Instruction manual

Table Of Contents

McQuay OM 920 75

Operator’s Guide

Temperature Control Configurations

Temperature control is based on a Control Type that may be set to either Zone or DAT.

When the Control Type is set to Zone temperature control, heating, compressors, and the

economizer are controlled to maintain the temperature of the zone at a desired setpoint. This

configuration is used on units equipped with constant volume supply fans. Compressors and

heating stages are staged to maintain space or return temperature. The number of compressors

is decreased when it is too cold and increased when it is too hot subject to stage timers. The

number of heat stages is decreased when it is too hot and increased when it is too cold subject

to stage timers.

When the Control Type is set to DAT, heating, compressors, and the economizer are controlled

to maintain the discharge air temperature at a desired setpoint. This configuration is typically

used on units equipped with variable air volume supply fans.

Heat/Cool Changeover

In general, a unit configured for discharge air temperature control either operates to deliver the

cooling discharge temperature set point using economizer and/or mechanical cooling or the

heating discharge air temperature set point using the heating equipment. Cooling and heating

never operate simultaneously. A unit configured for zone (or space comfort) control either

operates to maintain the Zone Cooling Set Point using economizer and/or mechanical cooling

or the Zone Heating Set Point using the heating equipment.

The Zone Cooling/Zone Heating setpoints can be set via the keypad/display or based on a

signal from an optional space temperature sensor with set point adjustment capability. The

following sections describe the unit heat/ cool changeover function.

Illustrative Heat/Cool Changeover Sequence

The following is an illustration of the heat/cool changeover function.

When the Control Temperature is below the zone heating setpoint by more than ½ the

deadband (point A), heating operation is enabled. Heating operation then remains enabled

until the control temperature begins to rise and rises above the zone heating setpoint by more

than ½ the deadband (point B), at which point heating operation is disabled and the unit enters

the fan only (or Min DAT) operating state. If the control temperature rises above the zone

cooling setpoint by more than ½ the deadband (point C) cooling operation is enabled. Cooling

operation remains enabled until the control temperature begins to fall below the zone cooling

setpoint by more than ½ the deadband (point D), at which point the unit returns or fan only (or

Min DAT) operating state.

Figure 7: Heat/Cool Changeover

Heating SPT

Cooling SPT

Cooling Enabled

Heating Enabled

Fan Only

Control Temperature

Time

Clg

Htg