MUA-SVX006B-EN
29
Operation
General Information
Refer to Separated Combustion Indoor Gas-Fired Duct Furnace Installation and Service Manual
for specific information regarding the heating operation of this unit.
All units are equipped with a intermittent pilot ignition system. The pilot is lit and extinguished
each cycle of unit heating operation.
On natural gas units, the ignitor will continue to spark and pilot gas will continue to flow until the
pilot flame is proven.
LP (propane) units are equipped with 100% lockout. The lockout function shuts off the main and
pilot gas valves if the pilot gas fails to ignite within 90 seconds of the onset of trial for ignition.
The gas control system operates at 24 VAC and is supplied by a step-down transformer found in
the electrical cabinet that will match the unit line voltage specified. See Figures 16 through 19
through for unit controls location.
Do not use a thermostatic fan control switch when either two-stage firing or modulated gas
controls are used.
Gas Controls System
The standard unit comes equipped for single stage operation. Unit heating operation is
accomplished at full input.
• Each duct furnace is equipped with an individual ignition control system which consists of the
following components: ignition control module, gas valve and pilot burner.
• The iiggnniittiioonn ccoonnttrrooll mmoodduullee is the heart of the ignition control system. This control initiates
all gas flow, provides means to light the pilot burner, proves and monitors the pilot burner
operation.
• The ggaass vvaallvvee consists of two operators which provide gas to the pilot and main burners.
Both operators are energized and de-energized by the ignition control module each heating
cycle.
• The ppiilloott bbuurrnneerr includes an ignitor and flame sense probe. The ignitor provides the spark,
originating at the ignition control module, to light the pilot. The ignition control module
proves and monitors the pilot flame through the flame sensing probe.
Sequence of Operation
All units follow the same basic sequence of operation and is as follows:
1. Thermostat calls for heat. Draftor relay energizes draftor with call for heat.
2. Pressure switch closes circuit to ignition control module.
3. Ignition control module energizes pilot operator of gas valve and initiates spark at ignitor of
the pilot burner.
4. Pilot burner ignition is proven to the ignition control by the flame sense probe and sparking
of the ignitor is discontinued.
5. Once pilot burner operation is proven, the ignition control module energizes the main burner
operator of the gas valve, allowing gas flow to the main burners.
6. Approximately 60 seconds after the main burners light, the fan time delay initiates fan
operation.
7. Thermostat is satisfied and call for heat is removed. Draftor relay de-energizes draftor with
removal of call for heat.
8. Ignition control module de-energizes pilot and main burner operators of the gas valve,
ceasing all gas flow.
9. Approximately two minutes after satisfying the thermostat, fan operation ceases.
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