Patent Publication Number: US-2013240172-A1

Title: Hydronic System and Control Method

Description:
FIELD OF THE INVENTION 
     This invention relates to hydronic systems for environmental control within structures. 
     BACKGROUND 
     Hydronic systems are hydraulically based systems for heating and cooling interior environments, such as office buildings, hospitals, apartment buildings and other edifices where there are many areas, isolated from one another, known as zones, which require individual control of the air temperature in each of the zones. The zones may correspond, for example, to the various rooms in a building. 
     Hydronic systems may comprise one or more boilers and chillers which are in fluid communication with a plurality of heat exchangers through a piping network. There may be, for example, one heat exchanger in each zone. The piping network carries a working fluid, for example water, between the boilers or the chillers and the heat exchangers in each zone. The working fluid is heated by the boiler or cooled by the chiller and flows to the heat exchangers, where heat is either imparted to or removed from the air in the zone depending on the difference between the actual air temperature and the desired air temperature. The heat exchangers may be, for example, variable air volume boxes which work in conjunction with a thermostat in the zone and the hydronic system controls to heat or cool the zone air as necessary to achieve the desired zone air temperature. Flow of the working fluid through each heat exchanger is controlled by a control valve associated with the heat exchanger which opens and closes to vary the mass flow rate of the working fluid through the heat exchanger in response to the demand for heating or cooling in each zone. 
     For proper operation of the hydronic system it is necessary to balance the flow of working fluid throughout the system so that all of the heat exchangers in all of the zones always have access to sufficient mass flow volume of the working fluid to achieve and maintain a desired zone air temperature for a particular set of design parameters peculiar to the location of the building and its thermal characteristics. Unless the system flow is balanced, the mass flow rate to each heat exchanger will naturally vary as a function of the head loss to each heat exchanger. The head loss for each heat exchanger will vary depending upon the length of the path from the pump to each heat exchanger, the friction encountered by the flow, and the height of the heat exchanger above the pump. System balancing involves using separate balancing valves positioned in series with the control valves associated with each heat exchanger to limit the flow of working fluid to an allowable maximum which ensures that no heat exchanger will be starved of working fluid. The hydronic system may be balanced, for example, by fully opening all of the control valves, pumping working fluid through the hydronic system to each of the heat exchangers, and setting each of the balancing valves so that the mass flow rate is the same to each heat exchanger in each zone. Using balancing valves to limit the maximum flow through each control valve ensures that each of the heat exchangers will always have a sufficient mass flow rate to achieve and maintain the desired air temperature in its zone regardless of the demand for working fluid in other zones in the hydronic system. 
     Hydronic systems according to the prior art which use both balancing valves and control valves are costly because they require at least two valves per heat exchanger. It is furthermore a challenge to balance hydronic systems, and they can suffer from inefficient operation depending upon what parameters are used to throttle the control valves. It is clear that advantages may be obtained by more efficient hydronic systems which use fewer valves, and which control the valves with efficient energy usage as a consideration. 
     SUMMARY 
     The invention concerns a hydronic system for controlling the air temperature in a plurality of zones. In one example, the hydronic system comprises a working fluid for effecting heat transfer and a first heat exchanger for imparting or removing heat to or from the working fluid. At least one of a plurality of first temperature measuring devices measures the air temperature in at least one of the zones. At least one of a plurality of second heat exchangers imparts or removes heat to or from air in the at least one zone. The at least one second heat exchanger is in fluid communication with the first heat exchanger. In one example embodiment, the at least one second heat exchanger comprises a valve controlling a mass flow rate of the working fluid through the at least one second heat exchanger. A second temperature measuring device measures a change in temperature of the working fluid across the at least one second heat exchanger. The example hydronic system may further comprise at least one pump in fluid communication with the first heat exchanger and the at least one second heat exchanger for pumping the working fluid therebetween. A controller, in communication with the at least one first temperature measuring device, the second temperature measuring device and the valve associated with the at least one second heat exchanger controls the valve for the at least one second heat exchanger in the at least one zone in response to signals from the first and second temperature measuring devices. 
     In an example hydronic system the first heat exchanger may comprise a boiler for adding heat to the working fluid, a chiller for removing heat from the working fluid, or a plurality of boilers and chillers. 
