SC-Z20
MODULATING ZONE CONTROL SYSTEM
OVERVIEW
The SC-Z20 is a commercial zone control system that allows a single HVAC unit to have up to 20
individual zones. Each zone is controlled by its own space thermostat and a modulating damper.
When a majority of zones are calling for cooling, the system operates in the cooling mode. The SC-
Z20 controls up to two stages of cooling to maintain a cooling discharge air setpoint. If a majority of
zones are calling for heating, the system changes from the cooling mode to the heating mode. The
SC-Z20 then controls up to two stages of heating to maintain a heating discharge air setpoint. When
the number of heating and cooling requests are equal, the system satisfies the cooling request first.
SEQUENCE OF OPERATION
The SC-Z20 controls the system fan, two stages of cooling and two stages of heating. In the
occupied mode, the fan runs continuously. Cooling stages are cycled to maintain a discharge air
temperature of approximately 54° F. Heating stages are cycled to maintain a discharge air
temperature of approximately 140° F. The control panel is equipped with two slide switches
designated “
” used to increase the number of zones that must be calling before the
Minimum Calls
equipment will come on. All stages of heating and cooling have a five minute minimum ON/OFF time.
There is also a ten minute time delay between first and second stage. The system will remain in
whatever mode it is in for a minimum of fifteen minutes. If equal heating and cooling calls take place,
cooling will receive priority. Equipment mode of operation is based on a “Majority Wins” control
algorithm.
UNOCCUPIED MODE
The panel has a set of clock terminals “ ” for occupied and unoccupied scheduling. When the clock
T
terminals are open, the system is placed in the unoccupied mode. All stages of heating or cooling are
turned off and the fan shuts off after a five minute delay. See wiring and programming instruction in
this manual for the SC-Z20-NS Night Stat.
WARNING
1. Turn power off to HVAC unit and control panel during installation to prevent serious injury from
electrical shock and/or damage to the system.
2. Use extreme care when making duct openings and handling sheet metal to avoid injury.
3. The SC-Z20 panel and all components must be installed in a conditioned space where the
ambient temperature will not exceed 150° F.
4. Install all components in a manner that provides access for test, check and startup.
CAUTION
1. Installation of this system must be in compliance with all applicable codes.
2. HVAC system must be properly sized and balanced to assure optimum system performance.
3. A bypass damper must be installed to maintain proper system static pressure.
4. All low voltage wire must be 18 gauge.
5. All wiring should be color coded and tagged for proper identification.
6. When landing wires to the control panel, zone thermostats and actuator boards, make sure wires
are snug, but do not over-tighten screw terminals.