How does the PEARL calculate where to place the outside damper during an economizer cycle?

During a cooling cycle, if the outdoor temperature is below the configured High Limit Shut Off, then the PEARL looks at the temperature difference (ΔT) between the room and the outside to decide if it should start an economizer cycle. If it decides that economizer should be active, it calculate what position to initially place the outdoor damper to so that the low limited mixed air setting is not exceeded. The outdoor damper modulates to prevent the mixed air temperature from exceeding this low limit, while being used to cool the room to the cool set point.

If the outdoor damper is unable to cool the room on its own, the PEARL will initiate a mechanical cooling cycle, while keeping the economizer active.

It is important the PEARL’s outdoor sensor is placed in a location where it is not being influenced but sun load or wind, so that it can accurately calculate this initial position. It is also important for the PEARL’s supply sensor to be reading an accurate supply temperature to prevent low limits from being met.

The default economizer low limited mixed air configuration is 56°F. This can be adjusted by Pelican Technical Support.

What position can I expect when the compressor(s) are running as well?

If the HVAC unit’s compressors are enabled while the PEARL has an economize cycle active, the PEARL will modulate the outside damper to prevent the supply air from exceeding the configured mixed air low limit. This is to prevent the compressor’s coils from freezing. In extremely cold situations, if the compressors are enabled during an economizer sequence, the PEARL might place the outdoor damper to 0% open.

Note: Compressors are enabled by the PEARL, during an economizer cycle, because the Pelican thermostat has communicated to the PEARL that the economizer is unable to maintain the room to at least within 1°F of the cool set point. In other words, the thermostat has communicated to the PEARL that the economizer is unable to cool the room on its own.

Does Pelican alarm if the mixed air gets colder than the configured low limit when economizer and compressors are active?

This is not a default alarm notification, but can be configured.

This configuration can be found in the Pelican Web-App > Admin > Thermostat Configuration > ‘select any thermostat‘ > Input Sensor > Function: Supply Temperature > Cool Safe Range.

The Cool Safe Range tells the thermostat that if the thermostat is in a cooling cycle (compressors are enabled) and the supply temperature is not within the configured temperature range within 4 minutes after the compressors were enabled, then to send an alarm.