RT-SVF25C-EN
19
Sequence of Operation
When the thermostat fan selection switch is set to the “On”
position, the (IGN) keeps the indoor fan relay coil (F)
energized for continuous fan motor operation.
Electromechanical Evaporator Fan
Operation (for Cooling Only Units)
When the thermostat fan selection switch is set to the
“Auto” position, the thermostat energizes the indoor fan
relay coil (F) to start the indoor fan motor (IDM). The fan
relay (F) de-energizes after the cooling requirement has
been satisfied. When the heating cycle is terminated, the
indoor fan relay (F) coil is de-energized with heater
contactors.
When the thermostat fan selection switch is set to the “On”
position, the thermostat keeps the indoor fan relay coil (F)
energized for continuous fan motor operation.
Economizer Set-Up
Adjusting the minimum position potentiometer located on
the unit economizer actuator (ECA) sets the required
amount of ventilation air.
Ambient temperature is controlling the economizing cycle
by sensing the outside air dry bulb temperature.
Table 20,
p. 17
lists the selectable dry bulb values by potentiometer
setting.
Electromechanical Control Cooling with
an Economizer
The economizer is utilized to control the zone temperature
providing the outside air conditions are suitable. Outside
air is drawn into the unit through modulating dampers.
When cooling is required and economizing is possible, the
unit economizer actuator (ECA) opens the economizer
damper. The ECA continues to modulate the economizer
damper open/closed to keep the mixed air temperature in
the 50ºF to 55ºF range.
The thermostat will close the Y2 contacts to turn on
contactor (CC1) if mechanical cooling is required.
If economizing is not possible, the ECA drives the damper
to the minimum position setpoint when the indoor fan
relay (F) is energized and allows mechanical cooling
operation.
Electromechanical Control Heating
Operation (for Cooling Only Units)
When the system switch is set to the “Heat” position and
the zone temperature falls below the heating setpoint, the
thermostat closes W1 contacts the first stage electric heat
contactor (AH or AH & CH) is energized. If the first stage of
electric heat can not satisfy the heating requirement, the
thermostat closes W2.
When theW2 contacts close, the second stage electric heat
contactor (BH) is energized, if applicable. The thermostat
cycles both the first and second stages of heat “On” and
“Off” as required to maintain the zone temperature
setpoint.
Electromechanical Control Heating
Operation (for Gas Units)
When the system switch is set to the “Heat” position and
the zone temperature falls below the heating setpoint, the
Ignition module (IGN) initiates a heat cycle.
Ignition Module Low, Medium and
High Heat
Two-stage (IGN) runs self-check (including verification that
the gas valve is de-energized). (IGN) checks the high-limit
switches (TC01 & TC02) for normally closed contacts, the
pressure switch (PS) for normally open contacts, and the
flame rollout (FR) switch for continuity. (IGN) energizes
inducer blower on high speed to check pressure switch
closure.
If the pressure switch is closed, the inducer blower starts
a 20 second pre-purge (15 seconds on high speed followed
by 5 seconds on low speed).
If the pressure switch (PS) is still open, the inducer blower
will continue to be energized on high speed until pressure
switch closure.
After pre-purge completes, the (IGN) energizes the first
stage of the gas valve, initiates spark for 2 seconds
minimum, 7 seconds maximum (ignition trial) and detects
flame and de-energizes spark. From this point, a fixed 45
second indoor blower delay on timing starts.
After the indoor blower delay on is completed, the (IGN)
energizes the indoor blower. The (IGN) enters a normal
operating loop where all inputs are continuously
monitored. If the first stage of gas heat can not satisfy the
heating requirement, the thermostat closes W2.The (IGN)
energizes the second stage of the gas valve and the second
stage of inducer blower.
When the zone thermostat is satisfied, the (IGN) de-
energizes the gas valve. The (IGN) senses loss of flame.
The (IGN) initiates a 5 second inducer blower post purge
and 90 second indoor blower delay off at current speed.
The (IGN) de-energizes the inducer blower at the end of the
post purge. The (IGN) de-energizes the indoor blower at
the end of the selected indoor blower delay off.
