Multi-circuit HVAC system

1. A heating, ventilation, and/or air conditioning (HVAC) system, comprising:

a first refrigeration circuit having a first compressor and a first condenser, wherein the first compressor is configured to urge refrigerant in the first refrigeration circuit in a direction upstream to downstream in the first refrigeration circuit;

a second refrigeration circuit having a second compressor and a second condenser, wherein the second compressor is configured to urge refrigerant in the second refrigeration circuit in a direction upstream to downstream in the second refrigeration circuit;

a conduit extending from a portion of the first refrigeration circuit that is downstream of the first condenser to a portion of the second refrigeration circuit that is upstream of the second compressor;

a valve disposed along the conduit and configured to manage refrigerant flow therethrough; and

a controller configured to operate the HVAC system in a first mode such that the first refrigeration circuit separately circulates the refrigerant in the first refrigeration circuit and the second refrigeration circuit separately circulates the refrigerant in the second refrigeration circuit, and configured to transition to a second mode such that refrigerant-sharing occurs between portions of the first and second refrigeration circuits, wherein, in response to a request to transition from the second mode to the first mode, the controller is configured to:

determine an amount of subcooling of refrigerant downstream of the second condenser;

compare the amount of subcooling to a threshold value of subcooling associated with the first mode; and

instruct opening of the valve upon a determination that the amount of subcooling is less than the threshold value of subcooling associated with the first mode.

2. The HVAC system of claim 1 , wherein the controller is configured to:

operate the first compressor and suspend operation of the second compressor during the second mode;

activate the second compressor after receiving the request to transition from the second mode to the first mode and prior to determining the amount of subcooling; and

deactivate the first compressor after receiving the request to transition from the second mode to the first mode and after the determination that the amount of subcooling is less than the threshold value of subcooling associated with the first mode.

3. The HVAC system of claim 2 , wherein the amount of subcooling is a first amount of subcooling, and wherein the controller is configured to:

determine a second amount of subcooling of refrigerant downstream of the second condenser after activating the second compressor;

compare the second amount of subcooling of refrigerant to the threshold value of subcooling; and

instruct closing of the valve upon a determination that the second amount of subcooling is greater than or equal to the threshold value of subcooling associated with the first mode.

4. The HVAC system of claim 3 , wherein the controller is configured to activate the first compressor after instructing closing of the valve.

5. The HVAC system of claim 1 , wherein the controller is configured to operate the first compressor and suspend operation of the second compressor during the second mode and, after receiving the request to transition from the second mode to the first mode:

detect a lubricant level in a second sump of the second compressor;

compare the lubricant level to a threshold value of lubricant associated with the first mode; and

instruct opening of a lubricant valve disposed along a lubricant conduit extending between the second sump a first sump of the first compressor upon a determination that the lubricant level is less than the threshold value of lubricant.

6. The HVAC system of claim 5 , wherein the controller is configured to:

deactivate the first compressor after comparing the lubricant level to the threshold value of lubricant and prior to instructing opening of the lubricant valve; and

activate the second compressor after instructing opening of the lubricant valve.

7. The HVAC system of claim 6 , wherein the lubricant level in the second sump is a first lubricant level, and wherein the controller is configured to, after activating the second compressor:

detect a second lubricant level in the second sump;

compare the second lubricant level to the threshold value of lubricant; and

instruct closing of the lubricant valve upon a determination that the second lubricant level is greater than or equal to the threshold value of lubricant.

8. The HVAC system of claim 5 , wherein the controller is configured to activate the second compressor upon a determination that the lubricant level is equal to or greater than the threshold value of lubricant.

9. The HVAC system of claim 1 , wherein the controller is configured to, before receiving the request to transition from the second mode to the first mode:

initiate operation of the HVAC system in the second mode based on a command to provide conditioned air to a building.

10. The HVAC system of claim 9 , further comprising:

a first bridging conduit extending from a portion of the first refrigeration circuit that is downstream of an evaporator to a portion of the second refrigeration circuit that is downstream of the evaporator; and

a second bridging conduit extending from a portion of the first refrigeration circuit that is upstream of the first condenser to a portion of the second refrigeration circuit that is upstream of the second condenser, wherein, to initiate operation of the HVAC system in the second mode, the controller is configured to:

instruct opening of a first bridging valve disposed along the first bridging conduit;

instruct opening of a second bridging valve disposed along the second bridging conduit; and

activate the first compressor of the first refrigeration circuit.

11. The HVAC system of claim 1 , wherein the controller comprises a plurality of separate controller devices.

12. A control system configured to control climate characteristics in a building via a multi-circuit system including a first refrigeration circuit having a first compressor and a first condenser, wherein the first compressor is configured to urge refrigerant in the first refrigeration circuit in a direction upstream to downstream in the first refrigeration circuit, and having a second refrigeration circuit having a second compressor and a second condenser, wherein the second compressor is configured to urge refrigerant in the second refrigeration circuit in a direction upstream to downstream in the second refrigeration circuit, the first and second refrigeration circuits sharing an evaporator, wherein the control system comprises a controller configured to:

receive a first command to transition operation of the multi-circuit system to a first mode of operation, in which the first and second refrigeration circuits operate separately, from a second mode of operation, in which refrigerant-sharing occurs between portions of the first and second refrigeration circuits;

determine an amount of subcooling of refrigerant downstream of the second condenser;

compare the amount of subcooling of refrigerant to a threshold value of subcooling associated with the first mode of operation; and

send a second command to open a valve to facilitate refrigerant flow along a conduit extending from a portion of the first refrigeration circuit downstream of the first condenser to a portion of the second refrigeration circuit upstream of the second compressor upon a determination that the amount of subcooling is less than the threshold value of subcooling.

