Unit Startup
106
RT-SVX24J-EN
(1800 & 2500 MBH Two Stage Natural Gas)
When a heating requirement exists, the Rooftop Module
(RTM) starts the supply fan and sends a request for heat to
the Heat Module. The Heat Module closes contacts and
starts the combustion blower motor through the
combustion blower relay.
The supply airflow switch and the combustion air switch
closes. Power is applied through the high limit cutout to
the Honeywell ignition control board.The ignition control
board begins the pre-purge timing cycle with the damper
in the light off position and the low fire start interlock is
closed
At the end of the pre-purge cycle, the ignition transformer
and the pilot solenoid valve are energized.This starts a 10-
second trial for pilot ignition.
When the pilot flame is established and sensed by the
flame sensing rod, the stage 1 of the main gas valve will
begin. The gas butterfly control valve is in the low fire
setting by the linkage arm connection between the
combustion air actuator and the butterfly valve.
The system will operate in the low heat mode until there
is an additional call for heat established by closing the
contacts on the Heat Module.
If the flame rod does not detect a pilot flame within the 10
second trial for ignition period, the ignition control board
will lockout. The combustion blower motor will continue
to operate as long as a heating demand exists and the
system switch is “On”.
Once locked out on flame failure, the IC board will not
reactivate the ignition/combustion control circuit until it is
reset manually. To do this, press the reset button on the
front of the IC board case.
A set of relay contacts is available for external use for heat
fail (Information Only).
Once the heating demand has been satisfied, the
combustion blower and the Honeywell ignition control
board is de-energized.
Modulating Gas Sequence of Operation
The control system for the rooftop units is wired to ensure
that the heating and cooling do not occur simultaneously.
Refer to the modulating heat wiring diagram that shipped
with the unit while reviewing the following sequence of
operation. As you review the sequence of operation, keep
the following in mind:
1. The furnace will not light unless the manual gas valves
are open and the control circuit switch is closed.
2. The control systems are wired to ensure that heating
and cooling cannot occur simultaneously.
3. The unit supply fans must run continuously so airflow
switch will stay closed.
4. Modulating Gas heat is available during both occupied
and unoccupied operation.
When there is a call for heat, the heat module energizes the
combustion blower which causes the combustion air flow
switch to close. The ignition control board will energize
providing that the indoor air flow switch, high limit, and
low and high pressure gas switches are closed.
The ignition control board then causes the combustion air
actuator to drive the inlet air damper to the fully open
position for a 30 second pre-purge. The pre-purge time
does not begin until the purge interlock switches are made.
After the pre-purge, the combustion air actuator drives the
inlet air damper and the gas butterfly control valve to a
nearly closed position for light off. When the Low fire
interlock switch is closed the ignition transformer is
energized, the igniter begins to spark and the pilot valve
opens.
This begins a 10-second trial for ignition period during
which the flame rod must detect the flame. If does not
detect a flame at the end of the period, it will shut down
and lock out the ignition/ combustion circuit.
Once the pilot flame has been established, the heat
module will open the main gas valve and auxiliary gas
valve. After the main flame is established, the pilot valve
closes. The ignition sequence is completed and the heat
module will drive the combustion air actuator to a firing
rate based on a 2-10 VDC signal.The gas butterfly control
valve will respond through the connecting linkage.
The heater will continue to run until the call for heat is
removed or a limit opens.
Following the completion of the call for heat, there is a 15-
second post-purge.
Flame Failure
In the event that (IC) board loses the “proof-of-flame”
input signal during furnace operation, it will lock out and
the must be manually reset (Combustion blower motor
continues to run as long as a heating requirement exists
and control circuit switch is ON.)
Once locked out on flame failure, the (IC) board will not
reactivate the ignition/combustion control circuit until it is
reset manually. To do this, press the reset button on the
front of the (IC) board case.
A set of relay contacts is available for external use for heat
fail (Information Only).
Note:
The modulating gas heaters are factory adjusted
for safe operation and to reach the nameplate rated
firing MBH for most areas of the country. The
proper air/gas ratio must be reached by the service
tech during startup..
Electric Heat Sequence of Operation
The control system for the rooftop units are wired to
ensure that heating and cooling do not occur
simultaneously. Refer to electric heat wiring diagrams that
shipped with the unit while reviewing the following
