM> are arranged as described in <FIG>.

However, <FIG> shows ambient air <NUM> as the low temperature heat source, which passes through Evaporator <NUM> causing the refrigerant to boil. The ambient air <NUM> enters the Evaporator <NUM> by evaporator heat source input line <NUM> and exits the Evaporator <NUM> by evaporator heat source output line <NUM>. The ambient air <NUM> could be sourced from outside or inside a building.

<FIG> additionally shows a hydronic (water or glycol-water mixture for example) loop <NUM> that transfers heat from the absorption heat pump to the load to be heated. <FIG> specifies the load as an Indoor Coil, which can be, for example, an air-coupled heat exchanger for space heating, a heat exchanger connected to a water tank to heat water or the like. The hydronic fluid, cooled by the load, may first be transferred to an optional Condensing Heat Exchanger (CHX) <NUM> by hydronic input line <NUM>. Condensing Heat Exchanger <NUM> serves to further cool the high temperature Desorber <NUM> heating source, to reduce losses (in this case, combustion of a carbon fuel is assumed). When the temperature of the hydronic fluid from hydronic input line <NUM> is below the dew point of the flue gases exiting the Desorber <NUM>, water vapor in the flue gas line <NUM> exiting the Desorber <NUM> can be condensed and transferred to the Condensing Heat Exchanger <NUM>, significantly decreasing energy losses from the flue gas.

After exiting the Condensing Heat Exchanger <NUM>, the hydronic fluid enters the Condenser <NUM> and Absorber <NUM> to collect the heat of condensation and absorption from the heat pump cycle. The hydronic can flow through the Condenser <NUM> and Absorber <NUM> in series, parallel, or some combination thereof. A parallel configuration is shown in <FIG>, where a portion of the hydronic flow enters the Condenser <NUM> via condenser hydronic flow input line <NUM>, while the remaining hydronic flow enters the Absorber <NUM> via absorber hydronic flow input line <NUM>. After exiting the Condenser <NUM> and Absorber <NUM> via condenser hydronic flow output line <NUM> and absorber hydronic flow output line <NUM>, the heated hydronic flow re- combines and travels to the load to be heated via hydronic fluid output line <NUM>. For a parallel flow arrangement, the percentage of the total hydronic flow that passes through the condenser <NUM> often depends on the particular cycle chosen, or the specific application. Generally, the hydronic flow split percentage is similar to the heating capacity ratio of the condenser/absorber, which is often around <NUM>% absorber.

The hydronic flow split percentage can be regulated by Control <NUM> which operates valve <NUM>. Optionally, the Control <NUM> may operate valve <NUM> depending on the temperature of the hydronic fluid entering or exiting the condensing heat exchanger <NUM>. For example, the Control <NUM> may reduce the flow rate of hydronic fluid entering or exiting the condenser relative to a total possible flow rate when the temperature is below a pre-determined value such as below a temperature of about <NUM>-<NUM> (<NUM>-<NUM>°F). To measure the temperature of the hydronic fluid, a temperature sensor 249a may be employed in hydronic input line <NUM> upstream of the condensing heat exchanger <NUM> or, alternatively, a temperature sensor 249b may be employed in hydronic input line <NUM> downstream of the condensing heat exchanger <NUM>.

<FIG> shows a valve <NUM> in the condenser hydronic flow input line <NUM> where it enters the Condenser <NUM>. The valve <NUM> can be of the ON/OFF or variable position type. The position of the valve <NUM> (full open, full closed, or somewhere in-between) is controlled by a controller <NUM> in response to the temperature of the hydronic fluid exiting the condensing heat exchanger <NUM> (or alternatively, entering the condensing heat exchanger <NUM>). Temperature sensor 249a and 249b (such as thermocouple, RTD, thermistor or other temperature measurement devices) are attached to the hydronic input line <NUM>, so that the temperature of the hydronic fluid entering or exiting the condensing heat exchanger <NUM> is known.

When the temperature of the hydronic fluid entering the Condenser <NUM> is below a pre-determined value, the controller <NUM> acts to close, or partially close the valve <NUM> to stop or reduce the flowrate of hydronic fluid through the condenser <NUM>. By stopping or reducing the hydronic flow rate through the Condenser <NUM>, the temperature of the refrigerant exiting the Condenser <NUM> via condenser hydronic flow output line <NUM> is increased, causing the high side pressure to increase. This control method keeps the high side pressure above a minimum value, ensuring that the pressure difference between the high and low side pressures is high enough to allow the working fluids to flow at a high enough rate through the pressure restrictions devices <NUM> and <NUM> to keep the heat pump cycle <NUM> operating properly.

