Chapter 5 Sequence of operations
In heating and cooling mode, the controller maintains the zone tempera- ture based on the active heating setpoint and the active cooling setpoint, respectively. The active heating and cooling setpoints are determined by the occupancy mode of the controller.
For two-pipe and four-pipe changeover units, normal heat/cool operation will not begin until the ability to conduct the desired heating or cooling operation is verified. This is done using the entering water temperature sampling function, for which a valid entering water temperature is required. When neither a hard-wired nor a communicated entering water temperature value is present on changeover units, the controller will operate in heating mode only and assume the coil water is hot. The sam- pling function is not used.
The entering water temperature sampling function is used only for changeover applications. It is used for information and troubleshooting only and does not affect the operation of the controller. (For more informa- tion, see “Entering water temperature sampling function.”)
Entering water temperature sampling function
The entering water temperature sampling function is used with two-pipe and four-pipe changeover units and requires a valid entering water tem- perature value. If the entering water temperature value is less than 5°F (2.8°C) above a valid zone temperature value for hydronic heating and greater than 5°F (2.8°C) below a valid zone temperature value for hydronic cooling, the sampling function is enabled. When the sampling function is enabled, the controller opens the main hydronic valve to allow the water temperature to stabilize. After 3 minutes, the controller again compares the entering water temperature value to the zone temperature value to determine if the desired heating or cooling function can be accomplished. If the entering water temperature value remains out of range to accomplish the desired heating/cooling function, the controller closes the main hydronic valve and waits 60 minutes to attempt another sampling. If the entering water temperature value falls within the required range, it resumes normal heating/cooling operation and disables the sampling function.
Fan operation
The Tracer ZN521 supports up to three fan speeds. Every time the fan is enabled, the fan will begin operation and run on high speed for a period of time (0.5 seconds for fan coils and 3 seconds for unit ventilators and blower coils) before changing to any other speed. This is done to provide adequate torque to start the fan motor from the off position. The fan will always operate continuously while either heating or cooling during occu- pied, occupied standby, and occupied bypass operation. During unoccu- pied operation, the fan will cycle between off and high regardless of the
