Systems and methods for freeze protection of a coil in an HVAC system

1. A freeze protection system for a heating, ventilation, or air conditioning (HVAC) system, the freeze protection system comprising:

a temperature sensor positioned at an inlet of a coil within which a liquid is at risk of freezing;

a fan or damper operable to control a velocity of an airflow across the coil; and

a controller comprising a processor and memory, the memory storing instructions that, when executed by the processor, cause the processor to:

obtain a measured temperature of the liquid at the inlet of the coil from the temperature sensor;

compare the measured temperature of the liquid at the inlet of the coil with a freeze prevention temperature threshold; and

engage a freeze prevention action comprising operating the fan or the damper to decrease the velocity of the airflow across the coil in response to a determination that the measured temperature of the liquid at the inlet of the coil is less than or equal to the freeze prevention temperature threshold.

2. The system of claim 1 , further comprising a valve operable to control a flow rate of the liquid through the coil;

wherein the freeze prevention temperature threshold is equal to a freeze temperature at which the liquid is predicted to freeze plus a safety temperature; and

wherein engaging the freeze prevention action comprises operating the valve to increase the flow rate of the liquid through the coil.

3. The system of claim 1 , wherein the freeze prevention temperature threshold is equal to a freeze temperature at which the liquid is predicted to freeze.

4. The system of claim 1 , wherein the freeze prevention temperature threshold is a multi-tiered threshold comprising a first freeze prevention temperature threshold and a second freeze prevention temperature threshold less than the first freeze prevention temperature threshold;

wherein the controller is configured to:

engage a first freeze prevention action in response to a determination that the measured temperature of the liquid at the inlet of the coil is less than or equal to the first freeze prevention temperature threshold; and

engage a second freeze prevention action, different from the first freeze prevention action, in response to a determination that the measured temperature of the liquid at the inlet of the coil is less than or equal to the second freeze prevention temperature threshold.

5. The system of claim 1 , further comprising a valve operable to control a flow rate of the liquid through the coil;

wherein the controller is configured to:

use a heat transfer model for the coil to calculate a threshold flow rate of the liquid through the coil sufficient to prevent the liquid from freezing within the coil as a function of the measured temperature of the liquid at the inlet of the coil; and

operate the valve such that the flow rate of the liquid through the coil is at least the threshold flow rate calculated using the heat transfer model.

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

determine whether the measured temperature of the liquid at the inlet of the coil is unreliable; and

in response to a determination that the measured temperature of the liquid at the inlet of the coil is unreliable, switch to using a measured temperature of an airflow with which the liquid exchanges heat within the coil instead of the measured temperature of the liquid at the inlet of the coil to determine whether to engage the freeze prevention action.

7. The system of claim 1 , wherein the freeze prevention action comprises causing stagnant liquid within the coil to begin flowing through the coil.

8. A freeze protection controller for a heating, ventilation, or air conditioning (HVAC) system, the freeze protection controller comprising:

a processor and memory, the memory storing instructions that, when executed by the processor, cause the processor to:

obtain a measured temperature of a liquid at an inlet of a coil within which the liquid is at risk of freezing from a temperature sensor positioned to measure the temperature of the liquid at the inlet of the coil;

use a heat transfer model for the coil to calculate a threshold flow rate of the liquid through the coil sufficient to prevent the liquid from freezing within the coil as a function of the measured temperature of the liquid at the inlet of the coil; and

operate a valve such that a flow rate of the liquid through the coil is at least the threshold flow rate calculated using the heat transfer model.

9. The controller of claim 8 , wherein the instructions cause the processor to:

compare the measured temperature of the liquid at the inlet of the coil with a freeze prevention temperature threshold equal to a freeze temperature at which the liquid is predicted to freeze plus a safety temperature, and

engage a freeze prevention action comprising operating the valve to increase the flow rate of the liquid through the coil, wherein the valve is operable to control the flow rate of the liquid through the coil.

