Modulating Gas Sequence Of Operation-Full And Limited Modulating Gas Furnace - Trane IntelliPak SAHL Series Installation, Operation And Maintenance Manual

Modulating Gas Sequence of Operation—Full and Limited Modulating 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 modulating heat wiring diagram that shipped with the unit while
reviewing the following sequence of operation. As you review the sequence of operations, keep
the following in mind:
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Whenever there is a call for heat, 1U50-K1 energizes and combustion blower motor 4B11 begins
to operate at High speed on the 850 and 1000 MBH heaters. The blower will operate on low speed
for the 500 MBH. A relay 4K119 in parallel with the main gas valve actuator control output, insures
the actuator will be open prior to proof of flame. This will force the combustion air actuator 4U82
to the open position, causing the auxiliary switch on 4U82 to close. This insures complete purging
of the combustion chamber during the 60 second purge cycle.
Ignition control IC board 4U18 will not energize, however, unless the supply air flow switch 4S38,
combustion air flow switch 4S25, high limit cutout 4S26, the auxiliary switch on combustion air
actuator 4U82 and the proof of closure switch on gas valve 4L22 are closed. These are all part of
the safety interlock system.
With all these conditions satisfied, the IC board energizes and initiates an internal 60 second pre-
purge time delay. When the pre-purge period expires, 4U18 energizes both the ignition transformer
4T7 and solenoid 4L9 on the intermittent pilot valve. At that point, 4U18 gives electrode 4E1
approximately 10 seconds to establish a pilot flame. (The presence of this flame is proven by flame
rod 4U19.)
If 4U18 does not detect a pilot flame at the end of this period, it will shut down and lock out the
ignition / combustion circuit.
If the pilot is ignited within 10 seconds, the IC board de-energizes the ignition transformer 4T7 and
electrode 4E1. At this point, relay 4K119 will energize, starting the combustion air actuator and the
furnace. The feedback signal from the discharge temperature sensor will cause the modulating
output from the heat module to change the damper position as required to maintain the outlet
temperature within the desired band.
Flame Failure
In the event that IC board 4U18 loses the "proof-of-flame" input signal during furnace operation,
it will make one attempt at reignite. If a flame is not reestablished within the 10 second trial period,
4U18 will shut down and lock out the ignition /combustion control circuit. (Combustion blower
motor 4B11 continues to run as long as a heating requirement exists and control circuit switch 4S24
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 are available for external use for heat fail (Information Only).
Note: The modulating gas heaters are factory adjusted for the proper air/gas ratio at minimum and
RT-SVX36G-EN
The furnace will not light unless the manual gas valves are open and the control circuit switch
4S24 is closed.
The control systems are wired to ensure that heating and cooling cannot occur simultaneously.
The unit supply fans must run continuously so air flow switch 4S38 will stay closed.
Modulating Gas heat is available during both occupied and unoccupied operation.
nameplate rated firing MBH for most areas in the country.
Unit Start Up
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