Detecting loss of charge in HVAC systems

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

an evaporator coil comprising an inlet for flow of refrigerant into the evaporator coil and an outlet for flow of the refrigerant out of the evaporator coil;

a first sensor operably coupled to the evaporator coil at a first position, the first sensor configured to monitor a first temperature of the refrigerant flowing in the evaporator coil at the first position, wherein the first position is adjacent to the inlet of the evaporator coil;

a second sensor operably coupled to the evaporator coil at a second position, the second sensor configured to monitor a second temperature of the refrigerant flowing in the evaporator coil at the second position, wherein the second position is downstream from the first position and the second position is located at between 10% and 90% of a length of a circuit of the evaporator coil; and

a controller communicatively coupled to the first sensor and the second sensor, the controller configured to:

receive, from the first sensor, a first signal corresponding to the first temperature;

receive, from the second sensor, a second signal corresponding to the second temperature;

determine, based on the received first and second signals, a temperature difference between the second temperature and the first temperature;

compare the determined temperature difference to a predefined threshold value; and

determine whether the temperature difference is greater than the predefined threshold value;

if the temperature difference is greater than the predefined threshold value, determine that a loss of charge has occurred in the HVAC system;

if the temperature difference is not greater than the predefined threshold value:

determine whether the first temperature is less than a second threshold value; and

if the first temperature is less than the second threshold value, determine that a rate of an airflow across the evaporator coil is low.

2. The system of claim 1 , wherein the second position is located at between 10% and 50% of a length of a circuit of the evaporator coil.

3. The system of claim 1 , wherein the controller is further configured to transmit an alert corresponding to the loss of charge to a user interface associated with the HVAC system.

4. The system of claim 1 , wherein the controller is further configured to:

determine that the first temperature is less than a second threshold value;

in response to determining that the first temperature is less than the second threshold value, determine that the HVAC system has experienced a critical loss of charge; and

transmit an alert corresponding to the critical loss of charge to a user interface associated with the HVAC system.

5. The system of claim 4 , wherein the controller is further configured to, in response to determining that the HVAC system has experienced the critical loss of charge, cause the HVAC system to shut down.

6. The system of claim 1 , the controller further configured to:

after determining that the rate of the airflow across the evaporator coil is low, transmit an alert indicating the low rate of the airflow across the evaporator coil.

7. The system of claim 6 , further comprising a blower configured to provide the airflow across the evaporator coil;

wherein the controller is communicatively coupled to the blower, the controller is further configured to, in response to determining that the rate of airflow across the evaporator coil is low:

determine whether a maximum airflow rate has been reached for the blower;

in response to determining the maximum airflow rate has not been reached, cause a speed of the blower to increase; and

in response to determining the maximum airflow rate has been reached, cause the HVAC system to shut down.

8. A method for detecting a loss of charge in a heating, ventilation, and air conditioning (HVAC) system, the method comprising:

receiving, from a first sensor, a first signal corresponding to a first temperature of refrigerant flowing in an evaporator coil of the HVAC system at a first position, wherein the first position is adjacent to an inlet of the evaporator coil; and

receiving, from a second sensor, a second signal corresponding to a second temperature of refrigerant flowing in the evaporator coil of an HVAC system at a second position, wherein the second position is downstream from the first position and the second position is located at between 10% and 90% of a length of a circuit of the evaporator coil;

determining, based on the received first and second signals, a temperature difference between the second temperature and the first temperature;

comparing the determined temperature difference to a predefined threshold value; and

determining whether the temperature difference is greater than the predefined threshold value;

if the temperature difference is greater than the predefined threshold value, determining that a loss of charge has occurred in the HVAC system;

if the temperature difference is not greater than the predefined threshold value:

determining whether the first temperature is less than a second threshold value; and

if the first temperature is less than the second threshold value, determining that a rate of an airflow across the evaporator coil is low.

9. The method of claim 8 , wherein the second position is located at between 10% and 50% of a length of a circuit of the evaporator coil.

10. The method of claim 8 , further comprising transmitting an alert corresponding to the loss of charge to a user interface associated with the HVAC system.

11. The method of claim 8 , further comprising:

determining that the first temperature is less than a second threshold value;

in response to determining that the first temperature is less than the second threshold value, determining that the HVAC system has experienced a critical loss of charge; and

transmitting an alert corresponding to the critical loss of charge to a user interface associated with the HVAC system.

12. The method of claim 11 , further comprising, in response to determining that the HVAC system has experienced the critical loss of charge, causing the HVAC system to shut down.

13. The method of claim 8 , further comprising:

after determining that the rate of the airflow across the evaporator coil is low, transmitting an alert corresponding to the low rate of the airflow.

14. The method of claim 13 , in response to determining that the rate of the airflow across the evaporator coil is low:

determining whether a maximum airflow rate has been reached for a blower of the HVAC system;

in response to determining the maximum airflow rate has not been reached, causing a speed of the blower to increase; and

in response to determining the maximum airflow rate has been reached, causing the HVAC system to shut down.

15. A controller for operating a heating, ventilation, and air conditioning (HVAC) system, the controller comprising:

an input/output interface configured to:

receive, from a first sensor, a first signal corresponding to a first temperature of refrigerant flowing in an evaporator coil of the HVAC system at a first position, wherein the first position is adjacent to an inlet of the evaporator coil; and

receive, from a second sensor, a second signal corresponding to a second temperature of refrigerant flowing in the evaporator coil of an HVAC system at a second position, wherein the second position is downstream from the first position and the second position is located at between 10% and 90% of a length of a circuit of the evaporator coil; and

a processor configured to:

determine, based on the received first and second signals, a temperature difference between the second temperature and the first temperature;

compare the determined temperature difference to a predefined threshold value; and

determine whether the temperature difference is greater than the predefined threshold value;

if the temperature difference is greater than the predefined threshold value, determine that a loss of charge has occurred in the HVAC system;

if the temperature difference is not greater than the predefined threshold value:

determine whether the first temperature is less than a second threshold value; and

if the first temperature is less than the second threshold value, determine that a rate of an airflow across the evaporator coil is low.

16. The controller of claim 15 , wherein the second position is located at between 10% and 50% of a length of a circuit of the evaporator coil.

17. The controller of claim 15 , wherein the processor is further configured to transmit an alert corresponding to the loss of charge to a user interface associated with the HVAC system.

18. The controller of claim 15 , wherein the processor is further configured to:

determine that the first temperature is less than a second threshold value;

in response to determining that the first temperature is less than the second threshold value, determine that the HVAC system has experienced a critical loss of charge; and

transmit an alert corresponding to the critical loss of charge to a user interface associated with the HVAC system.

19. The controller of claim 18 , wherein the processor is further configured to, in response to determining that the HVAC system has experienced a critical loss of charge, cause the HVAC system to shut down.

20. The controller of claim 15 , wherein the processor is further configured to:

in response to determining the rate of the airflow across the evaporator coil is low, determine whether a maximum airflow rate has been reached for a blower of the HVAC system, the blower configured to provide the airflow across the evaporator coil;

in response to determining the maximum airflow rate has not been reached, cause a speed of the blower to increase; and

in response to determining the maximum airflow rate has been reached, cause the HVAC system to shut down.