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Figure 68. Energy recovery wheel economizer operation
Outside Air Intake Damper
(Mist Eliminator not shown)
Exhaust Air,
Path to Exhaust Fan
Exhaust Air
Building Return Air
Bypass Damper
Gas Heating Sequence of Operation
Standard
Two Stage Gas Furnace
The control system for the rooftop units are wired to
ensure that the heating and cooling do not occur
simultaneously. Refer to the wiring diagram that shipped
with the unit while reviewing the following sequence of
operation.
Honeywell Ignition System
(850 & 1100 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.The combustion
blower motor starts on low speed through the normally
closed combustion blower relay contacts.
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 starts a pre-purge timing cycle. 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, stage 1 of the main gas
RT-SVX24J-EN
Outside Air Bypass Damper
Un-
Conditioned
Outside Air
Unit Return
Airflow
Return Air Damper
valve and the 60 seconds sequencing time delay relay is
energized.
The system will operate in the low heat mode until an
additional call for heat is established by closing the
contacts on the Heat Module.The sequencing time delay
relay will energize the combustion blower motor relay
which switches the combustion blower motor to high
speed and energizes the 2nd stage solenoid on the gas
valve after approximately 60 seconds.
If the flame rod does not detect a pilot flame within the 10
second trial for ignition period, the control will lockout. If
a flame failure occurs during operation, the gas valve, the
sequencing time delay relay, and the combustion blower
relay is de-energized.The system will purge and attempt
to relight the pilot. If a flame is not detected after this
attempt, the Honeywell ignition control will lock out.The
combustion blower motor will continue to operate as long
as a heating demand exists and the system switch is "On" .
Once the heating demand has been satisfied, the
combustion blower and the Honeywell ignition control
board is de-energized.
Note: The above sequence is the same for Propane.The
orifices are smaller and the manifolds are adjusted
to different values
Unit Startup
Path to
Filters
& Coil
Path to
Filters
& Coil
105
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