Liebert 8 Tons User Manual

The percent requirement is determined by the difference between the return air temperature and the 

temperature setpoint. As the return air temperature rises above the temperature setpoint, the per-

cent cooling required increases proportionally (from 0 to 100%) over a temperature band equal to the 

temperature sensitivity plus 1 degree Fahrenheit. The heating requirement is determined in a simi-

lar manner as the temperature decreases below the setpoint. With this control type the temperature 

at which the room is controlled increases as the room load increases. At full load the room would be 

controlled at a temperature equal to the setpoint plus the sensitivity.

The system will use the 3-ton (compressor #1) and 5-ton (compressor #2) compressors in an 8-ton sys-

tem. The control will determine the average cooling requirement updated every hour and select the 

lead compressor or, the user can select the lead compressor through the “Setup Operation” menu. At 

startup, the 3-ton compressor will be the lead compressor. The compressors will be staged on with hot 

gas bypass energized at 50 and 100% cooling requirements. The compressors will turn off at 75% and 

25% requirements. 

The chilled-water control valve is adjusted proportionally as the temperature control varies the 

requirement for cooling from 0% to 100%. This is based on the full valve travel time programmed in 

the “Setup Operation” menu.

When GLYCOOL is available, the temperature control will calculate a total cooling requirement of 

200%. Assuming that full GLYCOOL capacity is available, the GLYCOOL valve opens proportionally 

as the requirement for cooling rises from 0 to 100%. If the call for cooling continues to increase, the 

control will energize the compressors as needed to match the average cooling requirement. As long as 

GLYCOOL is available, the control will leave the valve 100% open. If GLYCOOL cooling is not avail-

able, the temperature control will operate the compressors in the same manner as the Multi-Step 

without GLYCOOL.

For 2-stage electric reheat, they are activated when the temperature control calculates a requirement 

of 50% and 100%.   They are deactivated when the requirement decreases to 75% (reheat 2) and 25% 

(reheat 1). 

The SCR (Silicon Controlled Rectifier) controller shall proportionally control the stainless steel 

reheats to maintain the selected room temperature. The rapid cycling made possible by the SCR con-

troller provides precise temperature control, and the more constant element temperature extends 

heater life. During operation of the SCR control, the compressor(s) operate(s) continuously. The heat-

ers are modulated to provide temperature control. If overcooling occurs, the compressor(s) will be 

locked off when the temperature drops to the low temperature alarm. SCR reheats are 15 kW.