Description of Operation

The CO2 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 CO2 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 CO2 DCV feature is provided and CO2 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 CO2 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 CO2 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.

McQuay OM 751

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American Standard UV05 manual Special Purpose Unit Modes, Discharge Air DA Heat State State, Low Limit State State F