Abstract:
A heat pump system ( 10 ) includes a fluid refrigerant compressor ( 12 ) and heat exchanger units ( 14, 16 ). Heat transfer tubings ( 36, 50, 58, 60, 66, 82 ) interconnect the fluid refrigerant compressor ( 12 ) and the heat exchanger units ( 14, 16 ) in a series relationship for carrying refrigerant fluid. Flow control valves ( 22, 24 ) are provided and are interconnected between a treated waste effluent source ( 20 ) such as a municipal or private treated waste effluent supply or reservoir and the heat exchanger unit ( 16 ) for controlling the amount of water flowing from the treated waste effluent source ( 20 ) to the heat exchanger unit ( 16 ). The flow control valves ( 22, 24 ) are responsive to the pressure at the outlet ( 12   b ) of the fluid refrigerant compressor ( 12 ) sensed at port ( 26 ) in the heat transfer tubing ( 58 ) to automatically optimize the operating condition of the heat pump system ( 10 ). To further optimize the heat pump system ( 10 ) in the heating mode, water from treated waste effluent source ( 20 ) flows through preheat exchanger ( 104 ) before entering heat exchanger unit ( 16 ).

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation-in-part of U. S. patent application Ser. No. 09/227,246 filed Jan. 8, 1999. 
    
    
     TECHNICAL FIELD OF THE INVENTION 
     This invention relates to heat pumps, and more particularly to a water to air heat pump system which is automatically thermodynamically balanced to operate at optimum conditions using a variety of water sources, primarily from municipal water mains or privately owned water systems and preferably using treated water effluent such as gray water and reuse water, with the intent of returning used water to the water main or other source with absolutely no contamination or reduction in volume of water with only a slight temperature change. 
     BACKGROUND OF THE INVENTION 
     Although the heat pump principle is not new, extensive use of this energy concept in practical devices has only been recently accomplished. Now that energy conservation is of prime importance, greater use of heat pump systems is being made to save energy and achieve lower initial costs of heating and cooling equipment. Heat pumping in its simplest terms is described as pumping heat from a low energy level to a high energy level and using the resulting heat for space and domestic water heating. 
     Many forms of heat pump systems have been devised. These systems normally include a refrigerant fluid compressor that is interconnected with two heat exchanger units. The two heat exchanger units are alternatively operated as evaporators or condensers depending upon the positioning of a directional control valve in the interconnecting refrigerant fluid conduit for heating or cooling modes of operation. One heat exchanger unit is associated with heating or cooling apparatus, such as a fan and coil type condenser or evaporator. The other heat exchanger unit is operated to either add heat to the system or remove heat by dissipation. 
     An efficient mechanism to add or draw heat away from the heat pump system is to transfer heat between the heat exchanger unit of the heat pump system and a circulating loop of a heat transfer fluid, usually water. The question then arises how the change in the temperature of the water used to cool or heat the heat exchanger unit of the heat pump system is accommodated. The heat loss or gain in the water must be compensated for, or the water temperature will increase or decrease, as the case may be, beyond tolerable levels. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention, a heat pump system is provided which substantially eliminates the problems heretofore associated with water source heat pump systems, including lack of compensation for temperature variations in the water source. Preferably, the water source is treated water effluent such as gray water and reuse water, which is returned to the municipal or other source from which it came without contamination or reduction in volume. 
     In accordance with the present invention, a heat pump system is provided having a fluid refrigerant compressor. The heat pump system further includes first and second heat exchanger units. Fluid conduit interconnects the fluid refrigerant compressor and the first and second heat exchanger units in a series relationship. 
