Instruction manual

76 McQuay OM 920
Operator’s Guide
Control Temperature
The “Control Temperature” is defined as the unit temperature input used to make the heat/cool
changeover decision. This determines whether or not cooling or heating is enabled.
The user may select Space Temperature, Return Temperature, or Outdoor Air Temperature for
both Rooftop units and Self Contained units and the Mixed Air Temperature for Self
Contained units. Normally either the Return or Space Temperature is selected as the Control
Temperature. Outdoor Air Temperature may be used as the Control Temperature for units with
DAT Cooling only or for units with both DAT Cooling and DAT Heating, but not for units
with either Zone Cooling Control or Zone Heating Control. The Mixed Air Temperature on
Self Contained units may be used as the Control Temperature for 100% Outdoor Air units with
DAT Cooling only or for 100% Outdoor Air units with both DAT Cooling and DAT Heating,
but not for units with Zone Cooling Control, Zone Heating Control, or Return Air Control.
The Mixed Air Temperature should not be selected as the Control Temperature for Self
Contained units with Return Air Control as erratic operation could result.
The Control Temperature Source automatically reverts from Return to Space if:
• The Return Air Sensor is not present and reliable AND
• A network or local Space Sensor is present and reliable
The Control Temperature Source automatically reverts from Space to Return if:
• The Space Sensor is not Reliable (A valid Space Temperature value is not provided via the
network and the Space Temperature sensor is either In alarm OR Not Present) AND
• The Return Air Sensor is Reliable (Present and Not in Alarm)
The Control Temperature Source automatically reverts from either Outdoor Air or Mixed Air
to Return if the Control Type is Zone (Not DAT). If the Return Air Temperature is not reliable,
the Control Temperature may then revert to Space as described above.
The Control Temperature Source automatically reverts from Mixed Air to Outdoor Air if Unit
Type is not SCU.
A Control Temperature Fault that shuts down the unit is initiated if any of the following is
true.
• The Return Air Sensor is not present and reliable AND
• Return Air is selected as the Control Temperature
• A network or local Space Sensor is not present and reliable AND Space is selected as the
Control Temperature
• The OAT Sensor is not present and reliable AND Outdoor Air is selected as the Control
Temperature
• The Mixed Air Temperature Sensor is not present or reliable AND Mixed Air is selected as
the Control Temperature
Zone Temperature Set Points
When the Use Tstat Setpoint parameter is set to No, the Zone Cooling Set points and the Zone
Heating Set points may be set through the keypad or via a network signal (all units). In this
case these set points are changed whenever the network or keypad value changes. When the
Use Tstat Setpoint parameter is set to Yes these set points can only be adjusted through the
zone thermostat (Zone control units). The parameter is adjustable via the following path:
Extended Menus/Zone Temp Setup/Use Tstat Spt. Heating and cooling set points must not
overlap. The Zone Heating Set points must be equal to or less than the Zone Cooling Set
points. If a conflict occurs from values entered via the keypad or network, the Zone Heating
Set point is automatically adjusted down to eliminate the conflict. When the Zone Cooling Set
point is changed by more than 0.5 degrees through the wall mounted sensor, the Zone Heating
Set point is raised or lowered the same amount so that the difference between the Cooling and
Heating set points do not change.