Abstract:
A control box kit for twinning fan coils in a heat pump or AC installation includes electro-mechanical isolation relays and auxiliary limit switches. Existing fan coil transformers are disconnected to avoid component failure due to high voltage wiring variations. A single transformer is connected to operate both systems. Isolation relays for the reversing valves and supplemental electric heaters in a heat pump system allow both heat pumps to have independent defrost cycles. The accessory transformer and isolation relays are packaged and pre-wired in a control box for easy connection in the field.

Description:
FIELD OF THE INVENTION 
     This invention relates generally to the field of kits for heating and cooling systems, and more particularly to a control box kit for twinning operation of a heating, cooling, or heat pump system. 
     BACKGROUND OF THE INVENTION 
     In a typical heating or cooling system, a controller or control circuit connects a thermostat to a heating or cooling device. The thermostat initiates a demand for heating or cooling which signals the heating/cooling device to turn on and off. The controller or control circuit receives the signal from the thermostat and controls the action of the heating/cooling device. Such heating/cooling devices include furnaces, air conditioners, combined furnace/air conditioner systems which share an air handling system, and heat pumps. 
     Heat pump systems use a refrigerant to carry thermal energy between a relatively hotter side of a circulation loop to a relatively cooler side of the circulation loop. Compression of the refrigerant occurs at the hotter side of the loop, where a compressor raises the temperature of the refrigerant. Evaporation of the refrigerant occurs at the cooler side of the loop, where the refrigerant is allowed to expand, thus resulting in a temperature drop. Thermal energy is added to the refrigerant on one side of the loop and extracted from the refrigerant on the other side, due to the temperature differences between the refrigerant and the indoor and outdoor mediums, respectively, to make use of the outdoor mediums as either a thermal energy source or a thermal energy sink. In the case of an air to water heat pump, outdoor air is used as a thermal energy source while water is used as a thermal energy sink. 
     The process is reversible, so the heat pump can be used for either heating or cooling. Residential heating and cooling units are bidirectional, in that suitable valve and control arrangements selectively direct the refrigerant through indoor and outdoor heat exchangers so that the indoor heat exchanger is on the hot side of the refrigerant circulation loop for heating and on the cool side for cooling. A circulation fan passes indoor air over the indoor heat exchanger and through ducts leading to the indoor space. Return ducts extract air from the indoor space and bring the air back to the indoor heat exchanger. A fan likewise passes ambient air over the outdoor heat exchanger, and releases heat into the open air, or extracts available heat therefrom. 
     In many cases, it is more cost effective or practical to install two residential-type units of moderate capacity than one commercial-type unit of large capacity. Twinned units typically operate off one thermostat. In a one-stage twinned system, both units turn on and off simultaneously. In a two-stage or multi-stage twinned system, the two units cycle separately in a prescribed manner. 
     Under certain operating conditions, frost builds up on a coil of the heat pump. Coil frosting results in lower coil efficiency while affecting the overall performance (heating capacity and coefficient of performance (COP)) of the unit. From time to time, the coil must be defrosted to improve the unit efficiency. In a twinned system, defrosting both units simultaneously is inefficient. 
     U.S. Pat. No. 5,316,073 discloses a twinning control for use on HVAC systems which is based on a microprocessor with LED indicators. Although such a control system works, the relative low volume of twinned fan coil installations does not justify developing a microprocessor based system. Making individual hand connections between thermostats, relays, fan coils, and heating/cooling units is cumbersome and time consuming. 
     SUMMARY OF THE INVENTION 
     Briefly stated, a control kit for twinning fan coils in a heat pump or AC installation includes electro-mechanical isolation relays and auxiliary limit switches. Existing fan coil transformers are disconnected to avoid component failure due to high voltage wiring variations. A single transformer is connected to operate both systems. Isolation relays for the reversing valves and supplemental electric heaters in a heat pump system allow both heat pumps to have independent defrost cycles. The accessory transformer and isolation relays are packaged and pre-wired in a control box for easy connection in the field. The accessory limit switches are integral safety devices to assure safe operation in the case of improper fan coil operation in heating mode. 
     According to an embodiment of the invention, a control box kit for twinning first and second units of an HVAC system, wherein the first and second units are either first and second air conditioning units or first and second heat pump units, and the system includes first and second fan coils and a thermostat, includes a control box; a single transformer which provides low voltage power to both of the units, the transformer disposed inside the control box; and connection wires inside the control box pre-wired for a preselected twinned-unit operating configuration such that an installer makes all field connections for the preselected twinned-unit operating configuration between the thermostat, the first and second fan coils, and the first and second units to the connection wires. 
     According to a feature of the invention, the preselected twinned-unit operating configurations include (a) single-stage operation of the first and second heat pump units; (b) multi-stage operation of the first and second heat pump units; (c) single-stage operation of the first and second air conditioning units; and (d) multi-stage operation of the first and second air conditioning units. 
     According to a feature of the invention, the control box kit further includes a terminal strip attached to the control box, the terminal strip having a plurality of terminal posts extending from outside the control box to inside the control box, wherein the connection wires are connected to the terminal posts inside the control box, and the field connections are made at the terminal strip outside the control box. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a wiring configuration for twinned heat pumps with single stage operation. 
     FIG. 2 shows a wiring configuration for twinned heat pumps with multi-stage operation. 
     FIG. 3 shows a wiring configuration for twinned air conditioners with single stage operation. 
     FIG. 4 shows a wiring configuration for twinned air conditioners with multi-stage operation. 
     FIG. 5 shows a perspective view of a control box used with the wiring configurations of FIGS. 1-4. 
     FIG. 6 shows a perspective view of a control box, with the lid removed, used with the wiring configurations of FIGS. 1-4. 
     FIG. 7 shows a top view of a control box, with the lid removed, used with the wiring configurations of FIGS. 1-4. 
     FIG. 8 shows a terminal strip used with the control box of FIGS. 5-7 to implement the wiring configurations of FIGS.  1 - 4 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The following nomenclature is used for the thermostat, fan coil, and heat pump connection terminals in FIGS. 1-4. 
     
