SCR Reheat—Tight Mode
In tight mode, cooling (compressors) and heating (SCR reheat) components operate simultaneously to provide maximum
temperature control.
NOTE: Tight mode is the factory default setting when available. Tight mode is not available on chilled water units,
units with variable speed fans or units with digital scroll compressors.
The compressor(s) respond to a 0% to 100% cooling requirement and the SCR responds to a 0% to 200% heating
requirement based on the temperature proportional band. Figure 3.16 on the facing page , illustrates SCR reheat on a system
with two single-step compressors operating in tight mode as follows:
• 70° setpoint
• 8° proportional band
• No deadband
The proportional band is divided evenly on each side of the setpoint, and an additional half of the proportional band is added
to the heating side for 200% heating.
• At 70° air temperature (heating demand 0%), SCR reheat operates at full capacity and compressor1 operates
continuously from 100% cooling demand to 200% heating demand.
• At 200% heating demand, compressor 1 deactivates until the temperature increases to 70° setpoint (0%
heating/cooling demand).
• As cooling demand increases from 0% to 100%, SCR reduces heating capacity proportionally and stops operating
at 100% cooling demand.
• At 100% cooling demand, compressor 2 starts and operates until the air temperature returns to 70° (0% cooling
demand) when it stops operation.
– at the same time –
SCR increases heating capacity as the cooling demand decreases from 100% to 0%.
• When temperature decreases below the setpoint, SCR operates at full capacity as the air temperature drops and
heating demand increases from 0% to 200%.
NOTE: Some cooling units are not suited for a strict, NO LOAD application and require a minimal load in the space.
Consult a Vertiv™ representative for verification.
3 Service Operation
64
Vertiv™ Liebert® iCOM™Installer/User Guide
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