Trane LO - Manual (Page 69)

UNT-IOM-6 69
Tracer ® ZN.520
On a transition from off to any other fan speed, the Tracer ® ZN.520
controller automatically starts the fan on high speed and runs the fan
at high speed for 0.5 seconds. This provides the ample torque
required to start all fan motors from the off position.
Tracer ® ZN.520 can sample the entering water temperature for 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 desired water temperature from the central plant.
When three-way valves are ordered with a Tracer ® ZN.520 control, the
controller is factory-configured to disable the entering water tempera-
ture sampling function, and the entering water sensor is mounted in
the proper location. Disabling entering water temperature sampling
eliminates unnecessary water flow through the main coil when three-
way valves are used.
The Tracer ® ZN.520 controller offers a control solution for two-way
valve applications that does not require special unit considerations,
such as those required by bleed lines. The controller includes an
entering water temperature sampling function that periodically opens
the two-way valve to allow temporary water flow, producing reliable
entering water temperature measurement.
.
Only units using the main hydronic coil for both heating and cooling
(2-pipe changeover and 4-pipe changeover units) use the entering
water temperature sampling function. Two-pipe changeover and 4-pipe
changeover applications allow the main coil to be used for heating
and for cooling; therefore, these applications require an entering water
temperature sensor.
The entering water temperature value is useful to the unit controller
only when heating or cooling is required. The entering water tempera-
ture must be five degrees above the space temperature for hydronic
heating and five degrees below the space temperature for hydronic
cooling. When water flows normally and frequently through the coil,
the controller does not invoke the sampling function because the
water temperature is satisfactory for the desired heating or cooling.
The controller invokes entering water temperature sampling only when
the measured entering water temperature is too cool to heat or too
warm to cool. Entering water is cold enough to cool when it is five
degrees below the measured space temperature. Entering water is
Fan Start on High Speed
Entering Water
Temperature
Sampling Function
Heating or Cooling Required