       
         
               
               
               
             
           
               
                   
                   
               
             
             
               
                   
                 R 
                 24-volt hot lead from transformer (fused) 
               
               
                   
                 C 
                 24 volt common lead from transformer 
               
               
                   
                 G 
                 Energizes indoor blower motor 
               
               
                   
                 Y/Y2 or Y 
                 Energizes outdoor unit contactor which controls 
               
               
                   
                   
                 compressor and fan motor 
               
               
                   
                 W/W1 or 
                 Energizes first stage heat on non-heat pump 
               
               
                   
                 W2 
                 systems or second stage heat on heat pump systems 
               
               
                   
                 O 
                 Energizes reversing valve on heat pumps only 
               
               
                   
                 T1 
                 24-volt hot lead from transformer (non-fused) 
               
               
                   
                   
               
             
          
         
       
     
     Referring to FIG. 1, a single-stage control diagram for a twinned heat pump system is shown. The system includes two heat pumps, one thermostat, and two indoor fan coils. In single-stage operation, both heat pumps turn on and off at the same time. 
     In one embodiment, a control system kit  10  is in kit form with all connections between a transformer  12  and relays R 1 , R 2 , and R 3  already made. Wires for connections to a thermostat  14 , fan coils FC 1 , FC 2 , and heat pumps HP 1 , HP 2  are preferably part of control system kit  10  and either labeled or color coded. The connections to thermostat  14 , fan coils FC 1 , FC 2 , and heat pumps HP 1 , HP 2  are made in the field by an installer. Thermostat  14  is preferably capable of at least one stage cooling and two stage heating, such as the Carrier Corporation Model TSTATCCNHP01-B. 
     The sequence of operation for the single-stage system of FIG. 1 is as follows. In the cooling mode, thermostat  14  connects R to O which energizes relay R 3 . The normally open contacts of relay R 3  close, thus energizing reversing valves (not shown) in heat pumps HP 1  and HP 2 . Thermostat  14  connects R to Y/Y 2 , energizing the contactor in each heat pump HP 1 , HP 2  via the Y connections. Thermostat  14  connects R to G, thus energizing the blower motor in each indoor fan coil FC 1 , FC 2 . In heating mode, thermostat  14  connects R to Y/Y 2 , energizing the contactor in each heat pump HP 1 , HP 2  via the Y connections. If the temperature in the space being heated continues to fall, thermostat  14  connects R to W/W 1 , energizing the electric heat relays via the normally closed contacts of relays R 1  and R 2 . Each heat pump HP 1  and HP 2  can enter defrost mode independently. A fixed or variable differential is preferably built into thermostat  14  between stages. In fan only mode, thermostat  14  connects R to G, energizing the blower motor in each indoor fan coil FC 1 , FC 2 . 
     Referring to FIG. 2, a multi-stage control diagram for a twinned heat pump system is shown. The system includes two heat pumps, one thermostat, and two indoor fan coils. In one embodiment, a control system kit  20  is in kit form with all connections between a transformer  12  and relays R 1 , R 2 , and R 3  already made. Wires for connections to a thermostat  22 , fan coils FC 1 , FC 2 , and heat pumps HP 1 , HP 2  are preferably part of control system kit  20  and either labeled or color coded. The connections to thermostat  22 , fan coils FC 1 , FC 2 , and heat pumps HP 1 , HP 2  are made in the field by an installer. Thermostat  22  is preferably capable of two stages of cooling and three stages of heating, such as the Carrier Corporation Model TSTATCCN2S01-B. 
