McQuay OM 751
21
Description of Operation
The CO
2
demand controlled ventilation function (optional) is active (see “CO2 Demand
Controlled Ventilation (optional)” on page 34), and the OA damper is adjusted as needed to
maintain the CO
2
set point.
Figure 15: Mech state operation (occupied mode and auto fan)
Discharge Air (DA) Heat State (State 4)
The DA Heat state is a “normal” state during Cool mode. The DA Heat state typically is active
when reheat is required to maintain DATS while maintaining the required OA damper
position. The DA Heat state can also be made active if the optional CO
2
DCV feature is
provided and CO
2
levels are high, requiring the OA damper to open beyond what is required
for economizer cooling.
When DA Heat state is active, the UVC uses the units heating capability as needed to maintain
VCLL. The CO
2
demand controlled ventilation function (optional) is active (see “CO2
Demand Controlled Ventilation (optional)” on page 34), and the OA damper is adjusted as
needed to maintain the CO
2
set point.
Low Limit State (State F)
The Low Limit state is a “non-normal” state during Cool mode. The Low Limit state typically
follows the DA Heat state when the UVC reaches 100% heat and still cannot maintain VCLL.
When the Low Limit state becomes active, the Low Limit PI-loop overrides the OAD
minimum position (see “Outdoor Air Damper Operation” on page 32) and adjusts the OAD
toward closed as necessary to maintain the DAT set point (see “Discharge Air Temperature
Control” on page 29).
Cant Cool State (State C)
The Cant Cool state is a “non-normal” state during Cool mode. The Cant Cool state typically
becomes active when both primary (economizer) and secondary (compressor) cooling are not
available (or they are disabled) or when an IAT, DAT or OAT sensor failure occurs.
When the Cant Cool state becomes active, no cooling is available.
Special Purpose Unit Modes
There are some additional UVC modes that are considered special purpose unit modes. These
special purpose modes include Pressurize, Depressurize, Purge, Shutdown, and Energy Hold
Off. These modes force the UVC to perform very specific and limited functions. Use these
with caution and only for short periods as needed.
