HVAC system with economizer switchover control

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

an airside economizer comprising a first air intake configured to receive return air from within a building, a second air intake configured to receive outdoor air from outside the building, and an air discharge configured to provide discharge air to the building;

a plurality of sensors configured to measure temperature and humidity of the return air and temperature and humidity of the outdoor air; and

a controller configured to:

identify a wet coil condition by calculating a temperature dewpoint of the return air based on the measured temperature and humidity of the return air;

comparing the temperature dewpoint of the return air with a temperature setpoint for the discharge air;

identifying the wet coil condition in response to the temperature dewpoint of the return air exceeding the temperature setpoint;

calculate an enthalpy of the return air based on the measured temperature and humidity of the return air;

calculate an enthalpy of the outdoor air based on the measured temperature and humidity of the outdoor air;

select either a normal cooling mode or free cooling mode based on the calculated enthalpy of the return air, the calculated enthalpy of the outdoor air, and whether a coil condition is the wet coil condition;

operate the airside economizer to provide the return air to the building as the discharge air in response to selecting the normal cooling mode; and

operate the airside economizer to provide the outdoor air to the building as the discharge air in response to selecting the free cooling mode.

2. The HVAC system of claim 1 , further comprising: identifying a dry coil condition by:

identifying the dry coil condition in response to the temperature dewpoint not exceeding the temperature setpoint; and

wherein the normal cooling mode or free cooling mode based on the calculated enthalpy of the return air, the calculated enthalpy of the outdoor air, and whether the coil condition is the wet coil condition or the dry coil condition.

3. The HVAC system of claim 2 , wherein in response to identifying the wet coil condition, selecting either the normal cooling mode or the free cooling mode comprises:

comparing the enthalpy of the return air with the enthalpy of the outdoor air;

selecting the free cooling mode in response to the enthalpy of the return air exceeding the enthalpy of the outdoor air; and

selecting the normal cooling mode in response to the enthalpy of the return air not exceeding the enthalpy of the outdoor air.

4. The HVAC system of claim 2 , wherein in response to identifying the dry coil condition, selecting either the normal cooling mode or the free cooling mode comprises:

predicting a first enthalpy of the discharge air under a first scenario in which the airside economizer is operated in the normal cooling mode;

calculating a first difference between the first enthalpy of the discharge air and the enthalpy of the return air;

predicting a second enthalpy of the discharge air under a second scenario in which the airside economizer is operated in the free cooling mode;

calculating a second difference between the second enthalpy of the discharge air and the enthalpy of the outdoor air; and

selecting either the normal cooling mode or the free cooling mode based on the first difference and the second difference.

5. The HVAC system of claim 4 , wherein the controller is configured to:

select the free cooling mode if the first difference exceeds the second difference; and

select the normal cooling mode if the first difference does not exceed the second difference.

6. The HVAC system of claim 4 , wherein the controller is configured to:

calculate a modified first difference by modifying the first difference to account for sensor error;

compare the modified first difference to the second difference; and

select either the normal cooling mode or the free cooling mode based on whether the second difference exceeds the modified first difference.

7. The HVAC system of claim 4 , wherein the controller is configured to:

predict the first enthalpy based on a first humidity ratio in which the discharge air is equal to a humidity ratio of the return air; and

predict the second enthalpy based on a second humidity ratio in which the discharge air is equal to the humidity ratio of the outdoor air.

8. A method of operating a heating, ventilation, or air conditioning (HVAC) system for a building, the method comprising:

receiving return air from within a building via a first air intake and outdoor air from outside the building via a second air intake;

measuring temperature and humidity of the return air and the outdoor air using a plurality of sensors;

identifying a dry coil condition, wherein identifying the dry coil condition comprises:

calculating a temperature dewpoint of the return air based on the measured temperature and humidity of the return air;

comparing the temperature dewpoint of the return air with a temperature setpoint for discharge air; and

identifying the dry coil condition in response to the temperature dewpoint being below the temperature setpoint;

calculating an enthalpy of the return air based on the measured temperature and humidity of the return air;

calculating an enthalpy of the outdoor air based on the measured temperature and humidity of the outdoor air;

selecting either a normal cooling mode or free cooling mode based on the calculated enthalpy of the return air, the calculated enthalpy of the outdoor air, and whether a coil condition is the dry coil condition; and

operating the HVAC system to provide the outdoor air to the building as the discharge air in response to selecting the free cooling mode.

9. The method of claim 8 , wherein identifying a wet coil condition comprises:

identifying the wet coil condition in response to the temperature dewpoint exceeding the temperature setpoint; and

wherein the normal cooling mode or free cooling mode based on the calculated enthalpy of the return air, the calculated enthalpy of the outdoor air, and whether the coil condition is the wet coil condition or the dry coil condition.