     In accordance with another aspect of the present invention, a heat pump system having two stages of operation for selectively heating and cooling is provided. The system includes a fluid refrigerant compressor having an inlet and an outlet and includes a refrigerant fluid. A first heat exchanger unit is operable as a condenser in the system for heating and as an evaporator in the system for cooling. A second heat exchanger unit includes a water source and is operable as an evaporator in the system for heating and as a condenser in the system for cooling mode. Fluid conduit is provided for interconnecting the fluid refrigerant compressor and the first and second heat exchanger units in a series relationship. The heat pump system further includes a valve interconnected in the fluid conduit intermediate the fluid refrigerant compressor and the first and second heat exchanger units for routing the refrigerant fluid in a first direction in the system heating mode of operation and in a second direction in the system cooling mode of operation through the first and second heat exchanger units in opposite directions. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     For a more complete understanding of the present invention and for further advantages thereof, reference is now made to the following Detailed Description, taken in conjunction with the accompanying Drawings in which: 
     FIG. 1 is a schematic block diagram of the present heat pump system; 
     FIG. 2 is a primary loop schematic; 
     FIG. 3 is a secondary loop pump piping schematic with base mounted pumps for 20 tons and up; and 
     FIG. 4 is a secondary loop pump piping schematic with in line pumps up to 200 tons. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to FIG. 1, a schematic block diagram of the present heat pump system is illustrated and is generally identified by the numeral  10 . Heat pump system  10  includes a fluid refrigerant compressor  12  and two heat exchanger units  14  and  16 . Heat exchanger unit  14  functions as a condenser in the heating mode and as an evaporator in the cooling mode of heat pump system  10  to heat or cool an air space  18 . Heat exchanger unit  16  functions as an evaporator in the heating mode of operation and as a condenser in the cooling mode of operation of heat pump system  10  for receiving heat or transferring heat to water circulating through heat exchanger unit  16 . 
     Water is supplied to heat exchanger unit  16  from a water source  20 . An important aspect of the present invention is that water source  20  is provided from a city, town or development water main. The water is then returned directly into water source  20  with no contamination or reduction in volume taking place. Preferably, the water source  20  used is a source of treated waste effluent such as gray water and reuse water. Treated waste effluent is water that has not yet been sufficiently purified and clarified to be considered potable water. Because of water conservation demands, and water treatment costs, considerations have been given to providing treated waste effluent water lines to a residence or business for use in non potable water needs, such as toilet flushing, lawn watering and the like. Also, water source  20  may comprise, for example, a well, stream or a body of water such as an ocean or lake. Additionally, water source  20  may comprise a closed system such as an above ground or underground water storage tank or underground piping loop system. 
     Treated waste effluent water includes reclaimed water that has been defined as Type I and Type II reclaimed water. Type I reclaimed water is for uses including irrigation or other uses in areas where the public may be present during the time when irrigation takes place or other uses where the public may come into contact with the reclaimed water. The following conditions apply to Type I reclaimed water: BOD 5  or CBOD 5  of 5 mg/L, Turbidity 3 NTU, Fecal Coliform 20 CFU/100 ml (geometric mean), and Fecal Coliform (not to exceed) 75 CFU/100 ml (single grab sample). Type II reclaimed water is for use where the public would not come into contact with the reclaimed water. The following conditions apply to Type II reclaimed water: BOD 5  20 mg/L, or CBOD 5  15 mg/L, Fecal Coliform 200 CFU/100 ml (geometric mean), and Fecal Coliform (not to exceed) 800 CFU/100 ml (single grab sample). The use of reclaimed water is regulated by the states. Reclaimed water projects can be found in most states, and reclaimed water is used for irrigation, cooling tower make up water, fire fighting, industrial processes, and road construction. Although normally less involved, permitting is similar to that required for the use of potable water, and main pipe materials and tapping procedures are the same. 
     Flow control valves  22  and  24  are disposed between water source  20  and the circulating input to heat exchanger unit  16 . Flow control valves  22  and  24  function to control the amount of water flowing from water source  20  to heat exchanger unit  16  depending upon the temperature of the water in water source  20 . Flow control valves  22  and  24  are actuated by refrigerant pressure through port  26  from heat transfer tubing  58 , which in turn are selectively actuated depending upon the mode of operation of heat pump system  10 . 
