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BCXC-SVX01B-EN 55
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Two- and Four-Pipe Changeover Operation
Tracer™ ZN controllers offer accurate and reliable unit changeover using 2-way valves and the
controller’s entering water temperature sampling function. Only units using the main hydronic coil
for both heating and cooling (2-pipe and 4-pipe changeover units) use the entering water
temperature sampling function.
Two-pipe and 4-pipe changeover applications require an entering water temperature sensor to
allow the main coil to be used for heating and cooling. This sensor is factory-provided and should
be field-installed on the entering water pipe.
The entering water temperature sampling function periodically opens the two-way valve to allow
temporary water flow, producing reliable entering water temperature measurement. To ensure
accurate unit changeover without sacrificing the benefits of 2-way, 2-position valves, Tracer™ ZN
controllers periodically test the entering water temperature on all hydronic main coil changeover
units. Hydronic heating/cooling changeover operation requires central plant operation, and the
unit controller must use an entering water temperature sensor to verify delivery of the correct water
temperature from the central plant.
Entering Water Temperature Sampling Function
The entering water temperature (EWT) must be five degrees above the space temperature for
hydronic heating and five degrees below the space temperature for hydronic cooling. When water
Table 23. Local fan switch enabled
Communicated Fan switch (local) Fan operation fan speed input
Off Ignored Off
Low Ignored Low
High Ignored High
Auto Off Off
Low Low
High High
Auto Auto (configured default, determined by heat/cool mode)
Table 24. Fan operation in heating and cooling modes
Fan mode
Heating Cooling
Occupied Unoccupied Occupied Unoccupied
Off Off Off Off Off
Low Low Off/high Low Off/high
High High Off/high High Off/high
Auto (continuous) Default fan speed Off/high Default fan speed Off/high