Instruction manual

112 McQuay OM 920
Operator’s Guide
Special Start Sequence for 100% Outdoor Air units with Gas Heat
A special start sequence is used for units 100% outdoor air units with gas heat. The special
start sequence applies to both Zone Control and DAT control units. If heat is required at unit
startup, the furnace enters a special burner startup sequence as the unit enters its Startup
operating state. Pre-firing the burner allows the gas heat pre-purge sequence to occur and the
burner to fire and warm up so that tempered air is available immediately when the fans start.
Once a 100% Outdoor Air unit equipped with Gas Heat completes one of the start up
sequences described below the gas heat operates the same for both return air units and 100%
Outdoor Air units.
The sequence described in the following paragraphs is initiated for a 100% OA Unit any time
during the Startup operating state when:
• Heating is enabled AND
• CtrlTemp is less than the Effective Zone Heating Setpoint* by ½ the ZoneHtgDeadband OR
• Heating is enabled AND
• OAT is less than the Min DAT Limit by the amount of the DAT Htg Deadband AND
• CtrlType = Zone
OR
• Heating is enabled AND
• Min DAT is set to Yes via the keypad AND
• OAT is less than the DATClgSpt by the amount of the DAT Htg Deadband
• CtrlType = DAC
*The Effective Zone Heating Setpoint equals the Zone Heating Setpoint if any of the
following three conditions are true. Otherwise it equals the Morning Warmup Heating
Setpoint:
• CtrlType = Zone
• Unit enters the Fan Only state with the CtrlTemp is equal to or greater than the
MWUHtgSetpoint
• Unit enters the Fan Only state with heating disabled.
The special start up sequence is initiated during Start Up period. The fans will remain off. The
main gas valve is energized so the burner starts during the Warmup Time (default = 240
seconds) and operates at Low Fire.
At the end of the Warmup Time, the modulating gas heating valve is set to a calculated
position. This calculated position is based on the “Application Discharge Air Temperature
Setpoint”, the Outdoor Air Temperature, and the Maximum Temperature Rise of the Gas Heat
Exchanger using the following formula:
Gas Heat Actuator Position = 100% x (Setpoint - Outdoor Air Temp) / Temp Rise
The Application Discharge Air Temperature Setpoint is the Zone Heating setpoint if the
Control Temperature initiated the Special Start sequence. If the OAT initiated the Special Start
sequence, the setpoint is the Min DAT Limit for Zone units and the Eff DAT Clg Spt for DAT
units. The Temp Rise is the Maximum Temperature Rise of the Gas Heat Exchanger that is
entered at the factory for the specific unit.
After the modulating gas heating valve is set to the calculated position, there is a HeatUpDelay
(default = 60 second) to allow the heat exchanger to heat up. After this delay, since the unit is
100% Outside Air, the unit immediately transitions from Recirculation to the Fan Only state.
As soon as the unit enters the Fan Only state, the unit will immediately transition to either the
Heating state or the MinDAT state.