HVAC system with economizer switchover control

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.

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.

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.