Trane Tracer ZN.520 Manual page 57

Sequence of operation
If the measured space temperature is 69°F and the active cooling setpoint is
72°F , the error between the space temperature and the setpoint is three degrees.
If the same error exists for 1 min (60 sec), the integration term is (3°FD60 sec)
or (180°F • sec).
The Tracer ZN.520 zone controller changes from heating to cooling and cooling
to heating when the integration term exceeds (900°F • sec). Along with
satisfying the integration for heating and cooling changeover, the measured
space temperature must fall outside the setpoint range. This means the space
temperature must be greater than the active cooling setpoint or lower than the
active heating setpoint.
Example: If the cooling setpoint is 75°F and the heating setpoint 70°F , any
space temperature greater than 75°F or less than 70°F is outside the setpoint
range.
Once the integration term is satisfied and the space temperature is outside the
setpoint range, the controller changes modes. However, before the unit's
heating or cooling capacity ramps up, the controller checks to make sure it is
capable of heating or cooling.
For some units, heating and cooling capability exists with local resources such
as electric heat or compressors. For these units, central heating or cooling plan
operation is not required for heating or cooling because they are capable of
providing their own local heating or cooling.
For 2-pipe changeover and 4-pipe changeover units with hydronic capacity,
heating and cooling is provided through hydronic. For those hydronic, central
heating or cooling plant operation is required for the unit to deliver heating or
cooling. To determine whether the central plant is providing the desired water
temperature, an entering water temperature sensor (either hardwired or
communicated) must be present.
The entering water must be five degrees or more above the space temperature
to allow hydronic heating, and five degrees or more below the space
temperature to allow hydronic cooling.
If the desired water temperature is available, the unit begins normal heating
and cooling operation. If the measured entering water temperature is not
adequate for the desired heating or cooling, the controller begins the entering
water temperature sampling logic.
The Tracer ZN.520 zone controller operates the modulating valves and dampers
based on a heating or cooling capacity calculated by the control algorithm. The
control algorithm calculates the heating or cooling capacity based on the
measured space temperature, the active setpoint and the discharge air
temperature. When the measured space temperature is within the active
heating and cooling setpoints, the heating and cooling capacity approaches
zero.
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