     The example hydronic system may further comprise a fan controlling a mass flow rate of the air through the at least one second heat exchanger, the fan being in communication with the controller, the controller controlling the fan in response to signals from the first temperature measuring device indicative of the air temperature in the at least one zone. 
     The example hydronic system may further comprise a means for measuring a mass flow rate of the working fluid through the at least one second heat exchanger, the controller being in communication with the means for measuring the mass flow rate and controlling the valve so as to limit the mass flow rate of the working fluid through the at least one second heat exchanger to a maximum value in response to signals from the means for measuring the mass flow rate of the working fluid. 
     The invention further comprises a method of operating a hydronic system for controlling the air temperature in a plurality of zones. An example method comprises:
         moving a working fluid through a plurality of heat exchangers, each heat exchanger being associated with a respective one of the zones;   for each of the zones, moving air from the zone through the heat exchanger associated therewith for transferring heat between the working fluid and the air in the zone;   for each of the heat exchangers, measuring a first temperature of the working fluid before heat is transferred between the working fluid and the air;   for each of the heat exchangers, measuring a second temperature of the working fluid after heat is transferred between the working fluid and the air;   for each of the heat exchangers, adjusting the mass flow rate of the working fluid through the heat exchanger so as to maintain a constant temperature difference between the first and second temperatures of the working fluid;   for each of the zones, measuring the air temperature in the zone; and   for each of the heat exchangers and each of the zones, adjusting the mass flow rate of air from the zone through the heat exchanger associated therewith so as to achieve and maintain a desired air temperature in the zone.       

     The example method may further comprise:
         for each of the heat exchangers, establishing a respective maximum permitted mass flow rate of the working fluid therethrough;   for each of the heat exchangers, measuring the mass flow rate of the working fluid therethrough; and   for each of the heat exchangers, adjusting the mass flow rate of the working fluid therethrough so that it is no greater than the respective maximum permitted mass flow rate.       

     Establishing the respective maximum permitted mass flow rate of the working fluid through each of the heat exchangers may comprise balancing a mass flow of the working fluid through the hydronic system so that the mass flow rate of the working fluid through each of the heat exchangers is sufficient to meet a maximum required heat load at all times during operation of the hydronic system. Heat load is the required heating or cooling of a heat exchanger to maintain a desired comfort level in a zone. 
     In another example of the method, the mass flow rate of the working fluid through each of the heat exchangers is adjusted so as to maintain a constant temperature difference of about 20° F. between the first and second temperatures of the working fluid. 
     In another example of the method, the mass flow rate of the working fluid through each of the heat exchangers is adjusted so as to maintain a constant temperature difference of about 40° F. between the first and second temperatures of the working fluid. 
     In another example of the method the mass flow rate of the working fluid through each of the heat exchangers is adjusted so as to maintain a constant temperature difference from about 20° F. to about 60° F. between the first and second temperatures of the working fluid. 
     In another example of the method, the mass flow rate of the working fluid through each of the heat exchangers is adjusted so as to maintain a constant temperature difference of about 10° F. between the first and second temperatures of the working fluid. 
     In another example of the method the mass flow rate of the working fluid through each of the heat exchangers is adjusted so as to maintain a constant temperature difference from about 10° F. to about 30° F. between the first and second temperatures of the working fluid. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1 and 1A  are schematic illustrations of example hydronic systems according to the invention; 
         FIGS. 2 ,  3 ,  3 A and  4  are schematic illustrations of alternate embodiments of components useable in the hydronic system according to the invention; and 
         FIG. 5  is a flow chart illustrating an example method of operating a hydronic system according to the invention. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  shows, in schematic form, an example hydronic system  10  according to the invention. Hydronic system  10  controls the air temperature in one or more zones  12 , for example, in rooms  14  on different floors  16  of an office building  18 . 
     Hydronic system  10  comprises a working fluid  20 , in this example, water, which is circulated by a pump  22  through a first heat exchanger  24  and one or more second heat exchangers  26 , the heat exchangers and pump being in fluid communication through a piping network  28 . The first heat exchanger  24  may comprise a boiler for imparting heat to the working fluid  20  to heat the zones  12 , or the first heat exchanger may be a chiller to remove heat from the working fluid when the zones are to be cooled. Hydronic systems  10  may have multiple chillers and boilers as required to heat and cool the zones, the first heat exchanger  24  generally representing these devices for imparting heat to or removing heat from the working fluid as needed for a particular design. 