By stopping or reducing the hydronic flow rate through the condenser <NUM> in this manner, higher than normal hydronic flow rate will pass through the Absorber <NUM>, causing the strong solution exiting the Absorber <NUM> via absorber hydronic flow output line <NUM> to exit at a lower temperature. Since the efficiency of the heat pump cycle <NUM> increases with decreasing temperature of the strong solution exiting Absorber <NUM> in line <NUM>, the extra flow rate through the Absorber <NUM> during periods when the control <NUM> is working to close or partially close the valve will cause the heat pump to operate at a higher efficiency.

The hydronic flow rate through the Condenser <NUM> can be controlled, based on the temperature of the hydronic fluid entering the Condenser <NUM>, using a simple ON/OFF valve, multi-position valve, or proportional valve (shown generically as <NUM>). An ON/OFF valve provides the simplest and least expensive option. If an ON/OFF valve is used, the control <NUM> can be configured to turn the valve <NUM> ON and OFF on pre-determined intervals, based on the temperature of the hydronic fluid. For example, if the hydronic fluid was very cold, the control <NUM> would act to keep the valve OFF for a longer duration that it was ON. As the hydronic temperature increases, the time intervals of the ON and OFF cycles can be adjusted accordingly (more time ON, less time OFF) to maintain an approximate minimum high side pressure based on the hydronic fluid temperature entering the Condenser <NUM>. When the hydronic temperature increases to a pre-determined value, the control <NUM> can keep the valve <NUM> open at all times, allowing "normal" heat pump operation. Note that during periods when the valve <NUM> is OFF (closed), all of the hydronic flow by-passes the Condenser <NUM> and flows through the Absorber <NUM>.

When a multi-position or proportional valve is used, the controller <NUM> may act to position the valve <NUM> in a predetermined position based on the hydronic temperature entering the Condenser <NUM>. For example, if the hydronic temperature is very cold, the valve may be set to a position more closed (by-passing more hydronic flow to the Absorber <NUM>), compared to when the hydronic temperature is warmer. When the hydronic temperature increases to a pre-determined value, the control <NUM> may keep the valve <NUM> open at all times, allowing "normal" heat pump operation.

When an absorption heat pump <NUM> has been off (not running), normally the high and low side pressures equalize to a pressure between the normal high and normal low. As the heat pump working fluids continue to cool, this equalized "system" pressure will continue to decay until reaching the saturation pressure based on the refrigerant concentration and the ambient temperature. When the heat pump <NUM> is activated from the off-state, it is desirable for the high side pressure to increase as quickly as possible, and the low side pressure to decrease as quickly as possible, for the heat pump <NUM> to reach maximum heating or cooling capacity as quickly as possible.

Therefore, during a start-up condition (the first <NUM> to <NUM> minutes after the heat pump cycle is activated, for example), an alternate valve control methodology may be utilized. Since the high side pressure will increase more quickly if the hydronic flow rate through the Condenser <NUM> is stopped or reduced, it is desirable for the control <NUM> to close the valve <NUM> for a period of time during the start-up period. When the valve is closed, the hydronic flow rate through the Absorber <NUM> will be increased, which allows the low side pressure to decrease faster.

During heat pump start-up periods, the time periods used by the controller <NUM> to turn the valve <NUM> ON or OFF based on the hydronic temperature entering the Condenser <NUM> may be different from steady-state operation use (or the position of the valve if a multi-position or proportional valve is used). For example, the controller <NUM> may be programmed to close (or partially close) the valve <NUM> for a pre-determined period of time during start-up, regardless of what the hydronic temperature is (although the period of time may be a function of the hydronic temperature). The ON/OFF time periods (or valve position) as a function of the hydronic temperature may be slightly different during start-up periods compared to steady-state conditions, such that the heat pump system <NUM> reaches maximum capacity and efficiency as fast as possible.

In <FIG>, a simplified view heat pump cycle <NUM> with a hydronically cooled Condenser <NUM> and Absorber <NUM> in parallel configuration is shown with a valve <NUM> located in the condenser hydronic flow input line <NUM> at the inlet to the Condenser <NUM>, a temperature sensor <NUM> located to measure the temperature of the hydronic entering the Condenser <NUM>, and a control <NUM> configured to control the position of the valve <NUM> in relation to the hydronic temperature. With this arrangement, the portion of the hydronic flow that does not pass through the Condenser <NUM> passes through the Absorber <NUM>, further cooling the working fluids in the Absorber <NUM>, increasing the heat pump efficiency.

