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Sequence of Operation
Electric Heat Protection
There are three automatic high temperature limit switches
that will trip when exposed to a high temperature. They will
reset automatically once the temperature drops into an
acceptable range. If any one limit switch trips, all stages of
heat are turned off. The limits are mounted vertically on the
heater terminal plate and all three have the same
temperature trip point.
A manual reset temperature limit is also mounted near the
top of the terminal plate and serves as backup protection.
When exposed to high temperature, it will turn all stages of
heat off until the reset button is manually reset. The electric
heat protections operate independently of the controller.
External Heat
The Symbio 800 controller supports external field supplied
heat sources. There is no factory wiring or coordination
between the controller and external heat. When External
Heat is configured, the supply fan operates as required in
heat modes and provides ventilation defined by the mode.
Single Zone-VAV units have an External Heat Supply Fan
Speed Setpoint (adjustable) that defines the supply fan
speed in Occupied Heat modes of operation. The supply
fan operates at maximum speed for all other heating
modes. Continuous and cycling fan modes are both
supported.
Multi Zone-VAV units operate the supply fan under duct
static pressure control, unless commanded to go to
Maximum Heat.
Unoccupied Cooling
Unoccupied Cooling mode is used when the building is
unoccupied and the space conditions are exceeding
temperature limits. The unit is normally off in unoccupied
mode. If a valid space temperature input rises above the
Unoccupied Cooling Setpoint, unit operation starts
unoccupied cooling to manage space temperature. The
controller operates in Discharge Air Control with ventilation
disabled while DX or economizer cooling capacity
increases to satisfy the Discharge Air Cooling Setpoint.
Multi Zone VAV units operate the supply fan with duct static
pressure control, other units operate the supply fan at
maximum speed. Cooling continues until space
temperature is 4°F less than the Unoccupied Cooling
Setpoint, the unit will then cycle off.
Unoccupied Cooling Mechanical Enable and Unoccupied
Cooling Economizing Enable are settings to enable or
disable cooling in unoccupied mode.
Unoccupied Heating
When the unit is in unoccupied mode and the valid space
temperature input falls below the Unoccupied Heating
Setpoint, unit operation starts unoccupied heating to
manage space temperature. The controller operates in
Discharge Air Control with ventilation disabled while
heating capacity increases to satisfy the Discharge Air
92
Heating Setpoint. The supply fan operates according to
heat type installed, see Heat Types for more information.
Heating continues until space temperature is 4°F greater
than the Unoccupied Heating Setpoint, the unit will then
cycle off.
Unoccupied Heating Enable is a setting to enable or
disable heating in unoccupied mode at the user interface.
External Control
Symbio 800 provides the ability for an external controller or
program to control supply fan, exhaust fan, and outdoor air
damper. This capability allows for custom programming
suited for specific application needs.
External Supply Fan Control
External supply fan control applies to variable volume
supply fan equipment. Supply fan operation must prove
running and any unit startup timing must expire before
external control is allowed. Supply Fan Speed Control
Enabled provides an enabled status when Supply Fan
Speed Setpoint is in control. The controller will annunciate
External Supply Fan Control on the user interface,
operating modes screen when active.
Supply Fan Speed Setpoint will only override the
controller's fan control in occupied Cool, Heat, and Fan
Only modes of operation. All other modes (such as Morning
Warm Up, Pre Cool, Daytime Warm Up) the supply fan is
operated under Symbio 800 control. Supply fan speed will
increase under Symbio 800 control according to heating
and cooling capacity requirements; space temperature
control unit configurations. Supply fan speed will decreases
under Symbio 800 control when high duct static pressure
limits are approached; discharge air control unit
configurations.
Table 38.
External supply fan control points
Object
Description
Name
Supply Fan
Enables supply fan speed setpoint control
Speed
Setpoint
Enable
Supply Fan
Supply fan speed is control via Supply Fan Speed
Speed
Setpoint. If disabled/inactive, speed control is under
Control
Symbio 800 control
Enabled
Supply Fan
Supply fan speed is being increased or decreased due
Speed
to a limit control action.
Limited
Supply Fan
External supplied supply fan speed setpoint value
Speed
Setpoint
Supply Fan
Active setpoint input to supply fan speed control
Speed
Setpoint
Active
Supply Fan
Indicates the requested speed to the supply fan, 0%
Speed
when fan is off.
Status
RT-SVX080A-EN
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