     Each second heat exchanger  26  is associated with a particular zone  12  and imparts heat to or removes heat from the air within its associated zone for controlling the zone air temperature.  FIG. 1  shows a plurality of zones  12 , each with an associated heat exchanger  26 . The hydronic system  10  is further described below with respect to one zone, it being understood that other zones using the system according to the invention will be identical. 
     Heat exchanger  26  may be, for example, a radiator, or a variable air volume box (VAV box)  30 . VAV box  30  comprises a fan  32  which draws zone air  34  from the associated zone  12  and passes it over coils  36  or other heat transfer surfaces through which the working fluid  20  is circulated. The zone air  34  may be heated or cooled as desired by conductive heat transfer between the zone air  34  and the coils  36 , the air, thus cooled or heated, being returned to the zone  12 . Fresh air  38  from outside of the zone  12 , usually ambient air from outside of the building  18 , is also drawn in to the VAV box and injected into the zone  12  with the zone air  34  that is recirculated within the zone. The volume of fresh air  38  added to a zone  12  is typically based upon occupancy type and the number of occupants and can range from 5-60 air exchanges per hour as recommended by the American Society of Heating, Refrigeration and Air Conditioning. 
     A first temperature measuring device  40  is associated with each zone  12 . Temperature measuring device  40  measures the air temperature within zone  12  and may be, for example, a thermostat, which generates a signal, such as an electrical voltage or current, indicative of the air temperature, or the difference between the air temperature and a desired air temperature in the zone. A second temperature measuring device  42  measures the temperature change of the working fluid across the heat exchanger  26  associated with the zone  12 . The temperature change “across the heat exchanger” means the difference in temperature between the working fluid  20  as it enters the heat exchanger (i.e., before heat is transferred between the working fluid and the zone air  34 ) and as it leaves the heat exchanger (i.e. after heat is transferred between the working fluid and the zone air), and is thus indicative of the heat transfer between the working fluid  20  and the zone air  34  and the fresh air  38  passing through the heat exchanger  26  in the zone  12 . 
     A valve  44 , associated with zone  12 , is positioned in the piping network  28  between the heat exchanger  24  and the heat exchanger  26 . Opening and closing of valve  44  is remotely controllable, and the valve is adjustable to act as a throttle to control the mass flow rate of the working fluid  20  to the associated heat exchanger  26 . Remote actuation of the valve  44  may be effected, for example, by an actuator  45 , such as a stepper motor mounted on the valve, and knowledge of the degree to which the valve is open or closed may be obtained using a positional encoder  47 , such as a rotary encoder, which generates a signal, for example, an electrical voltage or current, indicative of the position of the valve throttling member at or between its open and closed positions. 
     Valve  44  may also act to limit the mass flow rate through the heat exchanger to a maximum mass flow rate, determined, for example, by the requirements for balancing the working fluid flow to and from all of the heat exchangers  26  throughout the hydronic system  10 . The maximum permitted flow rate may also be limited to avoid excess noise and wear caused by high mass flow rates. One method of limiting the maximum mass flow rate through the heat exchanger  26  involves measuring the change in pressure, or pressure drop, of the working fluid  20  across the valve  44  associated with the heat exchanger. The mass flow rate of fluid through the valve (and hence through the heat exchanger) is proportional to this pressure drop. To this end, a pressure measuring device  46  is used to measure the working fluid pressure as it enters and leaves the valve  44 . The pressure measuring device  46  may be, for example a piezo-electric based device, which generates a signal, for example, an electrical voltage or current, indicative of the difference between the working fluid pressure as it enters and leaves the valve  44 . Knowing this pressure difference, the mass flow rate though the valve can be calculated using the formula M=C v (P in −P out ) 1/2 , where M is the mass flow rate, P in  is the working fluid pressure upon entering the valve, P out  is the working fluid pressure upon exiting the valve and C v  represents the particular flow characteristics of the valve, obtained from empirical measurements. Other devices may be used to determine the mass flow rate of the working fluid through the heat exchanger  26 . For example, as shown in  FIG. 2 , the pressure measuring device  46  may be used to measure the pressure difference across a fixed orifice  48 .  FIG. 3  illustrates the use of the pressure measuring device with venturi  49  positioned within the piping network  28  proximate to the heat exchanger  26 , the venturi being a well established device for accurately measuring flow rate. Alternately, mass flow rate may be measured using a hot wire anemometer  50  as shown in  FIG. 4 , the anemometer generating a signal, for example an electrical voltage or current, indicative of the mass flow rate. Other mass flow measurement devices, such as sonic-based devices, magnetic-based devices as well as spinning vane type devices may also be used. 