     Water flows from heat exchanger  16  through flow control valve  22  in the heating mode of operation or through flow control valve  24  in the cooling mode of operation of heat pump system  10  through a conduit  32  to water source  20 . Flow control valves  22  and  24  are pressure actuated and respond to pressure at port  26  in heat transfer tubing  58 . Pressure changes in heat transfer tubing  58  are transmitted via fluid lines  34   a  and  34   b  to flow control valves  22  and  24 , respectively. Flow control valve  22  operates to maintain a high level of energy in heat pump system  10  during the heating mode of operation and flow control valve  24  operates to maintain a low level of energy in the heat pump system  10  during the cooling mode of operation. 
     Fluid refrigerant compressor  12  is a positive displacement compressor which reduces the volume of refrigerant fluid such as, for example, freon gas through compression. Reduction of the volume of the refrigerant fluid also increases the temperature of the gas. For example, a 3.5 ton compressor develops gas having discharge temperature of approximately 219° F. and a discharge pressure of 298 p.s.i.g. The discharge gas from fluid refrigerant compressor  12  is in a condition referred to as superheat, meaning that the gas is at a high temperature and high pressure such that the temperature of the gas is above the temperature at which the gas will condense at that pressure. 
     The superheated refrigerant fluid flows through heat transfer tubing  36  to a domestic hot water coil  38  having an inlet  38   a  and outlet  38   b . Domestic hot water coil  38  is interconnected to a hot water tank  40  via conduits  42  and  44 . Water circulates under the control of a pump  46  disposed in conduit  44  between hot water tank  40  and domestic hot water coil  38 . The superheated gas flowing from fluid refrigerant compressor  12  via heat transfer tubing  36  through domestic hot water coil  38  loses part of its superheat to the water stored in hot water tank  40 . Domestic hot water coil  38  functions as a desuperheater to elevate the temperature of the water stored in hot water tank  40  regardless of the mode of operation of heat pump system  10 . 
     High pressure, high temperature refrigerant fluid in the form of a gas flows from outlet  38   b  of domestic hot water coil  38  via heat transfer tubing  50  to a reversing valve  52 . Reversing valve  52  includes a solenoid  54  whose operation is controlled by a thermostat  56 . Depending upon the mode of operation of heat pump system  10 , as controlled by thermostat  56 , solenoid  54  will be actuated to direct the high pressure, high temperature gas in one of two directions. Reversing valve  52  will direct refrigerant fluid via heat transfer tubing  58  to heat exchanger unit  16  in the cooling mode of operation of heat pump system  10  and via heat transfer tubing  60  to heat exchanger unit  14  in the heating mode of operation in the heat pump system  10 . 
     In the heating mode of operation of heat pump system  10 , the hot refrigerant fluid passes via heat transfer tubing  60  to port  14   a  of heat exchanger unit  14 . Heat exchanger unit  14  may comprise, for example, a fin type coil having fins  62 . The heat contained within the refrigerant fluid circulating within heat exchanger unit  14  is removed to the air stream passing through fins  62  by operation of a fan  64  to thereby heat the air space  18 . In passing through heat exchanger unit  14 , the refrigerant fluid gas condenses to the liquid phase thereby releasing its latent heat to heat exchanger unit  14  and, in turn, to air space  18 . 
     The refrigerant fluid now in the form of a liquid exits port  14   b  of heat exchanger unit  14  at high pressure and flows via heat transfer tubing  66  to a receiver tank  68 . Receiver tank  68  acts as an accumulator for excess liquid during periods of any load fluctuations. Interconnected within heat transfer tubing  66  is a thermostatic expansion valve  70  having a temperature and pressure sensor  72  at port  14   a  of heat exchanger unit  14 . Connected in parallel with thermostatic expansion valve  70  is a bypass valve  74 . In the heating mode of operation of heat pump system  10 , refrigerant fluid flows through bypass valve  74  to bypass thermostatic expansion valve  70 . 