10. The method of claim 9 , wherein in response to identifying the wet coil condition, selecting either the normal cooling mode or the free cooling mode comprises:

comparing the enthalpy of the return air with the enthalpy of the outdoor air;

selecting the free cooling mode in response to the enthalpy of the return air exceeding the enthalpy of the outdoor air; and

selecting the normal cooling mode in response to the enthalpy of the return air not exceeding the enthalpy of the outdoor air.

11. The method of claim 10 , wherein in response to identifying the dry coil condition, selecting either the normal cooling mode or the free cooling mode comprises:

predicting a first enthalpy of the discharge air under a first scenario in which the HVAC system is operated in the normal cooling mode;

calculating a first difference between the first enthalpy of the discharge air and the enthalpy of the return air;

predicting a second enthalpy of the discharge air under a second scenario in which the HVAC system operated in the free cooling mode;

calculating a second difference between the second enthalpy of the discharge air and the enthalpy of the outdoor air; and

selecting either the normal cooling mode or the free cooling mode based on the first difference and the second difference.

12. The method of claim 11 , wherein the method further comprises:

selecting the free cooling mode if the first difference exceeds the second difference; and

selecting the normal cooling mode if the first difference does not exceed the second difference.

13. The method of claim 11 , further comprising:

calculating a modified first difference by modifying the first difference to account for sensor error;

comparing the modified first difference to the second difference; and

selecting either the normal cooling mode or the free cooling mode based on whether the second difference exceeds the modified first difference.

14. A method for operating a heating, ventilation, or air conditioning (HVAC) system for a building, the method comprising:

receiving return air from within a building via a first air intake and outdoor air from outside the building via a second air intake;

measuring temperature and humidity of the return air and the outdoor air using a plurality of sensors;

identifying a wet coil condition or a dry coil condition;

calculating an enthalpy of the return air based on the measured temperature and humidity of the return air;

calculating an enthalpy of the outdoor air based on the measured temperature and humidity of the outdoor air;

selecting either a normal cooling mode or free cooling mode based on the calculated enthalpy of the return air, the calculated enthalpy of the outdoor air, and whether a coil condition is the wet coil condition or the dry coil condition;

operating the HVAC system to provide the outdoor air to the building as discharge air in response to selecting the free cooling mode:

predicting first enthalpy based on a first humidity ratio in which the discharge air is equal to a humidity ratio of the return air; and

predicting second enthalpy based on a second humidity ratio in which the discharge air is equal to the humidity ratio of the outdoor air.

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

one or more processors; and

one or more non-transitory computer-readable media storing instructions that, when executed by the one or more processors, cause the one or more processors to perform operations comprising:

identify a wet coil condition or a dry coil condition in response to a temperature dewpoint of return air exceeding a temperature setpoint; and

identifying the dry coil condition in response to the temperature dewpoint not exceeding the temperature setpoint;

calculate an enthalpy of the return air received from a building space based on a measured temperature and humidity of the return air;

calculate an enthalpy of outdoor air based on a measured temperature and humidity of the outdoor air;

select either a normal cooling mode or free cooling mode based on the calculated enthalpy of the return air, the calculated enthalpy of the outdoor air, and the identification of the wet coil condition or the dry coil condition;

operate equipment of the HVAC system to provide the return air to the building as discharge air in response to selecting the normal cooling mode; and

operate the equipment of the HVAC system to provide the outdoor air to the building as the discharge air in response to selecting the free cooling mode.

16. The controller of claim 15 , wherein identifying the wet coil condition or the dry coil condition comprises:

calculating the temperature dewpoint of the return air based on the measured temperature and humidity of the return air; and

comparing the temperature dewpoint of the return air with a temperature setpoint for the discharge air.

17. The controller of claim 16 , wherein in response to identifying the wet coil condition, selecting either the normal cooling mode or the free cooling mode comprises:

comparing the enthalpy of the return air with the enthalpy of the outdoor air;

selecting the free cooling mode in response to the enthalpy of the return air exceeding the enthalpy of the outdoor air; and

selecting the normal cooling mode in response to the enthalpy of the return air not exceeding the enthalpy of the outdoor air.

18. The controller of claim 16 , wherein in response to identifying the dry coil condition, selecting either the normal cooling mode or the free cooling mode comprises:

predicting a first enthalpy of the discharge air under a first scenario in which the HVAC system is operated in the normal cooling mode;

calculating a first difference between the first enthalpy of the discharge air and the enthalpy of the return air;

predicting a second enthalpy of the discharge air under a second scenario in which the HVAC system is operated in the free cooling mode;

calculating a second difference between the second enthalpy of the discharge air and the enthalpy of the outdoor air; and

selecting either the normal cooling mode or the free cooling mode based on the first difference and the second difference.

19. The controller of claim 18 , wherein the controller is configured to:

select the free cooling mode if the first difference exceeds the second difference; and

select the normal cooling mode if the first difference does not exceed the second difference.

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

calculate a modified first difference by modifying the first difference to account for sensor error;

compare the modified first difference to the second difference; and

select either the normal cooling mode or the free cooling mode based on whether the second difference exceeds the modified first difference.