When free cooling available and the SAT is greater than the operating VAV SAT setpoint, the dampers modulate to control the operating VAV SAT setpoint +/-0.5°F.
If the economizer position remains at 100% for 10 consecutive minutes and the SAT is greater than the operating VAV SAT setpoint +5°F, the first compressor output energizes. If the economizer position remains at 100% for another 5 minutes and the SAT is greater than the operating VAV SAT setpoint +5°F, the second compressor output energizes. If the economizer position remains at 100%, the process repeats every 5 minutes until all the compressors energize.
If the economizer position drops below 100% and does not reach the minimum position then returns to 100%, remains at 100% for 5 minutes, and the SAT is greater than the operating VAV SAT setpoint +5°F, the next available compressor energizes. If the economizer position remains at 100%, the process repeats every 5 minutes until all the compressors energize.
Any time the economizer remains at the minimum position for 5 consecutive minutes or the SAT is lower than the operating VAV SAT setpoint -5°F, the last energized compressor turns off. If it remains at the minimum position or the SAT remains lower than the operating VAV SAT setpoint -5°F, the compressors de-energize every 5 minutes until all the compressors are off.
If all compressor outputs de-engerize, the economizer modulates to control to operating VAV SAT Setpoint+/-0.5°F.
When the cooling demand ends the compressors de-energize immediately and the dampers return to operating minimum position.