     The high pressure refrigerant in the form of a liquid flows from receiver tank  68  via heat transfer tubing  82  to port  16   a  of heat exchanger unit  16 . Disposed within heat transfer tubing  82  is a thermostatic expansion valve  84  having a temperature and pressure sensor  86  interconnected at port  16   b  of heat exchanger unit  16 . Interconnected in parallel across thermostatic expansion valve  84  is a bypass valve  88  which is not utilized in the heating mode of heat pump system  10 . Also interconnected in heat transfer tubing  82  is a moisture indicator  80  which indicates sub-cooling with all liquid and no bubbles present in a sight glass contained in moisture indicator  80 . 
     Thermostatic expansion valve  84  functions to reduce the refrigerant fluid pressure from the system&#39;s high pressure side to the low pressure side of thermostatic expansion valve  84 , such that the refrigerant fluid flashes back to a vapor due to the rapid drop in pressure caused by thermostatic expansion valve  84 . Heat exchanger unit  16  functioning as an evaporator furnishes the heat required by the change of state of the refrigerant fluid. Where fluid refrigerant compressor  12  is a 3.5 ton unit, heat exchanger unit  16  would operate at a refrigerant fluid temperature of about 45° F. and at 82 p.s.i.g. pressure. The pressure within heat pump system  10  is thereby reduced from the pressure at outlet  12   b  of fluid refrigerant compressor  12 . Heat exchanger unit  16  acts as an evaporator and the heat required by the evaporation of the refrigerant fluid is furnished by the water circulating through heat exchanger  16  from water source  20 . 
     As the pressure in the heat transfer tubing  58  at port  26  varies due to the temperature of the water stored within water source  20 , the amount of water flowing from water source  20  to heat exchanger unit  16  will vary. The difference in the temperature of the water flowing from water source  20  into heat exchanger unit  16  affects the amount of heat removal that takes place in heat pump system  10  which is reflected through the balance of the system which will change the pressure in the heat transfer tubing  58  at port  26 . This pressure change will modulate flow control valve  22  in the heating mode of operation of heat pump system  10  so that the heat balance and thermodynamic balance of heat pump system  10  will automatically be adjusted. In the heating mode of operation of heat pump system  10 , as the temperature of water source  20  decreases, more water is necessary to maintain the thermodynamic balance within heat pump system  10  and therefore flow control valve  22  allows more water to circulate through heat exchanger unit  16  to thereby extract more heat from the circulating water. 
     An important aspect of the present system is the use of preheat exchanger  104  and the 3-way solenoid valve  98 . The purpose for the preheat exchanger  104  is to maintain the temperature of source water  20  entering the heat exchanger  16  above approximately 45° F. to enable the heat pump system  10  to function efficiently at low source water  20  temperature. Low source water  20  temperature will exist as low as 34° F. during the winter in city water mains, lakes, rivers and above-ground holding tanks, thus necessitating the use of preheat exchanger  104  which will raise the temperature of incoming water from 5° F. to 20° F. depending on the flow of water regulated by flow control valve  22 . 
     Preheat exchanger  104  is used in the heating mode of heat pump system  10 . This necessitates the use of 3-way solenoid valve  98  to bypass the preheat exchanger  104  in the cooling mode of heat pump system  10 . The 3-way solenoid valve  98  is controlled by the thermostat  99 . 
     During the heating mode of heat pump system  10 , the source water  20  flows via conduit  30 , through 3-way solenoid valve  98 , via conduit  30   a  through pre-heat exchanger  104 , via conduit  30   b , through check valve  76 , through circulating pump  78 , via conduit  30   d  to heat exchanger  16 . 
     During the cooling mode of heat pump system  10 , the source water  20  flows via conduit  30 , through 3-way solenoid valve  98 , via conduit  30   c , through circulating pump  78 , via conduit  30   d , to heat exchanger  16 . 
