COMRES triggers the compressor staging routine that controls the number of compressors energized. Units are equipped with 4 compressors piped in separate refrigerant circuits, and staged On/Off in a fixed sequential manner (compressor no. 1 through compressor no. 4). The compressor control routine uses a PID calculation to determine the percent- age of cooling required, from 1 to 100%. Demand for the PID calculation is determined from the supply air temperature and the supply air set point (SETPT06).
Compressor cooling (COMPRES) will be turned off for any of the following reasons:
•There is no condenser water flow (CDWF is Off).
•Economizer Freezestat (FREEZ) has been in alarm for more than 15 minutes.
•MODE changes to heat.
•OK-FAN turns off during normal shut down.
•During normal compressor operation the minimum on time is 5 minutes and the minimum off time is 5 min.
ECONOMIZER COOLING (50XJ) — The unit diverts con- denser inlet water flow through an optional economizer coil to precool evaporator entering airflow. This occurs when there is demand for the cooling, and the temperature at an Entering Water Temperature (EWT) thermistor is colder than the economizer start set point. Waterflow is controlled via two electronic water flow valves. This option also incorporates an Economizer Freeze Switch (EFS), located at the inlet of the economizer coil.
Economizer water flow is in series with the condensers allowing compressor operation while the economizer is operating.
If the Fan is on, and there is no demand for heat then the equipment mode (MODE) will be COOL and Cooling (COOLOK) will switch to ENABLE.
If the unit is configured for variable flow the Reverse/Head Pressure CTRL valve will open (otherwise it will already be open), and if there is condenser water flow (CDWF is YES) then the Fan + Condenser water flow point will become TRUE.
If the entering-water temperature is below the Economizer start set point in the configuration parameters table (Table 10), then the Mod. Econ Enabled point (ECONOK) will change to enable and the Economizer valve will modulate open to lower the Economizer control temp to the supply air set point (SETPT06) temperature. The economizer modulation is controlled by a PID loop and the Reverse/Head pressure control valve will modulate in reverse of the Economizer valve using the formula MVLV = 100 – ECONO.
ECONOMIZER COOLING (50BV) — The unit diverts con- denser inlet waterflow through an optional economizer coil to precool evaporator entering airflow. If the entering-water temperature is colder than the setting on the Aquastat, and the return-air temperature is warmer than the setting on the return air thermostat, the two-position diverting valve will direct water to the economizer coil.
Economizer water flow is in series with the condensers allowing compressor operation while the economizer is operating.
COOLING RESET (50XJ) — The controller can reset the supply air set point using these three methods:
•An external 0 to 10 volt input RESET
•The value of the space control point
•Linkage
The external 0 to 10 volt input reset is configured to produce a 0 to 20 degree supply air reset over the 2 to 10 volt range. If more than 1.8 volts is sensed on the input, this method of reset takes priority over other methods.
NOTE: The reset from all methods may be limited to 10 F or 15 F by changing the high end point of the custom voltage input from the default (20 F) to 15 F or 10 F.
Either the return air or a space temperature sensor will be used as the space control point. If this variable goes below the Occupied High set point in the HEAT/COOL MODE AND RESET set point (SETPT03), then for each degree that the Space control point is below the set point value the supply air set point will be reset by the value configured in the custom configuration RESET RATIO.
If Linkage is active, for each degree that the average occupied space temperature is below the average occupied cool set point, the supply air set point will be reset by the amount configured in the RESET RATIO. Reset will be limited to the maximum value the custom voltage input RESET can display.
COOLING RESET (50BV) — The 5k ohm temperature sen- sor will be used as the space control point. If this variable goes below the Occupied High set point in the HEAT/COOL MODE AND RESET set point (SETPT03), then for each degree that the Space control point is below the set point value the supply air set point will be reset by the value configured in the custom configuration RESET RATIO.
HEATING (50XJ) — The controller is configured to control two types of heat:
•A modulating 4 to 20 mA output Hot Water Valve (HWV) in the base unit, wired to the second module.
•Four stages of staged heat wired to a third, accessory module (PCB3).
For either method of heat to function, a space control point must be configured in the custom configuration. This control point comes from a return air sensor or space sensor, or from the average space temperature received through linkage.
Whenever the space control point is below the occupied or unoccupied heat set point the mode will change to heat and if unoccupied the fan will be started. For linkage, this occurs if the average space temperature is below the appropriate average heat set point.
Both heat control routines use a PID to calculate a supply air set point that will satisfy the heat demand in the space. The modulating output and the staged outputs will both operate at the same time to control an attached heat source, such as steam valves or electric heaters, to provide the supply-air temperature required.
The heat mode changes back to cool when the space control point is back above the occupied heat set point. For linkage, the mode changes back to cool when the average space tempera- ture is back above the average occupied heat set point.
When unoccupied heat is enabled the fan will be stopped and the heat turned off when the space control temperature is more than halfway above the difference between the occupied heat set point and the unoccupied heat set point. For example, if the occupied heat set point is 70 and the unoccupied heat set point is 60 the unit will come on for unoccupied heating below 60 F and turn off again above 65 F. The average occupied and unoccupied set points are used when linkage is active.
HEAD PRESSURE CONTROL (HPC) (50XJ) — In instal- lations where entering water temperature can fall below 55 F, where a water economizer (described above) is not installed, the HPC provides 1 or 2 electronic water flow control valves to vary flow to the condensers. Controlling the water flow maintains compressor discharge pressure above a minimum value, ensuring sufficient refrigerant flow out of the condenser and throughout the refrigerant circuit. Refrigerant pressure is measured at compressor circuit no. 1 by a Discharge Pressure Sensor (DPS).