13. The control system of claim 12 , wherein the controller is configured to:

operate the first compressor and suspend operation of the second compressor in the second mode;

activate the second compressor after receiving the first command and prior to determining the amount of subcooling; and

deactivate the first compressor after receiving the first command and after the determination that the amount of subcooling is less than the threshold value of subcooling.

14. The control system of claim 13 , wherein the amount of subcooling is a first amount of subcooling, and wherein the controller is configured to:

determine a second amount of subcooling of refrigerant downstream of the second condenser after activating the second compressor;

compare the second amount of subcooling of refrigerant to the threshold value of subcooling; and

send a third command to close the valve to block refrigerant flow along the conduit upon a determination that the second amount of subcooling is greater than or equal to the threshold value of subcooling.

15. The control system of claim 12 , further comprising a lubricant conduit extending from a first sump of the first compressor to a second sump of the second compressor, wherein controller is configured to operate the first compressor and suspend operation of the second compressor during the second mode of operation and, after receiving the first command:

detect a lubricant level in the second sump;

compare the lubricant level to a threshold value of lubricant; and

send a third command to open a lubricant valve disposed along the lubricant conduit upon a determination that the lubricant level is less than the threshold value of lubricant.

16. The control system of claim 15 , wherein the controller is configured to:

deactivate the first compressor after comparing the lubricant level to the threshold value of lubricant and prior to sending the third command to open the lubricant valve; and

activate the second compressor of the second refrigeration circuit after sending the third command to open the lubricant valve.

17. The control system of claim 16 , wherein the lubricant level in the second sump is a first lubricant level, and wherein the controller is configured to, after activating the second compressor:

detect a second lubricant level in the second sump;

compare the second lubricant level to the threshold value of lubricant; and

send a fourth command to close the lubricant valve upon a determination that the lubricant level is greater than or equal to the threshold value of lubricant.

18. The control system of claim 15 , wherein the controller is configured to activate the second compressor upon a determination that the lubricant level is equal to or greater than the threshold value of lubricant.

19. The control system of claim 12 , wherein the controller is configured to, before receiving the first command:

receive a third command to provide conditioned air to the building; and

adjust the operation of the multi-circuit system to the first mode of operation based on the third command.

20. A control system configured to control climate characteristics in a building via a multi-circuit system including a first refrigeration circuit having a first compressor and a first condenser, wherein the first compressor is configured to urge refrigerant in the first refrigeration circuit in a direction upstream to downstream in the first refrigeration circuit, and having a second refrigeration circuit having a second compressor and a second condenser, wherein the second compressor is configured to urge refrigerant in the second refrigeration circuit in a direction upstream to downstream in the second refrigeration circuit, the first and second refrigeration circuits sharing an evaporator, wherein the control system comprises a controller configured to:

receive a first command to transition operation of the multi-circuit system from a hybrid operation to a conventional operation;

detect a lubricant level in a second sump of the second compressor;

compare the lubricant level to a threshold value of lubricant associated with the conventional operation;

determine that the lubricant level is less than the threshold value of lubricant;

send a second command to open a lubricant valve disposed along a lubricant conduit that extends from the second sump to a first sump of the first compressor;

determine an amount of subcooling of refrigerant downstream of the second condenser;

compare the amount of subcooling of refrigerant to a threshold value of subcooling associated with the conventional operation;

determine that the amount of subcooling is less than the threshold value of subcooling; and

send a third command to open a refrigerant valve disposed along a refrigerant conduit extending from a first portion of the first refrigeration circuit that is downstream of the first condenser to a second portion of the second refrigeration circuit that is upstream of the second compressor.

21. The control system of claim 20 , wherein the controller is configured to:

operate the first compressor and suspend operation of the second compressor during the hybrid operation;

deactivate the first compressor after comparing the lubricant level to the threshold value of lubricant and prior to sending the second command to open the lubricant valve; and

activate the second compressor after sending the second command to open the lubricant valve.

22. The HVAC system of claim 21 , wherein the lubricant level in the second sump is a first lubricant level, and wherein the controller is configured to, after activating the second compressor:

detect a second lubricant level in the second sump;

compare the second lubricant level to the threshold value of lubricant associated with the conventional operation; and

send a fourth command to close the lubricant valve upon a determination that the second lubricant level is greater than or equal to the threshold value of lubricant.

23. The HVAC system of claim 20 , wherein the amount of subcooling is a first amount of subcooling, and wherein the controller is configured to, after sending the third command:

determine a second amount of subcooling of refrigerant downstream of the second condenser;

compare the second amount of subcooling of refrigerant to the threshold value of subcooling associated with the conventional operation; and

send a fourth command to close the refrigerant valve disposed along the refrigerant conduit upon a determination that the second amount of subcooling is greater than or equal to the threshold value of subcooling associated with the conventional operation.