RT-SVX51L-EN
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When the W2 contacts close, the second stage electric
heat contactor (BH or BH and DH) 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.
Heating Operation (for Gas Units) -
Standard Economizer
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 and TC01) 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 cannot 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.
Table 8.
Ignition module diagnostics
Steady light
Module is powered up, but no active call for
heat.
Blinking at
continuous steady
rate
Active call for heat.
One blink
Loss of communication.
Two blinks
System lockout (failure to ignite, no spark,
low/ no gas pressure, etc.)
Table 8.
Ignition module diagnostics (continued)
Three blinks
Pressure switch (no vent air flow, bad CBM,
closed at initial call for heat). Auto reset.
Four blinks
High limit (excessive heat in combustion
chamber, low airflow). Auto reset.
Five blinks
Flame sensed and gas valve not energized
or flame sensed and no call for heat.
Six blinks
Flame rollout (CBM failure, incorrect gas
pressure, and incorrect primary air). Requires
manual reset of the switch.
Seven blinks
W1 and W2 swapped (electromechanical 3-
10 tons units).
Drain Pan Condensate Overflow Switch
(Optional)
The Condensate Overflow Switch (COF) is utilized to
prevent water overflow from the drain pan. The float switch
is installed on the corner lip of the drain pan. When the
condensate level reaches the trip point, the COF relay
energizes and opens the 24 Vac control circuit which
disables the unit. Once the 24 Vac control circuit is opened,
a delay timer will prevent unit start-up for three minutes.
Low Ambient Operation (Optional)
During low ambient operation, outside air temperature
below 50°F, the Low Ambient Controller will cycle the
outdoor fan motor "Off" and "On" based on discharge
pressure. The indoor fan motor (IDM) will continue to
operate and the outdoor fan will return to normal operation
once the ambient temperature is above 50°F.
Two-Speed Indoor Fan Control
For units equipped with two-speed indoor fan control,
standard unit operation for cooling and heating will
continue and all unit functions will operate normally, except
for the control of the indoor fan speed. All units configured
with any type of heating will heat the supply air and space
as normal utilizing full speed indoor fan by energizing the
appropriate outputs. The two-speed indoor fan unit will only
utilize two speeds during the cooling operation modes
described below.
Cooling Operation
For cooling operation, the unit will operate at Low Fan
Speed during Fan Only, Economizer Only, and the first
stage of compressor unit modes by Default. When the unit
receives a call for cooling, if the economizer is enabled, the
unit will energize the associated Low Speed output and
begin to modulate the OA Damper open above minimum
position up to 100% as necessary. By default, the supply
fan will remain at low speed for the duration of Economizer-
Only cooling. If the supply fan is on when the first stage
compressor is energized, it will remain energized or
energize at low speed for the duration of Cool 1.
