All logic diagrams based on Indoor Set Point = 77°F.

1. Compressor and Heater Operation

Note: compressor may engage 3min delay when the units are powered to prevent rapid compressor cycling.

1.1 Lead Compressor

The lead compressor turns on at setpoint +2 (configurable at C02 under E Menu).

Note: when outdoor temp is lower than indoor temp, the lead compressor will start with a delay ranging from 10s to 300s proportional to the temperature difference. This is to give the system a chance to use free cooling before turning on the compressor.

The lead compressor turns off at 6 degrees below turn on point (configurable at C01  under E Menu).

1.2 Lag Compressor

The lag compressor turns on at setpoint + 7.4 with 1 min delay (configurable at F38 under L05 Menu).

The lag compressor turns off 7.2 degrees below turn on point (configurable at F39 under L05 Menu).

1.3 Lead Heater

The lead heater turns on at setpoint -27 (configurable at H02  under E Menu).

The lead heater turns off 3 degrees above the turn on point (configurable at H01  under E Menu).

1.4 Lag Heater

The lag heater turns on at setpoint -27 with 1 minute delay (configurable at F36  under L05 Menu).

The lag heater turns off 3 degrees above the turn on point (configurable at F37 under L05 Menu).

2. Free Cooling (Economizer) Operation

2.1 Conditions for Free Cooling

The following list of environment conditions must ALL be satisfied for the units to engage in free cooling.

· Room Temp – Outdoor Temp ≥ 3.6 (configuration at U06 under L04 Menu)

· Indoor Humidity ≤ 85% (configurable at U04 under L04 Menu)

· Room Temp > Lead Compressor turn off point

· Outdoor temp < Lead Compressor turn on point

· Dirty air filter not active. (Default requirement in 13B55 and up. For lower revisions, configure F24 from No to Yes to add this requirement)

2.2  100% Free Cooling

Free Cooling starts at low speed when indoor temperature reaches above lead compressor turn off point. The fan speed increases as the indoor temperature rise. If the indoor temperature can be maintained be the lead compressor turn on point, the unit will be in 100% free cooling mode. 100% free cooling mode uses ~10% of the power required to run compressors

2.3  Partial Free Cooling

If all the conditions for free cooling are met and the indoor temp exceeds lead compressor turn on point, the partial free cooling mode will engage where free cooling and compressor operates at the same time to conserve energy. The working principle of partial free cooling is that since outdoor air is cooling than indoor return air, bringing in outdoor air across an operating evaporator will result in a lower supply air temperature.

Note: When the compressor is on, the variable speed supply fan will always operate at 100% speed.

3  Damper Operation During Low Outdoor Temperature

The outside-air damper will begin to close when the supply air temperature falls below 51.8°F (configurable at U54 under L04 Menu). When the damper is partially closed, the outside air will mix with the return air to prevent cold shocking equipment inside.