Free cooling uses unconditioned outside air to cool the space directly. The Economizer Controller enables or disables free cooling after it judges which control mode is active. It also uses hysteresis to ensure a smooth switchover.
Depending on the sensors that are used, there are four different control modes. In different control modes, the assessed conditions are different.
Control Mode | Used Sensors | Enable Free Cooling? |
|---|---|---|
Control Mode 1 - Fixed Dry-bulb | OA (outside air) temperature sensor and MA (Mixed Air) temperature sensor | The outside-air dry-bulb temperature is compared with the set temperature shutoff setpoint (2TEMP OFF). If the outside-air dry-bulb temperature is below the temperature shutoff setpoint, the outside air is used to meet all or part of the cooling demand. |
Control Mode 2 - Differential Dry-bulb (Dual Dry- Bulbs) | OA temperature sensor, RA (Return Air) temperature sensor and MA temperature sensor | The outside-air dry-bulb temperature is compared with the return-air dry-bulb temperature. If both OAT and RAT are higher than the temperature high limitation (2THL), free cooling is prohibited. If OAT or RAT is lower than the temperature high limitation and the outside-air dry-bulb temperature is lower than the return-air dry-bulb temperature, the outside air is used to meet all or part of the cooling demand. |
Control Mode 3 - Combination Fixed Enthalpy and Fixed Dry-bulb Control | OA temperature and humidity sensor, and MA temperature sensor | The outside-air dry-bulb temperature and enthalpy are compared with the set temperature and enthalpy shutoff setpoints. If the outside-air enthalpy is lower than the set enthalpy shutoff setpoint (2ENTH OFF), and the outside-air dry bulb temperature is lower than the temperature shutoff setpoint, the outside air can be used for economizing. |
Control Mode 4 - Combination of Differential Enthalpy and Fixed Dry-bulb | OA temperature and humidity sensor, RA temperature and humidity sensor, and MA temperature sensor | The outside-air dry-bulb temperature and enthalpy are compared with the temperature shutoff setpoint and return-air enthalpy. If both OA enthalpy and RA enthalpy are higher than the enthalpy high limitation (2EHL), free cooling is prohibited. If OA enthalpy or RA enthalpy is lower than the enthalpy high limitation, outside-air enthalpy is lower than the return-air enthalpy, and the outside-air dry-bulb temperature is lower than the set temperature shutoff setpoint, then outside air can be used for economizing. |
Default hysteresis setting
Hysteresis setting (DB) between OAT and temperature shutoff setpoint defaults to 2 °F (non-changeable).
The temperature shutoff setpoint decides when to disable free cooling when the outside air temperature increases. However, once free cooling is disabled after the increased temperature is higher than the temperature shutoff setpoint, the hysteresis setting decides when to enable free cooling. When the temperature decreases and drops 2 °F (hysteresis setting) below the temperature shutoff setpoint, free cooling is enabled.
Damper modulation during free cooling
Once outside air is suitable for free cooling, the controller modulates the damper based on MAT (mixed air temperature, default) or OAT (outside air temperature).
If MAT is used when free cooling is enabled, MAT setpoint ('3MAT SET', configurable under Advanced Settings) is used for MAT modulating. When MAT falls below the anti-freeze setpoint (3FRZ PROT), the damper either fully closes or opens to the minimum position ('3FREEZE POS', configurable under Advanced Settings).
- If MAT is lower than MAT setpoint, the damper is modulated to maintain MAT setpoint, towards fully closes or opens to the minimum position based on occupancy status if MAT continues dropping.
- If MAT is in the range [MAT setpoint, (MAT setpoint + neutral zone band (1 °F by default, non-changeable))], the damper position remains no change.
- If MAT is higher than (MAT setpoint + neutral zone band), the damper opens towards fully open.
- If MAT is 10 °F higher than MAT setpoint, the damper fully opens to 100%.
If OAT is used when there is a cooling demand, the damper can be opened to different positions depending on different outside air temperatures:
- If outside air is higher than 50 °F but lower than the temperature shutoff setpoint, the damper is fully open.
- If outside air is higher than OAT lockout setpoint but lower than 50 °F, linear modulation is applied when only Cooling Stage 1 input (Y1I) is ON. Result of the following formula indicates the damper's open position:
((OAT - OAT Lockout Setpoint) / (50 - OAT Lockout Setpoint)) * (80% - MIN POS) + MIN POS- Note: After the Controller receives the second cooling demand, the damper fully opens regardless of the modulating logic. After fan delay (2FAN DLY) time runs out, mechanical cooling turns on.