Sterling RT - Manual (Page 15)

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GAS CONTROLS
SINGLE STAGE CONTROL
Gas heating units are factory provided with an automatic
single-stage gas valve. This valve is an on/off type
control, typically activated by a low voltage single-stage
thermostat.
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) allows the heat
exchanger to come up to operating temperature.
At this time, the fan time delay relay closes and
activates the fan controls.
8. The unit continues to fire until the thermostat is
satisfi ed and no longer calls for heat.
9. The main and pilot valves close.
TWO-STAGE CONTROL
Optional two-stage control is provided with a two-stage
gas valve capable of firing at 100% and 50% of rated
input. Ignition at a low fi re (50% of the unit's rated input)
and the unit is typically controlled by a voltage two-stage
thermostat.
With power applied to the unit, this system operates in
the following manner:
1. The fi rst stage of the thermostat call 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 open to low fi re.
6. Main burners are lit at 50 percent of unit's rated
input.
7. The fan time delay relay (optional) allows the heat
exchanger to come up to operating temperature. At
this time, the fan time delay closes and activates the
fan motor.
8. If additional heat is required, the second stage of the
thermostat calls for heat.
9. The main gas valve opens to full fire. The main
burners are now at full fire. The unit continues a
full fi re until the second stage of the thermostat is
satisfi ed and no longer call for heat.
10. The main valve closes to low fi re. The main burners
are now at low fi re. The unit continues at low fi re until
the fi rst stage of the thermostat is satisfi ed and no
longer calls for heat.
11. The main and pilot valves closes.
12. The fan time delay remains closed, keeping the fan
operating to dissipate residual heat from the heat
exchanger. At this time, the fan time delay relay
opens and deactivates the fan motor.
ELECTRONIC MODULATING CONTROL
Units with electronic modulating control are provided
with an electronic modulating valve capable of fi ring from
100 percent to 40 percent of rated input. Ignition is at
full fi re (100 percent of unit's rated input). The electronic
modulating valve is controlled by a room thermostat or
duct thermostat with remote setpoint adjustment which
modulates the gas input from 100 percent to 40 percent
of rated input.
An optional override room thermostat is available
for use with the duct thermostat. The override room
ther mostat allows full fire and overrides the duct
thermostat when the room temperature falls below the
override room thermostat's setpoint.
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 operating
temperature. At this time the fan time delay relay
closes and activates the fan motor.
8. The unit is controlled by the electronic thermostat
which modulates the unit from 100 to 40 percent of
unit's rated input. The electronic thermostat can be
a duct sensing device or a room sensing device.
An amplifi er 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 fi ring 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 fi ring is required, the
increased voltage closes a relay which closes the
automatic gas valve.
9. When the sensing device is satisfi ed, the main and
pilot valves close.
10. The fan time delay relay remains closed keeping
the fan motor operating to dissipate residual heat
from the heat exchanger. At this time, the fan time
delay relay opens and deactivates the fan motor.