     Heat for preheat exchanger  104  is supplied by hot water that was heated by superheated refrigerant in domestic hot water coil  38 . This hot water flows from domestic hot water coil  38  via conduit  42 , through preheat exchanger  104 , via conduit  42   a , to hot water tank  40 . 
     Refrigerant fluid in the form of gas flows from port  16   b  of heat exchanger unit  16  via heat transfer tubing  58  back to reversing valve  52  at low pressure. Reversing valve  52  now causes the refrigerant fluid to flow via heat transfer tubing  102  to suction line accumulator  100  for any excess liquid during periods of any load fluctuation. The low pressure gas flows via heat transfer tubing  90  to a filter-drier  92  which cleans and dries the refrigerant fluid for return to inlet  12   a  of fluid refrigerant compressor  12  via heat transfer tubing  94 . 
     In the cooling mode of operation of heat pump system  10 , the refrigerant fluid from fluid refrigerant compressor  12  passes through domestic hot water coil  38  which functions in the same manner as in the heating mode of operation of heat pump system  10  previously described. The high pressure, high temperature refrigerant fluid passes through heat transfer tubing  50  to reversing valve  52 . Reversing valve  52  functions in the cooling mode to route the flow of refrigerant fluid through heat transfer tubing  58  to heat exchanger unit  16  at port  16   b . Heat exchanger unit  16  now functions as a condenser, such that the refrigerant fluid gives up its heat to the water circulating within heat exchanger unit  16  and thereby becomes a liquid at high pressure. The thermodynamic balance of heat pump system  10  is maintained by flow control valve  24  modulating the water flow from water source  20  in response to pressure and temperature variations of the heat transfer tubing  58  via port  26 . 
     The refrigerant exiting port  16   a  of heat exchanger unit  16  in the form of a liquid passes to receiver tank  68  via bypass valve  88  and heat transfer tubing  82 . The refrigerant fluid then passes through moisture indicator  80  and then heat transfer tubing  66  to thermostatic expansion valve  70 . Thermostatic expansion valve  70  functions to reduce the pressure of the refrigerant fluid from, for example, 298 p.s.i.g. at 219° F. to 82 p.s.i.g. pressure at 45° F. for the example where a 3.5 ton fluid refrigerant compressor  12  is utilized. 
     With the reduction in pressure caused by thermostatic expansion valve  70 , the refrigerant fluid passes from a liquid state to a gas state. The gas passes through heat exchanger unit  14  from port  14   b  to port  14   a . Heat exchanger unit  14  now functions as an evaporator to thereby remove heat passing through the fins  62  and thereby cool air space  18 . 
     The refrigerant fluid is then transferred by heat transfer tubing  60  to reversing valve  52  which diverts the refrigerant fluid in the form of a gas at low pressure via heat transfer tubing  102  to suction line accumulator  100  and via heat transfer tubing  90  to filter-drier  92 . The gas then flows from filter-drier  92  via heat transfer tubing  94  to inlet  12   a  of fluid refrigerant compressor  12 . 
     It therefore can be seen that in the cooling mode of operation of heat pump system  10 , flow control valve  24  functions to modulate the flow of water from water source  20  to ensure the proper flow of water based upon the amount of heat to be extracted or transferred to the water within water source  20 . Flow control valve  24  is operated in response to pressure at port  26  from heat transfer tubing  58  to ensure that heat pump system  10  is contained in an equilibrium to operate at optimum efficiency conditions. As the temperature of the water from water source  20  decreases, less water is needed for heat exchanger unit  16  in the cooling mode of operation of heat pump system  10 . It therefore can be seen that flow control valves  22  and  24  operate in opposite directions for increasing or decreasing the flow of water from water source  20  to heat exchanger unit  16 . 
     It therefore can be seen that the present heat pump system  10  operates to place the system automatically in thermodynamic balance to operate at optimum operating conditions independent of the temperature of the water or air utilized as the energy source. 