     A shown in  FIG. 1 , each valve  44  has associated with it a zone controller  52 , such as a microprocessor or a programmable logic controller, which controls the operation of the valve  44  and the heat exchanger  26  as described below. Zone controllers  52  can also be programmed to store the valve characteristics C v  (or the venturi or hot wire anemometer flow characteristics) and the maximum permitted flow rate for the valve. These parameters are useful both to balance the flow of working fluid to all valves  44  throughout the hydronic system  10  as well as for control of each valve individually during operation of the system. 
     In one embodiment, shown in  FIG. 1 , the zone controller  52  is in communication with fan  32  of the VAV box  30  (when present), the temperature measuring device  40  (measuring the temperature of the zone air  34 ), the temperature measuring device  42  (measuring the temperature change of the working fluid across the heat exchanger  26 ), the pressure measuring device  46  (measuring the mass flow rate of the working fluid  20  through the heat exchanger  26 ), the valve actuator  45  and the positional encoder  47  of the valve  44 . The controllers  52  of the various zones are also in communication with a building controller  54  that controls the heat exchanger  24  and the pump  22  in response to signals from the various zone controllers  52 . Building controller  54  may also be a microprocessor or programmable logic controller. Communication between the various components and their respective controllers  52  as well as between the zone controllers  52  and building controller  54  is effected over communication lines  56 , which may represent, for example, hardwired electrical lines, or wireless links. Using resident software and the communication lines  56 , controller  52  controls operation of fan  32  and valve  44  through its associated actuator  45 . Controller  52  furthermore receives signals from the positional encoder  47  associated with valve  44 , the pressure measuring device  46  (or the hot wire anemometer  50 ), the temperature measuring device  40  and the temperature measuring device  42  to effect efficient climate control of a zone  12  within the building  18  as described below. In another system embodiment, shown in  FIG. 1A , the temperature measuring device  40 , which measures the temperature of the zone air  34 , is in direct communication with the fan  32  of the VAV box  30 , thereby affording a different control relationship between the zone controller  52  and the VAV box as described below. The other communication links in this embodiment remain the same as described above with reference to  FIG. 1 . 
     Method of Balancing the Hydronic System 
     Hydronic systems must be “balanced” to ensure effective operation. “Balancing” as used herein refers to adjusting the mass flow rate of the working fluid throughout the hydronic system so that there is sufficient flow available to all zones which will meet the required heat load of each zone when all zones demand maximum mass flow for heating or cooling of the zones. “Heat load” is the amount of heating or cooling required to maintain the desired comfort level in a zone. Balancing is necessary because, if the flow of working fluid is otherwise uncontrolled, the heat exchangers  26  in zones  12  remote from the pump  22  will naturally receive a lower mass flow rate than the heat exchangers in zones proximate to the pump. This unequal flow rate results from head losses due to friction and potential energy differences among zones at different heights and at different distances from the pump. 
     The example hydronic system  10  as described above may be used to efficiently balance itself without the need for additional balancing valves normally associated with hydronic systems. An example balancing method comprises using the controller  54  to operate pump  22  and heat exchanger  24  to pump the working fluid  20  to the heat exchangers  26  in each zone  12  in the building  18 . Controllers  52  in each zone  12  then use signals from their respective pressure measuring devices  46  to measure the instantaneous mass flow rate through their respective valves  44  to their associated heat exchangers  26 . Knowing these flow rates, the controllers  52  send control signals to the actuators of their respective valves  44 , opening or closing the valve to the degree required to achieve the flow through every valve  44  to meet the heat load for every zone  12  in the system  10 . This is an iterative process which is controlled by the resident software of the controllers  52  and the process converges on a set of valve settings which determine, for each valve  44 , the maximum permitted mass flow rate through that valve. The maximum permitted mass flow rate for each valve  44  is recorded in the zone controller  52  associated with each valve  44 , and that information is used during system operation to limit the maximum flow rate of working fluid through a particular valve. The information may be recorded, for example, as a pressure difference across the valve or an orifice or venturi associated with the valve, a reading from the hot wire anemometer associated with a valve, or a particular position of the valve throwing member as reported by the positional encoder  47  associated with the valve  44 . The hydronic system  10  thus uses the valves  44  for establishing and maintaining hydronic system balance and obviates the need for separate balancing valves which would otherwise be positioned in series with each valve  44  and set so as to permit a mass flow rate no greater than the maximum permitted for a particular heat exchanger  26  and thereby override the valve  44  if it calls for a greater mass flow than is permitted during system operation. 