     The sequence of operation for the multi-stage system of FIG. 2 is as follows. In cooling mode, thermostat  22  connects R to O energizing relay R 3 . The two normally open contacts of relay R 3  close, energizing the reversing valve in each heat pump HP 1 , HP 2 . Thermostat  22  connects R to Y 1 , energizing the contactor in heat pump HP 1 , i.e., first stage cooling. Thermostat  22  connects R to G energizing the blower motor in each indoor fan coil FC 1 , FC 2 . If the temperature in the conditioned space continues to rise, thermostat  14  connects R to Y/Y 2  which energizes the contactor in HP 2 , i.e., second stage cooling. A fixed or variable differential is preferably built into thermostat  14  between stages. In heating mode, first stage, thermostat  22  connects R to Y 1 , energizing the contactor in heat pump HP 1 . Thermostat  22  connects R to G energizing the blower motor in each indoor fan coil FC 1 , FC 2 . If the temperature continues to fall in the conditioned space while operating in first stage heating, thermostat  14  connects R to Y/Y 2  which energizes the contactor in heat pump HP 2 , i.e., second stage heating. A fixed or variable differential is preferably built into thermostat  22  for all stages. In heating mode, third stage, thermostat  22  connects R to W/W 1 . 24 volts from W/W 1  is fed through the normally closed contacts of relays R 1  and R 2  to both W 2  terminals of fan coils FC 1 , FC 2 . The electric heat relays are energized bringing on supplemental heat. Note that in first stage heating, one heat pump and both indoor fan coils are operating. Entering second stage heating adds the second heat pump, while entering third stage heating adds the supplemental heat source. 
     In the defrost mode for heat pump HP 1 , the defrost control in heat pump HP 1  energizes the reversing valve when defrost is needed. The defrost control also sends 24 volts to the W 2  terminal of heat pump HP 1  and to relay R 1 . Relay R 1  is energized, thus closing its normally open contacts and opening its normally closed contacts. 24 volts is fed from R to the W 2  terminal of fan coil FC 1 . The electric heat relay is energized bringing on supplemental heat during defrost. In the defrost mode for heat pump HP 2 , the defrost control in heat pump HP 2  energizes the reversing valve when defrost is needed. The defrost control also sends 24 volts to the W 2  terminal of heat pump HP 2  and to relay R 2 . Relay R 2  is energized, thus closing its normally open contacts and opening its normally closed contacts. 24 volts is fed from R to terminal W 2  terminal of fan coil FC 2 . The electric heat relay is energized bringing on supplemental heat during defrost. Each heat pump HP 1  and HP 2  can enter defrost mode independently. 
     In fan only mode, thermostat  22  connects R to G energizing the blower motor in each indoor fan coil FC 1 , FC 2 . 
     Referring to FIG. 3, a single-stage control diagram for a twinned air conditioner system is shown. The system includes two air conditioners, one thermostat, and two indoor fan coils. In single-stage operation, both air conditioners turn on and off at the same time. In one embodiment, a control system kit  30  is in kit form with all internal kit connections already made. Kit  30  preferably includes a transformer  12 . Wires for connections to a thermostat  32 , fan coils FC 1 , FC 2 , and air conditioners AC 1 , AC 2  are preferably part of control system kit  30  and either labeled or color coded. The connections to thermostat  32 , fan coils FC 1 , FC 2 , and air conditioners AC 1 , AC 2  are made in the field by an installer. Thermostat  32  is preferably capable of at least one stage of cooling and heating, such as the Carrier Corporation Model TSTATCCNAC01-B. 
     The sequence of operation for the single-stage system of FIG. 3 is as follows. In the cooling mode, thermostat  32  connects R to Y/Y 2 , energizing the contactor in each air conditioner AC 1 , AC 2  via the Y connections. Thermostat  14  connects R to G, thus energizing the blower motor in each indoor fan coil FC 1 , FC 2 . If, as is common, air conditioners AC 1 , AC 2  are part of a heating/cooling system that includes resistance heating (or hot water heat), the system is capable of going into heating mode. In heating mode, thermostat  32  connects R to W/W 1 , energizing the electric heat (via W 2 ) in each fan coil FC 1 , FC 2 . In fan only mode, thermostat  14  connects R to G, energizing the blower motor in each indoor fan coil FC 1 , FC 2 . 
     Referring to FIG. 4, a multi-stage control diagram for a twinned air conditioner system is shown. The system includes two air conditioners, one thermostat, and two indoor fan coils. In one embodiment, a control system kit  40  is in kit form with all internal kit connections already made. Kit  40  preferably includes a transformer  12 . Wires for connections to a thermostat  42 , fan coils FC 1 , FC 2 , and air conditioners AC 1 , AC 2  are preferably part of control system kit  40  and either labeled or color coded. The connections to thermostat  42 , fan coils FC 1 , FC 2 , and air conditioners AC 1 , AC 2  are made in the field by an installer. Thermostat  42  is preferably capable of two stages of cooling and two stages of heating, such as the Carrier Corporation Model TSTATCCN2S01-B. 