     With reference now to FIGS. 2-4, a heat pump system  190  forming a second embodiment of the present invention will be described. Heat pump system  190  is designed for use in an environment, such as an office, which has multiple heat pumps  10  to condition different areas of the building. Of course, heat pump system  190  can also be used with just a single heat pump  10 . The heat pump system  190  has a primary loop for water flow from the water source  20  which flows through the primary side of a dedicated heat exchanger  212 . A secondary loop containing a heat transfer liquid, such as water with anti-freeze, flows through the secondary side of the heat exchanger  212  and is connected to the heat exchanger units  16  of each of the heat pumps  10  in series. One advantage of such a system is that in a structure with different climate zones, it may be only necessary to remove heat from one zone and transfer it to another zone by use of the liquid in the secondary loop. In such a case no net heat transfer would occur between the primary loop and the secondary loop and circulation would not be required in the primary loop, allowing the primary loop pump to be off to save energy. 
     FIG. 2 illustrates the primary loop with a water main  200  which is connected to the water source  20 . Taps  202  and  204  are made into the water main  200 , with the supply tap  202  being upstream of the return tap  204 . It is desirable to place the supply and return taps as far apart as possible, to avoid short circuiting of the primary loop water. Typically, the taps have a head loss of 1-9 feet. 
     A supply line  206  extends from the supply tap  202  to the inlet of a supply pump  208 . The pump acts to draw water from the water main  200  and discharge it into discharge line  210  which enters the inlet side  214  of a heat exchanger  212 . The water flows through the inlet side  214  of the heat exchanger and is discharged into return line  216  for return to the water main at return tap  204 . The inlet side  214  does not permit flow of the water from the water main  200  to the outlet side  218  of the heat exchanger. Only thermal energy can be transferred between the inlet side  214  and the outlet side  218 , thus protecting the water in the water main from any contamination from the system. Temperature probes  220 - 226  can be used in the discharge line  210  and return line  216  and in the inlet line  228  and return line  230  of the outlet side  218  to monitor the temperatures. Preferably, supply line  206  is sized for 1-3 foot head loss per 100 feet equivalent length. 
     With reference now to FIG. 3, a typical secondary loop pump piping for base mounted pumps, 20 tons and up is illustrated. The discharge line  230  extends from the outlet side  218  of heat exchanger  212  to the heat pump or pumps  10  (not shown) to provide heat to or remove heat from the heat exchanger units  16  as needed. From the heat pumps  10 , the working fluid in the secondary loop, typically water, passes through return  250  to an air separator  252 , which acts to remove any air from the working fluid and discharge it through high capacity air vent  254  with valve. 
     A working fluid make-up system  256  is also used. If working fluid is lost from the system, it will be replaced by working fluid in the make up water line  258 , which passes through a reduced pressure principle back flow preventor  260 , a ball valve  262  and a strainer  264 . The make-up fluid then flows through a line  266  having an automatic fill valve  268  and a water meter  270  to monitor make-up flow. A by pass line  272  with a bypass valve  274  can be used to permit water make-up flow to the separator without passing through the water meter  270 , if desired. 
     The air separator  252  also preferably mounts a pressure gauge  276  with a shut off valve  278  and boiler drain  280 . The air separator also preferably has a drain  282  with cap. 
     From the separator  252 , the working fluid passes through line  300  to the inlets  302  and  304  of primary pump  306  and back-up pump  308  which circulate the working fluid in the secondary loop. Line  300  is also connected to a bladder type expansion tank  310  through a ball valve  309  with the handle locked open. The expansion tank  310  prevents over pressure conditions in the secondary loop. The design operating pressures at the tank should be 20 psi minimum and 27 psi maximum. The tank  310  has a pressure bladder therein that is compatible with propylene glycol and has a high capacity air vent  311  with valve  313 . Line  300  is also connected to the inlet of a by-pass feeder  312 . 
