Trane GRAA Installation, Operation And Maintenance Manual page 30

G G a a s s C C o o n n t t r r o o l l s s
With power applied to the unit, this system operates in the following manner:
1. The thermostat calls for heat.
2. The pilot valve opens.
3. The ignitor sparks continuously to ignite the pilot.
4. The sensor proves pilot ignition and shuts off the ignitor.
5. With the pilot lit, the main gas valve opens.
6. Main burners are lit at 100 percent of unit's rated input.
7. The fan time delay relay (optional on duct furnaces) allows the heat exchanger to come up to
8. The unit is controlled by the electronic thermostat which modulates the unit from 100 to 40
9. When the sensing device is satisfied, the main and pilot valves close.
10. The fan time delay relay remains closed keeping the fan motor operating to dissipate residual
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operating temperature. At this time the fan time delay relay closes and activates the fan
motor.
percent of unit's rated input. The electronic thermostat can be a duct sensing device or a
room sensing device. An amplifier receives an electrical signal from the thermostat and
converts this into a working voltage. This working voltage determines the position of the
modulating valve. With no voltage applied to the valve, the valve will be full open and full fire
will occur. As increasing voltage is applied to the valve, the valve will modulate closed. At
approximately 12 volts DC, the valve will be at it's minimum fire position. As temperature
drops, the voltage also drops causing the relay to reopen the valve. The unit will continue to
cycle in this manner until either an increase in the unit's firing rate is required or the sensing
device is satisfied and no longer calls for heat. If the voltage continues to increase, indicating
a further reduction in the unit's firing is required, the increased voltage closes a relay which
closes the automatic gas valve.
heat from the heat exchanger. At this time, the fan time delay relay opens and deactivates the
fan motor.
MUA-SVX004B-EN