Johnson Controls RC Series Engineering Manual page 10

RC SERIES ENGINEERING GUIDE
Controls cont.
The user selects either "Heat/Cool" or "Fan Only" on the mode switch,
then either "High" or "Low" at the fan speed switch. The temperature
can be controlled by rotating the thermostat control knob.
Figure 6: Unit Mounted Control
The "Fan Only" setting provides constant fan operation.
In the "Heat" mode, a call for heat by the thermostat closes the
compressor contactor contacts, energizing the compressor, which
will run until the thermostat is satisfied.
In the "Cool" mode, a call for cooling by the
thermostat energizes the reversing valve and closes the
compressor contactor contacts, energizing the compressor, which
will run until the thermostat is satisfied.
The emergency electric heat operation in the "Heat/Cool" mode
is subject to the setting of the internally mounted mode switch.
The optional, factory installed electric heat will operate when the
internally mounted mode switch is in the "Emergency Heat" mode.
In the "Heat" mode, a call for heating energizes the fan and electric
heater contactor, energizing the electric heat elements and fan,
which will run until the thermostat is satisfied. When the internally
mounted mode switch is in the "Normal/Boilerless" mode the unit
operates in its normal "Heat/Cool" operation, unless there is an
aquastat controller. When the normally open circuit of the aquastat
closes and the unit is in the heating mode, it will switch to the
"Emergency Heat" condition until the thermostat is satisfied or the
aquastat opens restarting the compressor.
If either the low or high pressure safety switches are opened, the
compressor and reversing valve are disabled by the lockout relay.
Unit operation will resume only after the voltage to the unit is
interrupted or the mode switch is placed in the "Off" position.
If the electric heat limit switches are opened, the electric heat
is disabled.
Optional Versatec Microprocessor
Control Features
The Versatec microprocessor board provides control of the entire
unit as well as outputs for status modes, faults and diagnostics.
The control system is a microprocessor-based control board that
is located in the unit control box. This feature is available for either
unit mounted controls or optional remote wall mounted thermostat.
A 9-pin low voltage terminal strip provides all necessary terminals
for the wall mounted thermostat.
Startup
The unit will not operate until all the inputs and safety
controls are checked for normal operating conditions.
Fault Retry
All faults are retried twice before finally locking the unit out to
prevent nuisance service calls.
Component Sequencing Delays
Components are sequenced and delayed for optimum unit
performance.
Short Cycle Protection
The control allows a minimum on or off time of 5 minutes for short
cycle protection.
Load Shed
A grounded signal to common or connecting 24 VAC to the LS
terminal places the controller into the load shed mode. The
compressor will become disabled and the fan will start upon a
thermostat call for heating or cooling.
Emergency Shutdown
A grounded signal to common or connecting 24 VAC to the ES
terminal places the controller into the emergency shutdown mode.
The compressor and fan operation are suspended while in the
emergency shutdown mode.
Condensate Overflow Protection
The board incorporates an impedance liquid sensor at the top of
the condensate drain pan. Upon a continuous 30-second sensing
of the condensate, the cooling operation of the unit is suspended.
Safety Controls
The microprocessor board receives separate signals from a high
pressure switch for safety, a low pressure switch to prevent loss of
refrigerant charge and a low suction temperature thermistor for freeze
sensing. Upon a continuous 30-second measurement of the fault
(immediate for high pressure), compressor operation is stopped.
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