Catalogue

UNT-PRC001-EN 49
Controls
Control Features
The following control functions are standard features on units with Tracer ZN010, ZN510, ZN520,
or UC400.
Entering Water Temperature Sampling Function
A system that uses a two-way control valve option might not sense the correct entering water
temperature during long periods when the control valve is closed. If the demand for heating or
cooling does not exist for a long period, the entering water will eventually approach ambient
temperature.
Using the entering water temperature sampling function, the controller provides accurate two-pipe
system changeover—without sacrificing the benefits of two-way control valves. Also, it eliminates
inefficient bleed or bypass lines that can allow unnecessary waterflow through the system.
This function periodically samples the entering water temperature by opening the hydronic valve.
The valve opens for 20 seconds to allow the water temperature to stabilize. Then the controller
reads the entering water temperature for up to three minutes to see if the correct water temperature
is available for the selected operating mode.
The entering water temperature 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 correct water temperature for the operating mode is available, the unit begins normal
heating or cooling operation. If the correct water temperature is not available, the controller closes
the control valve and waits 60 minutes before attempting to sample the entering water temperature
again.
A factory-mounted thermistor senses the entering water temperature on changeover
cooling/heating coil units. If the fan-coil has a factory-mounted piping package, the sensor is
strapped to the entering water pipe. If the fan-coil does not have a piping package, the sensor is
coiled in the end pocket for mounting on customer-supplied piping. This sensor must detect
accurate water temperature for proper changeover.
Automatic Heat/Cool Mode Determination
The controller automatically determines whether heating or cooling is needed—based on space
and system conditions. Utilizing a proportional/integral (PI) control algorithm to maintain the space
temperature at the active heating or cooling setpoint. The controller measures the space
temperature and active setpoint temperature to determine the unit’s heating or cooling capacity
(zero to 100 percent).
Occupied/Unoccupied Operation
The occupancy input utilizes a binary switch (i.e. motion sensor, timeclock, etc.) that allows the zone
sensor to utilize its unoccupied internal setpoints.
Random Start
This feature randomly staggers multiple unit start-up to reduce electrical demand spikes.
Warm-up
The two-position fresh air damper option closes during the occupied mode when the space
temperature is three degrees or more below the heating setpoint temperature. The damper
remains closed during warm-up until the space temperature is within two degrees of the heating
setpoint temperature.
Cool-down
The two-position fresh air damper option closes during the occupied mode when the space
temperature is three degrees or more above the cooling setpoint temperature. The damper
remains closed during cool-down until the space temperature is within two degrees of the cooling
setpoint temperature.
Control Features
UNT-PRC001-EN.book Page 49 Tuesday, May 8, 2012 10:54 AM