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Chapter 5 Sequence of operations
46 CNT-SVX07C-EN
Two-position control of a modulating
outdoor air damper
The Tracer ZN521 does not support a two-position outdoor air damper
actuator. However, a modulating outdoor/return air damper actuator can
be used for two-position control. Two-position control can be accomplished
by not providing an outdoor air temperature (neither hard-wired nor com-
municated) to the controller, and by setting the damper minimum posi-
tion (using the Rover service tool) to the desired value (typically, 100%).
Face-and-bypass damper operation
The Tracer ZN521 can control a face-and-bypass damper to modulate a
percentage of air to the face of the coil(s) and around (bypass) the coil(s) to
maintain zone comfort. For two-pipe changeover applications, if the con-
troller requests heating and hot water is available, the face-and-bypass
damper modulates to the face position. If the controller requests heating
and hot water is not available, the face-and-bypass damper remains in
the bypass position, and water sampling may be initiated (see “Entering
water temperature sampling function” on page 40). For four-pipe
changeover applications, both heat and cool capacity are assumed to be
available.
Face-and-bypass, isolation-valve operation
A Tracer ZN521 with a face-and-bypass damper controls on/off isolation
valve(s) to prevent unwanted water flow through the coil(s) when no air-
flow is passing over the coil (100% bypass). In two-pipe applications, the
isolation valve stops water flow, preventing radiant heat (heating mode)
and excessive condensate (cooling mode) from the coil. In four-pipe appli-
cations, the isolation valves are used to prevent simultaneous heating
and cooling.
The heating or cooling isolation valve opens whenever capacity is
requested (>0%) as the face-and-bypass damper begins to modulate
toward the coil face. The isolation valve closes when capacity returns to
0%.
DX cooling operation
The Tracer ZN521 supports one stage of direct expansion (DX) compres-
sor operation for cooling only.
Note:
The controller does not use the DX compressor and economizing
simultaneously. Not doing so prevents short cycling from occur-
ring if the entering air temperature is too low for the evapora-
tor coil to operate as designed.