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Modulating outdoor/return air dampers
CNT-SVX07C-EN 43
Two-position valve operation
The Tracer ZN521 supports two-position valves with a single binary out-
put for each valve. Controllers used for two-pipe applications support
heating, cooling, or heat/cool changeover with a single valve/coil. Control-
ler used for four-pipe applications support cooling or heat/cool changeover
with a main valve/coil, and heating only with an auxiliary valve/coil.
Isolation-valve operation
See “Face-and-bypass damper operation” on page 46.
Two-pipe operation
For two-pipe applications, the Tracer ZN521 can be configured as heating
only, cooling only, or heat/cool changeover. The coil can be used as the pri-
mary heating source and/or the primary cooling source. If present, an
electric heating element can be used only as the primary heating source
(instead of hydronic or steam heating). A changeover unit requires a valid
entering water temperature value—either communicated or hard-wired—
to operate properly (see “AI1: Entering water temperature” on page 17
and “Entering water temperature sampling function” on page 40).
Four-pipe operation
For four-pipe applications, the Tracer ZN521 can be configured as heat/
cool or heat/cool changeover. The main coil can be used as the primary
cooling source or the primary heating/cooling source. The auxiliary coil
can be used only as the primary heating source, not as a second stage of
heating. During normal operation, the controller never uses the main coil
and auxiliary coil simultaneously. A changeover unit requires a valid
entering water temperature value—either communicated or hard-wired—
to operate properly (see“AI1: Entering water temperature” on page 17
and “Entering water temperature sampling function” on page 40). Elec-
tric heat control is not available on four-pipe applications.
Modulating outdoor/return air dampers
The Tracer ZN521 operates the modulating outdoor/return air dampers
according to the following factors:
• Occupancy mode
• Outdoor air temperature (communicated or hard-wired sensor)
• Zone temperature
• Setpoint
• Discharge air temperature
• Discharge air temperature setpoint
The minimum position for an outdoor air damper is configurable using
the Rover service tool for occupied and occupied standby modes and for