Fan Start on High Speed

sequence of

Operation operation

On a transition from off to any other fan speed, the TracerZN520 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.

Entering Water Temperature Sampling Function

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.

When three-way valves are ordered with a Tracer ZN520 control, the controller is factory-configured to disable the entering water temperature 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 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 mea- sured space temperature. Entering water is warm enough to heat when it is five degrees above the measured space temperature.

When the controller invokes the entering water temperature sampling function, the unit opens the main hydronic valve for no more than three minutes before consid- ering the measured entering water temperature. An initial stabilization period is allowed to flush the coil. This period is equal to 30 seconds plus ½ the valve stroke time. Once this temperature stabilization period has expired, the controller compares the entering water temperature against the effective space temperature (either hardwired or communicated) to determine whether the entering water can be used for the desired heating or cooling. If the water

temperature is not usable for the desired mode, the controller continues to com- pare the entering water temperature against the effective space temperature for a maximum of three minutes.

The controller automatically disables the entering water temperature sampling and closes the main hydronic valve when the measured entering water exceeds the high entering water temperature limit (110°F). When the entering water temperature is warmer than 110°F, the controller assumes the entering water temperature is hot because it is unlikely the coil would drift to a high temperature unless the actual loop temperature was very high.

If the entering water temperature is unusable — too cool to heat or too warm to cool — the controller closes the hydronic valve and waits 60 minutes before initializing another sampling. If the controller determines the entering water temperature is valid for heating or cooling, it resumes normal heating/ cooling control and effectively disables entering water temperature sampling until it is required.

Electric Heat Operation

The Tracer ZN520 controller supports one or two-stage electric heat operation for heating. To control the space temperature, electric heat is cycled to control the discharge air temperature. The rate of cycling is dependent upon the load in the space and the temperature of the incoming fresh air from the economizer (if any). Two-pipe changeover units with electric heat use the electric heat only when hot water is not available.

Manual Fresh Air Damper

Units with the manual fresh air damper option ship with the damper in the closed position, which is adjustable from zero to 100% in 25% increments. To adjust the position, first remove the air filter to expose the damper stop screw on the control panel end. Relocate the stop screw to the appropriate position. Then loosen the stop screw wingnut and adjust the linkage.

Economizer Damper Option

With a valid outdoor air temperature (either hardwired or communicated), Tracer ZN520 uses the modulating economizer damper as the highest priority source of cooling. Economizer operation is only possible through the use of a modulating damper.

Economizing is possible during the occupied, occupied standby, unoccupied, and occupied bypass modes.

The controller initiates the economizer function if the outdoor air temperature is cold enough to be used as free cooling capacity. If the outdoor air temperature is less than the economizer enable setpoint (absolute dry bulb), the controller modulates the outdoor air damper (between the active minimum damper position and 100%) to control the amount of outdoor air cooling capacity. When the outdoor air temperature rises 5°F above the economizer enable point, the control- ler disables economizing and moves the outdoor air damper back to its predeter- mined minimum position based on the current occupancy mode or communi- cated minimum damper position.

Dehumidification Dehumidification is possible when mechanical cooling is available, the heating capacity is located in the reheat position, and the space relative humidity setpoint is valid.The controller starts dehumidifying the space when the space humidity exceeds the humidity setpoint.

The controller continues to dehumidify until the sensed humidity falls below the setpoint minus the relative humidity offset.The controller uses the cooling and reheat capacities simultaneously to dehumidify the space. While dehumidify- ing, the discharge air temperature is controlled to maintain the space tem- perature at the current setpoint.

A typical scenario involves high humidity and high temperature load of the space.The controller sets the cooling capacity to 100% and uses the reheat capacity to warm the discharge air to maintain space temperature control. Dehumidification may be disabled via Tracer or configuration.

56	UNT-SVX07A-EN