     The discharge lines  314  and  316  of pumps  306  and  308  connect to the return line  228  to the heat exchanger  212  through triple duty valves  318 , pressure relief valves  320  and butterfly valves  322 . Vibration absorbers  324  isolate the vibration of pumps  306  and  308  from the remainder of the system. The outlet line  326  of the by-pass feeder  312  also connects to the return line. The pressure relief valves should be set to a pressure at least 10 psi below, and preferably 20 psi below, the pressure in the primary loop with water from the water source  20 . This acts to prevent a leak from the secondary loop into the primary loop. 
     With reference to FIG. 4, a typical secondary loop pump piping for in line pumps up to 200 tons is illustrated. Many components are the same as shown in FIG.  3  and are identified with the same reference numerals. However, in the piping of FIG. 4, the line  300  connects to the inlet line  350  of a single circulating pump  352  which circulates the working fluid in the secondary loop. A shut off valve  354  and strainer  356  with boiler drain are positioned in the inlet line  350 . The outlet line  358  of the pump  352  connects to the return line  228  and includes a pressure relief valve  360 . Union or pump flanges  362  permit the pump  352  to be removed for servicing and replacement. The outlet line  326  of the by-pass feeder  312  is connected directly into the return line  228 . 
     For optimal system efficiency, the source water temperatures should be between 45 F. (In the heating season) and 80 F. (In cooling season). Water main temperatures are usually within this range, but must be verified to ensure efficient, reliable system operation. If water temperatures are outside this range, the system will still operate, but additional flow will be needed, and system efficiency will be reduced. 
     To minimize pumping power consumption and cost, the total supply and return head loss should be ten feet or less. 
     The heat exchanger  212  is a key element in the system, which ensures that no cross contamination of the water supply system can take place. To prevent the possibility of cross contamination by mixing of water between the primary and secondary sides of the heat exchanger, vented double wall construction is preferred. All components of the heat exchanger in contact with the water source  20  should be of materials rated and approved for such use. That requirement includes the gasket material in a plate and frame heat exchanger and the soldering or brazing material in a tube in tube heat exchanger. Also, the secondary loop should preferably be at a pressure of 20 or more psi lower that the pressure in the primary loop to reduce the chances of contamination. 
     The primary loop pump  208  circulates water from the main, through the intermediate heat exchanger  212 , and back to the main. It must be constructed of materials rated and approved for use in contact with the water from water source  20 . The pump should be selected based on the desired flow rate and total head loss of the primary loop. A variable frequency drive(VFD) on the pump  208  can save operating costs on pumps of 5 HP and greater. One suggested control sequence is to control the VFD by a temperature sensor in the circulated water leaving the heat exchanger in line  216 . On a rise in leaving water temperature above 75 F. (Adjustable) the pump speed shall increase to maintain 75 F. (Adjustable) leaving water temperature. A dead band would exist between 75 F. (Adjustable) and 50 F. (Adjustable) where the pump shall not run. On a fall in leaving water temperature below 50 F. (Adjustable), the pump speed increases to maintain 50 F. (Adjustable) leaving water temperature. As noted, when multiple heat pumps  10  are connected to the secondary loop, it may be possible to transfer heat from just one conditioned zone to another to maintain both zones at the desired temperature. In such an event, the primary loop pump would not need to be operated and the temperature of the water in the primary loop would be in the dead band. 
     In the secondary loop of the system, it may be desirable to use antifreeze. For piping, PVC and copper are commonly used. Steel pipe is not recommended in the secondary loop because dielectric unions are necessary where transitioning to other metals and water treatment chemicals are necessary for corrosion prevention. Pipes should be sized to minimize pumping cost, so design head losses between 1 and 3 feet head per hundred feet equivalent length are desirable. 
     Whereas the present invention has been described with respect to specific embodiments thereof, it will be understood that various changes and modifications will be suggested to one skilled in the art and it is intended to encompass such changes and modifications as fall within the scope of the appended claims.