     Method of System Operation 
     In an example method of operating hydronic system  10 , the controller  54  commands pump  22  to move the working fluid  20  through the heat exchanger  24 , where heat is added or removed from the working fluid depending upon whether the zones  12  are to be heated or cooled. This example method describes system operation for heating the zones with reference to the system shown in  FIG. 1 , the method for cooling being similar. The description is further directed to one zone, it being understood that similar operational actions are being carried out contemporaneously for many or all zones associated with the hydronic system  10 . 
     Temperature measuring device  40  (a thermostat) measures the temperature of the zone air  34  in a particular zone  12  and compares it to a desired temperature for that zone. If the measured temperature is below the desired temperature, measuring device  40  signals zone controller  52 , transmitting information that the actual temperature of air  34  in zone  12  is below the desired temperature. In response to this signal, the zone controller  52  signals actuator  45  which opens valve  44 . Valve  44 , being in fluid communication with both heat exchanger  24  and the heat exchanger  26  associated with the zone  12 , permits working fluid  20  to flow to the heat exchanger  26 . When the heat exchanger  26  is a VAV box  30  (as opposed to a radiator), zone controller  52  also activates fan  32  to force zone air  34  though the heat exchanger  26  where heat is transferred from the working fluid  20  to the air  34  through conduction between the air and the coils  36  before it is discharged back into the zone  12 . Fresh or “make-up” air  38  is also drawn from the ambient, forced through the heat exchanger  26  and into the zone  12 . The zone controller  52  receives signals from the temperature measuring device  42 , which measures the temperature difference of the working fluid  20  across the heat exchanger  26 , i.e. the temperature difference between the working fluid  20  before heat is transferred between it and the zone air  34 , and after heat is transferred between the working fluid  20  and the zone air  34 . The zone controller  52  uses this information to adjust the mass flow rate of the working fluid  20  through the heat exchanger  26  by adjusting the valve  44  so as to maintain the temperature difference of the working fluid  20  across the heat exchanger constant. Maintaining this constant temperature difference allows the heat exchanger  24 , which supplies heat to the working fluid  20 , to operate more efficiently than if other parameters were used as a criterion for controlling the mass flow rate. When the system is in the heating mode of operation, temperature differences from about 20° F. to about 60° F. are considered practical, with a constant temperature difference of about 20° F. or about 40° F. being advantageous for the efficient operation of modern boilers used in large scale climate control systems. When the system is in the cooling mode of operation, temperature differences from about 10° F. to about 30° F. are considered practical, with a constant temperature difference of about 10° F. or about 20° F. being advantageous for the efficient operation of modern chillers used in large scale climate control systems. Thus the zone controller  52  adjusts the valve  44  by opening and closing its throttling member as required to maintain the desired constant temperature difference across the heat exchanger  26 . The mass flow rate of the working fluid  20  through the heat exchanger  26  will fluctuate in response to the heat transfer from the working fluid to the zone air  34  as the zone air passes through the heat exchanger  26  and is recirculated through the zone  12 . The zone controller  52  receives signals from the temperature measuring device  40  which it uses to control the mass flow rate of zone air  34  through the heat exchanger  26  by controlling the operation of fan  32 , thereby controlling the air temperature within the zone  12 . Depending upon the nature of the control regime, the fan  32  may run at a constant speed, shutting off when the desired zone temperature is reached or exceeded by a set amount, or it may run at varying speeds, decreasing the mass flow rate of zone air  34  through the heat exchanger  26  as the desired zone temperature is approached. 
