Trane IntelliPak SAHL Series Installation, Operation And Maintenance Manual page 102

Unit Start Up
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
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 K1 contacts and starts the combustion
blower motor (4B11). The combustion blower motor starts on low speed through the normally
closed combustion blower relay (4K33) contacts.
The supply airflow switch (4S38) and the combustion air switch (4S25) closes. Power is applied
through the high limit cutout (4S26) to the Honeywell ignition control board (4U18). The ignition
control board (4U18) starts a pre-purge timing cycle. At the end of the pre-purge cycle, the ignition
transformer (4T7) and the pilot solenoid valve (4L9) are energized. This starts a 10 second trial for
pilot ignition. When the pilot flame is established and sensed by the flame sensing rod (4U19), stage
1 of the main gas valve (4L7) and the 60 seconds sequencing time delay relay (4DL6) is energized.
The system will operate in the low heat mode until there is an additional call for heat is established
by closing the K3 contacts on the Heat Module.
On Discharge Temperature Control systems, the sequencing time delay relay (4DL6) will energize
the combustion blower motor relay (4K33) which switches the combustion blower motor to high
speed and energizes the 2nd stage solenoid on the gas valve (4L7) after approximately 60 seconds.
If the flame rod (4U19) 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 (4L7), the
sequencing time delay relay (4DL6), and the combustion blower relay (4K33) 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 (4U18) will lock out. The combustion blower motor will continue to
operate as long as a heating demand exists and the system switch (4S24) is "On" .
Once the heating demand has been satisfied, the combustion blower and the Honeywell ignition
control board (4U18) is de-energized.
Propane Gas
Units that operate on propane gas after 1990 have two (2) additional controls that affect the
combustion blower motor operation and the sequence of the gas valve operation.
With the post purge time delay relay (4DL4), the additional service switch (4S24), and the additional
115 volt control relay (4K31) installed, the sequence of operation is as follows:
Power is applied to the Honeywell ignition control board (4U18) through the high limit switch
(4S26). The Honeywell ignition control board (4U18) will sequence through its pre-purge timing
and pilot ignition sequence to The post purge time delay relay (4DL4) delays the starting of the
combustion blower motor by approximately 60 seconds. Once the timing has elapsed, the
combustion blower motor will start, closing the combustion air switch (4S25).
Energize the control relay (4K31) and the sequence time delay relay (4DL6). 24 volts is applied from
the new service switch (4S24) through the normally open control relay (4K31) contacts to energize
the 1st stage solenoid on the gas valve (4L7).
On an additional call for heat, the K3 contacts on the Heat Module will close to energize the
combustion blower relay (4K33) which switches the combustion blower motor to high speed and
closes its normally open contacts allowing 24 volts to energize the 2nd stage on the gas valve (4L7).
Once the heating demand has been satisfied, the Honeywell ignition control board (4U18) and the
post purge time delay relay (4DL4) is de-energized. The combustion blower motor will continue to
operate for approximately 15 seconds to purge the heat exchanger on the "Off" cycle.
102 RT-SVX36G-EN