MUA-SVX003B-EN
29
Operation
General Information
Refer to Outdoor Rooftop 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 Figure 10, p. 26 for unit
controls location.
Do not use a thermostatic fan control switch when either two-stage fi ring or modulated gas
controls are used.
Gas Control 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.
All units follow the same basic sequence of operation and is as follows:
1. Thermostat calls for heat. PPoowweerr vveenntteedd uunniittss oonnllyy: Drafter relay energizes drafter with call
for heat.
2. Ignition control module energizes pilot operator of gas valve and initiates spark at ignitor of
the pilot burner.
3. Pilot burner ignition is proven to the ignition control by the flame sense probe and sparking
of the ignitor is discontinued.
4. 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. PPoowweerr vveenntteedd uunniittss oonnllyy:
In addition, drafter operation must be proven by drafter end switch before ignition control
module energizes main burner operator of gas valve.
5. Approximately 60 seconds after the main burners light the fan time delay initiates fan
operation.
6. Thermostat is satisfied and call for heat is removed. PPoowweerr vveenntteedd uunniittss oonnllyy: Drafter relay
de-energizes drafter with removal of call for heat.
7. Ignition control module de-energizes pilot and main burner operators of the gas valve,
ceasing all gas flow.
8. Approximately two minutes after satisfying the thermostat, fan operation ceases.
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