In <FIG>, a heat pump cycle <NUM> with a similar Absorber-Condenser parallel configuration is shown, with the addition of a second valve 447b located in the absorber hydronic flow input line <NUM>. The position of both valves (447a and 447b) is controlled by a controller <NUM> connected to a temperature measurement device <NUM> located to measure the hydronic temperature entering the Condenser <NUM>-Absorber <NUM>. With this configuration, the hydronic flow rate through the Absorber <NUM> may be stopped or reduced depending on the hydronic fluid temperature, with the portion of the hydronic flow not passing through the Absorber <NUM> passing through the Condenser <NUM> (which acts to further cool the Condenser <NUM> and reduce the high side pressure). This arrangement may be beneficial for systems that require delivery of very hot hydronic temperatures to the load. When the hydronic temperature increases to a pre-determined temperature, indicative of a maximum high side pressure allowable by the heat pump components, the control <NUM> can act to stop or reduce the hydronic flow through the Absorber <NUM>, allowing the Condenser <NUM> to run cooler and limit the high side pressure. As noted before, if the valve 447a and/or 447b is of the ON/OFF type, the controller <NUM> can act to turn the valve 447a and/or 447b ON/OFF at different periods based on the hydronic temperature. This arrangement, by reducing the hydronic flow through the Absorber <NUM>, may decrease the efficiency of the heat pump, but it will allow the heat pump to provide higher hydronic temperatures to the load if required. Alternatively, flow through the condenser can also be controlled using valve 447b. During normal operation, valve 447b is set to a semi-open position. When it is desired to decrease the flow through hydronic flow through the condenser, valve 447b is opened, which reduces the pressure loss of the hydronic fluid flowing through the absorber leg relative to the condenser leg, thereby increasing the flow through the absorber and decreasing flow through the condenser.

In another example of a heat pump cycle, flow through the condenser can also be controlled using valve 447b. During normal operation, valve 447b is set to a semi-open position. When it is desired to decrease the flow through hydronic flow through the condenser, valve 447b may be opened. This reduces the pressure loss of the hydronic fluid flowing through the absorber leg relative to the condenser leg thereby increasing the flow through the absorber and decreasing flow through the condenser.

In <FIG> shows a heat pump cycle <NUM> with an Absorber <NUM> and Condenser <NUM> tied to a hydronic loop in series configuration (Condenser <NUM> first is shown). In this case, the valve <NUM> is located in a by-pass line <NUM> around the Condenser <NUM>. During normal operation, the valve <NUM> may be closed, thereby forcing all of the hydronic flow through the Condenser <NUM>. If the hydronic temperature entering the Condenser <NUM> is below a pre-determined value (or during a heat pump start-up sequence), the controller <NUM> can open (or partially open) the valve <NUM>, allowing all or a portion of the hydronic flow to by-pass the Condenser <NUM> via by-pass line <NUM>. With this arrangement, if the valve is open, the temperature of the hydronic entering the Absorber <NUM> will be lower compared to a closed valve, thereby increasing cooling in the Absorber <NUM> and increasing heat pump efficiency.

Claim 1:
A method of controlling a sorption heat pump comprising, in an absorption heat pump cycle, a desorber (<NUM>, <NUM>) in fluid connection with a rectifier (<NUM>, <NUM>), the rectifier (<NUM>, <NUM>) being in fluid connection with a hydronically-cooled condenser (<NUM>, <NUM>, <NUM>, <NUM>), the hydronically-cooled condenser (<NUM>, <NUM>, <NUM>, <NUM>) being in fluid connection with a refrigerant heat exchanger (<NUM>, <NUM>), the refrigerant heat exchanger (<NUM>, <NUM>) being in fluid connection with an evaporator (<NUM>, <NUM>), the evaporator being in fluid connection with the hydronically-cooled absorber (<NUM>, <NUM>, <NUM>, <NUM>), the absorber (<NUM>, <NUM>, <NUM>, <NUM>) and condenser (<NUM>, <NUM>, <NUM>, <NUM>) being arranged in parallel with respect to the hydronic flow, the method comprising:
a) measuring the temperature of hydronic fluid entering the condenser (<NUM>, <NUM>, <NUM>, <NUM>) the method being characterised by comprising:
b) controlling a flow rate of the hydronic fluid entering the condenser (<NUM>, <NUM>, <NUM>, <NUM>) in response to the temperature measured in a) to reduce the flow rate of hydronic fluid entering the condenser (<NUM>, <NUM>, <NUM>, <NUM>) relative to a total possible flow rate and increase the flow rate of the hydronic fluid entering the absorber (<NUM>, <NUM>, <NUM>, <NUM>) when the temperature is below a pre-determined value, wherein the flow rate of the hydronic fluid entering the condenser is controlled by closing or opening a ON/OFF valve.