Tracer® ZN.520
| 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 considering 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 control- |
| ler continues to compare 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® ZN.520 controller supports 1- or |
| 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 |
| units with electric heat use the electric heat only when hot water is |
| not available. |
Fresh Air Damper Options
70
Manual
Units with the manual fresh air damper option ship with the damper in the closed position, which is adjustable from zero to 100 percent in
25 percent increments. To adjust the position, first remove the air filter