10. The controller of claim 8 , wherein the instructions cause the processor to:

compare the measured temperature of the liquid at the inlet of the coil with a freeze prevention temperature threshold equal to a freeze temperature at which the liquid is predicted to freeze,

engage a freeze prevention action comprising operating a fan or a damper to decrease a velocity of an airflow across the coil, wherein the fan or the damper are operable to control the velocity of the airflow across the coil.

11. The controller of claim 8 , wherein the instructions cause the processor to:

compare the measured temperature of the liquid at the inlet of the coil with a multi-tiered threshold comprising a first freeze prevention temperature threshold and a second freeze prevention temperature threshold less than the first freeze prevention temperature threshold;

engage a first freeze prevention action in response to a determination that the measured temperature of the liquid at the inlet of the coil is less than or equal to the first freeze prevention temperature threshold; and

engage a second freeze prevention action, different from the first freeze prevention action, in response to a determination that the measured temperature of the liquid at the inlet of the coil is less than or equal to the second freeze prevention temperature threshold.

12. The controller of claim 8 , wherein the instructions cause the processor to:

determine whether the measured temperature of the liquid at the inlet of the coil is unreliable; and

in response to a determination that the measured temperature of the liquid at the inlet of the coil is unreliable, switch to using a measured temperature of an airflow with which the liquid exchanges heat within the coil instead of the measured temperature of the liquid at the inlet of the coil to determine whether to engage a freeze prevention action.

13. The system of claim 1 , wherein the freeze prevention action comprises causing stagnant liquid within the coil to begin flowing through the coil.

14. A method for preventing a liquid from freezing inside a coil of a heating, ventilation, or air conditioning (HVAC) system, the method comprising:

measuring a temperature of an airflow with which the liquid exchanges heat within the coil;

comparing the measured temperature of the airflow with a freeze prevention temperature threshold; and

engaging a freeze prevention action in response to a determination that the measured temperature of the airflow is less than or equal to the freeze prevention temperature threshold.

15. The method of claim 14 , further comprising equaling the freeze prevention temperature threshold to a freeze temperature at which the liquid is predicted to freeze plus a safety temperature, wherein engaging the freeze prevention action comprises operating a valve to increase a flow rate of the liquid through the coil, wherein the valve is operable to control the flow rate of the liquid through the coil.

16. The method of claim 14 , further comprising equaling the freeze prevention temperature threshold to a freeze temperature at which the liquid is predicted to freeze, wherein engaging the freeze prevention action comprises operating a fan or damper to decrease a velocity of an airflow across the coil, wherein the fan or the damper are operable to control the velocity of the airflow across the coil.

17. The method of claim 14 , further comprising:

establishing the freeze prevention temperature threshold as a multi-tiered threshold comprising a first freeze prevention temperature threshold and a second freeze prevention temperature threshold less than the first freeze prevention temperature threshold;

engaging a first freeze prevention action in response to a determination that the measured temperature of the airflow is less than or equal to the first freeze prevention temperature threshold; and

engaging a second freeze prevention action, different from the first freeze prevention action, in response to a determination that the measured temperature of the airflow is less than or equal to the second freeze prevention temperature threshold.

18. The method of claim 14 , further comprising using a heat transfer model for the coil to calculate a threshold flow rate of the liquid through the coil sufficient to prevent the liquid from freezing within the coil as a function of the measured temperature of the airflow; and

operating a valve such that a flow rate of the liquid through the coil is at least the threshold flow rate calculated using the heat transfer model, wherein the valve is operable to control the flow rate of the liquid through the coil.

19. The method of claim 14 , further comprising:

determining whether a measured temperature of the liquid at the inlet of the coil is unreliable; and

in response to a determination that the measured temperature of the liquid at the inlet of the coil is unreliable, switching to using the measured temperature of an airflow with which the liquid exchanges heat within the coil instead of the measured temperature of the liquid at the inlet of the coil to determine whether to engage the freeze prevention action.