     The sequence of operation for the multi-stage system of FIG. 4 is as follows. In cooling mode, first stage, thermostat  42  connects R to Y 1 , energizing the contactor in air conditioner AC 1 . Thermostat  42  connects R to G energizing the blower motor in each indoor fan coil FC 1 , FC 2 . If the temperature continues to rise in the conditioned space while operating in first stage cooling, the system enters cooling mode, second stage. A fixed or variable differential is preferably built into thermostat  42  between stages. In cooling mode, second stage, thermostat  42  connects R to Y/Y 2 , energizing the contactor in air conditioner AC 2 . Note that in first stage cooling, one air conditioner and both indoor fan coils are operating, while entering second stage cooling adds the second air conditioner. If, as is common, air conditioners AC 1 , AC 2  are part of a heating/cooling system that includes resistance heating (or hot water heat), the system is capable of going into heating mode. In heating mode, thermostat  42  connects R to W/W 1  which energizes the electric heat relay in FC 1 . W/W 1  also energizes relay R 1  which connects R to G via R 1  contacts, thus energizing blower motors in fan coils FC 1  and FC 2 . If the temperature continues to fall in the conditioned space while operating in first stage heating, thermostat  42  connects R to O/W 2 , energizing the electric heat relay (via W 2 ) in fan coil FC 2 . In fan only mode, thermostat  42  connects R to G energizing the blower motor in each indoor fan coil FC 1 , FC 2 . 
     In the embodiments of FIGS. 1-4, auxiliary limit switches ALS 1  and ALS 2  are shown in series between transformer  12  and R. Primary limit switches are conventionally part of an HVAC system which provides heating. In the event of blower failure and the consequent heat buildup, the primary limit switches trip due to the excessive heat. Typically, primary limit switches automatically reset when the temperature drops. During twinned operation, however, if blower failure occurs in one fan coil but not the other, it is possible that recirculation of air from the second fan coil is sufficient to keep the primary limit switch from tripping. Therefore, as an additional safety precaution, auxiliary limit switches ALS 1  and ALS 2  are attached to the respective blower housings. Auxiliary limit switches ALS 1  and ALS 2  are preferably manual reset switches instead of automatic reset switches. 
     Referring to FIGS. 5-7, a control box  50  is preferably of metal. Transformer  12 , circuit breaker CB, and relays R 1 , R 2 , and R 3  are shown mounted into control box  50 . A portion of the front panel of control box  50  is cut away, permitting a terminal strip  52  to be fastened to control box  50  by a pair of fasteners such as screws  54 . Terminal strip  52  includes a plurality of terminal posts  56  held in place by a non-conductive plate  58 . Each terminal post  56  preferably includes a screw post  60  on one end and either one or two standard male terminal blade connectors  62  on the other. The wires from control system kits  10 ,  20 ,  30 ,  40  preferably are terminated with female terminal connectors  64  to facilitate attachment to terminal posts  56 . The wires shown in FIGS. 5-7 are for illustrative purposes only and are not intended to reflect any particular wiring arrangement. 
     Referring to FIG. 8, each of the screw posts  60  of terminal strip  52  are preferably labeled with the external connections to be made. An example of standardized labels are shown, along with a legend, which correspond to the legends of FIGS. 1-4. The field installer affixes control box  50  in any suitable location within a cabinet of the system being installed, but preferably to one of the fan coils in a suitable location, and runs wires from terminal strip  52  to the thermostat, indoor fans, heat pumps, and/or air conditioners as necessary to implement one of the configurations of FIGS. 1-4. The wires necessary for the field installation external of control box  50  are optionally packaged along with control box  50 . 
     In an alternative embodiment, the transformer, relays, and terminal connectors are mounted on a printed circuit board (PCB), with all the wiring incorporated into the PCB traces. 
     While the present invention has been described with reference to a particular preferred embodiment and the accompanying drawings, it will be understood by those skilled in the art that the invention is not limited to the preferred embodiment and that various modifications and the like could be made thereto without departing from the scope of the invention as defined in the following claims.