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McQuay OM 751 17
Description of Operation
Auto Mode
Auto mode is provided so that the UVC can be set to automatically determine if heating or
cooling is required. Auto mode is the default power-up UVC mode. Auto mode is made up of
the Heat and Cool modes. When the UVC is set to auto mode, the UVC automatically
determines which mode (Heat or Cool) to use.
Heat Mode (Super State)
When in Heat mode, the UVC will use auxiliary heat (if field provided and field connected to
the unit) as needed to maintain the effective heating setpoint (see “Space Temperature Set
Points” on page 26). The keypad/display or a network connection can be used to force the unit
into the Heat mode. Additionally, the UVC when set to Auto mode can automatically force the
unit into the Heat mode as needed. When the UVC is in Auto mode, it is “normal” for the
UVC to “idle” in Heat mode when there is no need to switch to another mode.
The Heat mode super state consists of UVC states: Heat [5], Low Limit [E], and Cant Heat
[B].
When the Heat mode super state becomes active, the UVC automatically determines which of
the Heat Mode states to make active based upon the transitions for each state.
Figure 11: Heat mode super state diagram
Heat State (State 5)
The Heat state is the “normal” state during Heat mode. When the Heat state becomes active,
the UVC will (within State) continually calculate the DATS (“Discharge Air Temperature
Control” on page 29) required to maintain the effective heat setpoint (see “Space Temperature
Set Points” on page 26). The calculated DATS will not be allowed to go above DAHL. The
UVC will use auxiliary heat (if field provided and field connected to the unit) as needed to
maintain the current DATS.The auxiliary heat binary output will be used as needed. The Heat
Timer (3-minutes fixed) will begin counting. The CO
2
demand controlled ventilation function
will be active, if the unit is equipped for CO
2
control (see “CO2 Demand Controlled
Ventilation (optional)” on page 34), and the OA damper will be adjusted as needed to maintain
the CO
2
setpoint. The UVC will remain in this state until one of the transition out conditions
become true, or until one of the super state transition out conditions becomes true.
Heat
Low Limit
Cant Heat
5
E
B
Transition
point
Heat Mode
Super State
UVC Mode
≠
Heat AND
UVC Mode
≠
Auto
OR
UVC Mode
=
Auto AND Space =
Warm AND Heat Pl = Sat Low
UVC Mode = Heat
OR
UVC Mode
=
Auto AND
Space
≠
Warm
Heat Pl = Sat Hi
(2 minutes)
AND
DAT<VCLL
LLPl = Sat Lo
(2 minutes)
AND
DAT<(VCLL + 1.8F)
Heat = Available
AND
Low Limit = Inactive
Heat = Available
AND
Low Limit = Inactive
Heat
≠
Available
Heat
≠
Available