     It is conceivable, however, that the zone controller  52 , using only the change in temperature of the working fluid across the heat exchanger  26  to directly control the valve  44  may, under some circumstances, command its valve  44  to open to a degree at which the mass flow rate of the working fluid  20  through the valve will exceed the maximum permitted mass flow rate established during balancing of the system  10 . This cannot be permitted, because it may result in some heat exchangers being starved for working fluid and therefore unable to control the air temperature in their associated zone and not meet their heat load as various heat exchangers compete for the working fluid and the system flow regime becomes out of balance. To avoid this situation, the controller  52  uses signals from the pressure measuring device  46  to adjust the valve  44  to limit the degree to which it may open so as not to exceed the maximum permitted mass flow rate established for the valve  44  during balancing of the system. In another embodiment, the controller  52  may use the output from the hot wire anemometer  50  associated with its valve  44 , which also provides signals indicative of the instantaneous mass flow rate of the working fluid through the valve. 
       FIG. 5  is a flow chart which illustrates an example method for controlling the hydronic system disclosed herein. As noted, the method comprises:
         moving a working fluid through a heat exchanger;   moving air from at least one zone through the heat exchanger for transferring heat between the working fluid and the air;   measuring a first temperature of the working fluid before heat is transferred between the working fluid and the air;   measuring a second temperature of the working fluid after heat is transferred between the working fluid and the air;   adjusting a mass flow rate of the working fluid through the heat exchanger so as to maintain a constant temperature difference between the first and second temperatures of the working fluid;   measuring the air temperature in the at least one zone; and   adjusting the mass flow rate of the air from the at least one zone through the heat exchanger so as to achieve and maintain a desired air temperature in the at least one zone.       

     The method may also include:
         establishing a maximum permitted mass flow rate of the working fluid through the heat exchanger (for example, by balancing the hydronic system);   measuring the mass flow rate of the working fluid through the heat exchanger; and   adjusting the mass flow rate of the working fluid through the heat exchanger so that it is no greater than the maximum permitted mass flow rate.       

     In another embodiment of a method for operating a hydronic system as shown in  FIG. 1A , the temperature measuring device  40  (a thermostat) measures the temperature of the zone air  34  in a particular zone  12  and compares it to a desired temperature for that zone. If the measured temperature is below the desired temperature, measuring device  40  signals fan  32  of VAV box  30  to turn on and heat the zone air  34 . In response to this signal, the fan  32  signals the zone controller  52  that the fan is on and that heat is needed at the VAV box  30 . In response to the signal from the fan  32  the zone controller  52  signals actuator  45  which opens valve  44 . Note that in the previous method embodiment (system of  FIG. 1 ), fan  32  was slaved to respond to signals from the master, zone controller  52 . However, in this embodiment (system of  FIG. 1A ), zone controller  52  is slaved to the operation of fan  32 , which can have processing capability, in the form of a microprocessor or programmable logic controller associated with it. Similar to the previously described method, valve  44  is in fluid communication with both heat exchanger  24  and the heat exchanger  26  associated with the zone  12 , and permits working fluid  20  to flow to the heat exchanger  26 , VAV box  30 . In response to the signal from temperature measuring device  40 , fan  32  forces zone air  34  though the heat exchanger  26  where heat is transferred from the working fluid  20  to the air  34  through conduction between the air and the coils  36  before it is discharged back into the zone  12 . Fresh or “make-up” air  38  is also drawn from the ambient, forced through the heat exchanger  26  and into the zone  12 . The zone controller  52  receives signals from the temperature measuring device  42 , which measures the temperature difference of the working fluid  20  across the heat exchanger  26 , i.e. the temperature difference between the working fluid  20  before heat is transferred between it and the zone air  34 , and after heat is transferred between the working fluid  20  and the zone air  34 . The zone controller  52  uses this information to adjust the mass flow rate of the working fluid  20  through the heat exchanger  26  by adjusting the valve  44  so as to maintain the temperature difference of the working fluid  20  across the heat exchanger constant. Maintaining this constant temperature difference allows the heat exchanger  24 , which supplies heat to the working fluid  20 , to operate more efficiently than if other parameters were used as a criterion for controlling the mass flow rate. As described above, the controller  52  again uses signals from the pressure measuring device  46  to adjust the valve  44  to limit the degree to which it may open so as not to exceed the maximum permitted mass flow rate established for the valve  44  during balancing of the system. 
     Hydronic systems according to the invention, and methods of operating such hydronic systems are expected to operate more efficiently and provide a comfortable environment with lower energy expenditure. They are expected to be less costly because they use half the number of valves as prior art systems and they are expected to be easier to balance.