Trane Tracer ZN.520 Manual page 66

Sequence of operation
2-position valve operation
Tracer ZN.520 zone controller supports 2-position valves for hydronic heating
and cooling operation. The main valve/coil is used for heating only, cooling
only, or heat/cool changeover. The auxiliary valve/coil provides only heating for
the following applications:
I 4-pipe heat/cool
I 4-pipe heat/cool changeover
The auxiliary valve/coil is capable of providing only heating (4-pipe operation).
Typical 2-pipe changeover configurations allow for both cooling (summer
mode) and heating (winter mode) with the main valve/coil.
Four-pipe configurations normally utilize the main valve/coil as the cooling
source and the auxiliary valve/coil as the heating source. The controller also
supports main coil changeover for 4-pipe configurations.
To control the space temperature, 2-position valves are cycled to control the
discharge air temperature. The rate of cycling is dependent upon entering water
temperature, the load in the space, and the temperature of the incoming fresh
air from the economizer (if any).
Face and bypass isolation valve operation
Face and bypass units may use isolation valves to prevent unwanted water flow
in the coil when no air flow is present through the coil (bypass). In 2-pipe
applications, the isolation valve stops water flow, preventing radiant heat and
excessive condensate from the coil.
In 4-pipe applications, the isolation valves are used to prevent conflicting
capacities (hot and chilled water) from entering the coil at the same time. Only
one isolation valve allows water flow at a time during normal operation.
Two-pipe operation
When the 2-pipe changeover unit is configured with electric heat, the controller
uses the electric heat if the entering water temperature is not appropriate for
hydronic heating.
Two-pipe heating-only units and all units with a heat control request (provided
by a communicated heat/cool request) are capable of providing only heating.
Two-pipe cooling-only units and all units with a cool control request (provided
by a communicated heat/cool request) are capable of providing only cooling.
Two-pipe changeover units capable of both hydronic heating and hydronic
cooling determine their mode based on the following sequence (summarized in
Table 37 on page 65):
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