Sterling RT - Manual (Page 14)

14
IV. OPERATION
GENERAL INFORMATION
Refer to Outdoor Rooftop Gas-Fired Duct Furnace
Installation and Service Manual for specifi c 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 fl ow until the pilot fl ame 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 stepdown transformer found in the electrical
cabinet that will match the unit line voltage specifi ed. See
Figure 5 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 ignition control module is the heart of the ignition
control system. This control initiates all gas fl ow, provides
means to light the pilot burner, proves and monitors the
pilot burner operation.
The gas valve 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 pilot burner includes an ignitor and fl ame 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 fl ame sensing probe.
All units follow the same basic sequence of operation
and is as follows:
1. Thermostat calls for heat.
Power vented units only: Draftor relay energizes
draftor 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 fl ame 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 fl ow to the main burners.
Power vented units only: In addition, draftor
operation must be proven by draftor 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 satisfi ed and call for heat is removed.
Power vented units only: Draftor relay de-energizes
draftor with removal of call for heat.
7. Ignition control module de-energizes pilot and main
burner operators of the gas valve, ceasing all gas fl ow.
8. Approximately two minutes after satisfying the
thermostat, fan operation ceases.
OPTIONAL GAS CONTROLS
These units are available with optional gas controls which
will give either multi-stage or modulating operation. The
sequence of operation of those units equipped with these
optional gas controls differs only at what rate each duct
furnace’s main burners ignite and operate at or the order
of duct furnace fi ring for those units equipped with two or
three duct furnaces.
Two Stage Units: Two stages of heating; fi rst stage is
40% of full rate, second stage is 100% of full rate. Main
burner ignition is at fi rst stage only.
Three Stage Units: Dual duct furnace models: First
furnace is equipped with two stage heating; second
furnace is equipped with a single stage heating but fi red
independantly giving three stages of heating.
Triple duct furnace models: Each duct furnace is equipped
with single stage heating but fi red independently, giving
three stages. Each stage is 33% of the unit’s full input rate.
Duct furnace one will always light fi rst and disengage last.
Four Stage Units: Dual duct furnace models only. Each
duct furnace is equipped with two stage heating but fi red
independently, giving four stages. Each stage is 25% of
the units full input rate. Duct furnace one will always light
fi rst and disengage last.