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16 McQuay OM 751
Description of Operation
Emergency Heat Mode (Super State)
The Emergency Heat mode is provided for situations where the UVC is in a mode that does
not normally allow heating, such as OFF, Cool, Night Purge, or Fan Only. If Emergency Heat
mode is enabled, the UVC can automatically force itself into the Emergency Heat mode from
OFF, Cool, Night Purge, Fan Only, Purge, Pressurize, De-pressurize, and Shutdown.
Emergency Heat mode consists of UVC states: Full Heat [7] and Cant Heat [D].
Software model 05 does not have the primary or secondary heating devices. The UVC uses
auxiliary heat (if field provided and field connected) when emergency heat is required.
When the Emergency Heat mode becomes active, the UVC automatically determines which
state to make active, Full Heat [7], or Cant Heat [D], based on the transitions for each of those
states.
Figure 10: Emergency heat state diagram
Full Heat State (State 7)
The Full Heat [7] state is the “normal” state that the UVC goes into when Emergency Heat
mode is active. It is activated when the space temperature is lower than the EHS.
When Emergency Heat mode becomes active, the UVC goes into 100% heating until the space
temperature raises to the EHS plus a fixed differential (5.4°F/3°C). In the Emergency Heat
mode, the space fan is set to high speed, and the OA damper operates normally.
If the UVC automatically forces itself into the Emergency Heat mode from another mode
(e.g., Cool, Fan Only, etc.), then the UVC returns to the appropriate unit mode once the space
temperature rises to the EHS plus a fixed differential (5.4°F/3°C).
The UVC monitors the DAT to ensure it does not exceed DAHL. If the DAT does exceed
DAHL, then heating is set to 0% for a minimum of 2-minutes (fixed) and until the DAT drops
36°F (20°C) fixed differential below DAHL.
Cant Heat State (State D)
The Cant Heat [D] state is a “non-normal” state that the UVC can go into when Emergency
Heat mode is active. An IAT or DAT sensor fault during Emergency Heat mode causes the
UVC to make this state active.
When the Cant Heat state becomes active, the space fan remains at high speed as set during
the Full Heat state.
The UVC will remain in the Cant Heat state until heat becomes available.
Full Heat
Cant Heat
7
D
UVC Mode
≠
Emergency Heat
UVC Mode
=
Emergency Heat
Transition
point
Heat
≠
Available
Heat
=
Available
Emergency Heat Mode
Super State