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A system and a method are provided for returning oil in a heating, ventilation, and air conditioning (HVAC) system by determining when an ambient zone temperature is equal to or less than a temperature limit and reversing a direction of refrigerant flow of the HVAC system based on the length of time

A system and a method are provided for returning oil in a heating, ventilation, and air conditioning (HVAC) system by determining when an ambient zone temperature is equal to or less than a temperature limit and reversing a direction of refrigerant flow of the HVAC system based on the length of time. Also, a system and a method are provided for monitoring the length of operation in a first and a second heating mode and for reversing a direction of refrigerant flow based on the length of time operation occurs in the first and second heating modes. A system and a method having a first compressor, a first heating mode, a second heating mode, a defrost mode, and a controller is disclosed. The controller selectively controls initiation of the defrost mode in response to an HVAC system operating in a heating mode of operation for a predetermined amount of time.

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1. A heating, ventilation, and air conditioning (HVAC) system, comprising: a processor configured to determine when an ambient zone temperature is equal to or less than a first temperature limit;a processor configured to calculate a first length of time using a first timer, the first length of time

1. A heating, ventilation, and air conditioning (HVAC) system, comprising: a processor configured to determine when an ambient zone temperature is equal to or less than a first temperature limit;a processor configured to calculate a first length of time using a first timer, the first length of time being the length of time that a first compressor operates at a first capacity to deliver refrigerant to a first circuit of a first heat exchanger and during which operation the ambient zone temperature is equal to or less than the first temperature limit;a processor configured to initiate reversing a direction of refrigerant flow of the HVAC system when the first length of time is equal to or greater than a first time limit;a processor configured to determine when the ambient zone temperature is equal to or less than a second temperature limit, the second temperature limit having a value higher than the value of the first temperature limit;a processor configured to calculate a second length of time using a second timer, the second length of time being the length of time that the first compressor operates at a second capacity to deliver refrigerant to the first circuit of the first heat exchanger, the second capacity being different than the first capacity, and during which operation the ambient zone temperature is equal to or less than the second temperature limit; anda processor configured to initiate reversing the direction of refrigerant flow of the HVAC system when the second length of time is equal to or greater than a second time limit. 2. The HVAC system of claim 1, further comprising: a processor configured to set a value of the first timer to zero in response to the direction of refrigerant flow being reversed. 3. The HVAC system of claim 1, wherein the reversing of the refrigerant flow is accomplished by operating the HVAC system in a defrost mode. 4. The HVAC system of claim 1, further comprising: a processor configured to calculate a third length of time using a third timer, the third length of time being the length of time that a second compressor operates. 5. The HVAC system of claim 4, further comprising: a processor configured to set a value of at least one of the first timer and the second timer to zero when the value of the third timer is equal to or greater than a third time limit. 6. The HVAC system of claim 5, further comprising: a processor configured to set a value of at least one of the first timer and the second timer to zero when the value of the third timer is equal to or greater than a fourth time limit;wherein the fourth time limit is greater than the third time limit. 7. The HVAC system of claim 6, wherein the first temperature limit is a value between about 6° F. to about 42° F., the second temperature limit is a value between about 42° F. to about 60° F., the first time limit is a value between about 1 hour to about 6 hours, the second time limit is a value between about 5 hours to about 9 hours, the third time limit is a value between about 5 minutes to about 20 minutes, and the fourth time limit is a value between about 3 minutes to about 20 minutes. 8. The HVAC system of claim 6, wherein the first temperature limit is a 28° F., the second temperature limit is 45° F., the first time limit is 3 hours, the second time limit is 8 hours, the third time limit is 15 minutes, and the fourth time limit is 5 minutes. 9. A heating, ventilation, and air conditioning (HVAC) system, comprising: a processor configured to determine when an ambient zone temperature is equal to or less than a first temperature limit;a processor configured to calculate a first length of time using a first timer without respect to a temperature of an ambient zone heat exchanger, the first length of time being the length of time that a first compressor operates at a first capacity and during which operation the ambient zone temperature is equal to or less than the first temperature limit;a processor configured to initiate reversing a direction of refrigerant flow of the HVAC system when the first length of time is equal to or greater than a first time limit;a processor configured to determine when the ambient zone temperature is equal to or less than a second temperature limit;a processor configured to calculate a second length of time using a second timer without respect to the temperature of the ambient zone heat exchanger, the second length of time being the length of time that the first compressor operates at a second capacity and during which operation the ambient zone temperature is equal to or less than the second temperature limit; anda processor configured to initiate reversing a direction of refrigerant flow of the HVAC system when the second length of time is equal to or greater than the second time limit;wherein the first temperature limit is lower than the second temperature limit and the first capacity is lower than the second capacity.