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START-UP PROCEDURE
Air Shutter Adjustment
Proper operation provides a soft blue flame with a well-defined
inner core. A lack of primary air will reveal soft yellow-tipped
flames. Excess primary air produces short, well-defined flames
with a tendency to lift off the burner ports. For both natural
and propane gas, the air shutters can be adjusted to control
the burner flame height. The air shutters can be accessed
by reaching behind the gas valve in Figure 11.2. The larger
models may require the removal of the manifold (see Manifold
Assembly Removal).
Natural Gas Flame Control
Control of burner flames on duct furnaces utilizing natural gas
is achieved by resetting the primary air shutters (See Figure
19.2) to either increase or decrease primary combustion air.
Prior to flame adjustment, operate duct furnace for about fifteen
minutes. The main burner flame can be viewed after loosening
and pushing aside the gas designation disc on the side of the
burner box.
To increase primary air, loosen the air shutter set screws and
move the air shutters closer to the manifold until the yellow-
tipped flames disappear. (See Figure 19.2 for air shutter and
heat exchanger support locations.) To decrease primary air,
move the air shutters away from the manifolds until flames
no longer lift from burner ports, but being careful not to cause
yellow tipping. Retighten set screws after adjustment.
Propane Gas Flame Control
An optimum flame will show a slight yellow tip. Prior to flame
adjustment, operate furnace for at least fifteen minutes. Loosen
air shutter set screws and move the air shutters away from the
manifold to reduce the primary air until the yellow flame tips
appear. Then increase the primary air until yellow tips diminish
and a clean blue flame with a well defined inner cone appears.
Control Operating Sequence
IMPORTANT
To prevent premature heat exchanger failure, with all control
systems, a blower starting mechanism must be provided so
that the blower is running or energized within 45 seconds of
the gas control operation.
Indoor gravity and power vented duct furnaces are supplied
with intermittent pilot systems with continuous retry, which both
the main burner and pilot burner are turned off 100% when the
thermostat is satisfied. On a call for heat, the system will attempt
to light the pilot for 70 seconds. If the pilot is not sensed for
any reason, the ignition control will wait for approximately six
minutes with the combination gas control closed and no spark.
After six minutes, the cycle will begin again. After three cycles,
some ignition controllers lockout for approximately one hour
before the cycle begins again. This will continue indefinitely until
the pilot flame is sensed or power is interrupted to the system.
Note: Gas Control Options (see page 14) could change the
listed sequence of operation based on their function.
The descriptions given are for the basic duct furnace.
12
Single Furnace Controls
Staged Control (Digit 12=1 or 2):
These units utilize a single- or two-stage combination gas valve,
an ignition control, and a low voltage thermostat.
Electronic Modulating Control (Digit 12=4, 7, or 8):
These units utilize a single-stage combination gas valve, an
electronic modulating gas valve, a modulating amplifier, an
ignition control, and one of the following:
• Modulating room thermostat
• Modulating duct thermostat with remote temperature set point
adjuster
• Building Management System (BMS) signal by others (an
inverted signal where 0 VDC or 4 mA is high fire and 10 VDC
or 20 mA is low fire).
The control operating sequence for all units is as follows:
1. The thermostat calls for heat. For BMS controlled units, the
BMS closes a heat enable contact at the unit.
2. Model Digit 3=P Only - The power exhauster relay is
energized starting the power exhauster motor. Once the
motor has reached full speed, the differential pressure switch
closes. The power exhauster pre-purge time delay relay then
closes after 20 to 40 seconds and energizes the gas control
circuit.
3. The pilot valve opens and the spark igniter sparks in an
attempt to light the pilot. (If the unit was not provided with a
time delay relay, the blower starts).
4. Once the pilot is lit, the flame sensor proves the pilot and
stops the spark igniter from sparking.
5. The main gas valve is opened and the main burner is
controlled as follows:
a. Single-Stage Units: The main burner is lit to 100% full fire.
b. Two-Stage Units: The main burner is lit to 50% fire. If
the temperature at the thermostat continues to fall, the
thermostat will call for high stage heat and the main burner
is lit to 100% full fire.
c. Modulating Thermostat (Room or Duct): The main
gas valve is opened 100% and the burner firing rate is
modulated between 40% and 100% full fire. A resistance
signal (8000 to 12000 ohms) in the thermostat is converted
by the modulating amplifier to an inverted DC voltage
(0VDC for high fire to 12 VDC for low fire). The output
voltage is applied to the modulating gas valve to control
the gas flow to the main burner. The modulating valve is
modulated open or closed based on the voltage from the
amplifier (less gas flow required = higher voltage, more gas
flow required = lower voltage).
Note: When modulating duct sensing is utilized, a room
override thermostat can be added. When the room override
calls for heat, the burner modulates to full fire operation
until the room override is satisfied. The unit then reverts
back to duct sensing control. When equipped with both,
either the duct sensor or the room override thermostat can
call for heat.
d. BMS Signal: The main gas valve is opened 100% and
the burner firing rate is modulated between 40% and 100%
full fire. A BMS 0-10VDC or 4-20mA signal (inverted, such
that 0 VDC or 4 mA is high fire and 10 VDC or 20 mA is
low fire) is converted by the signal conditioner/modulating
amplifier into an inverted DC voltage (0VDC for high fire to
12 VDC for low fire). The output voltage is applied to the
modulating gas valve to control the gas flow to the main
burner. The signal conditioner can accept a 0-10 VDC
signal when all the dip switches are in the "OFF" position
5-564.4
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