Form I-XE/CRGB/RPB, Mfg No. 131782 Rev 4, Page 27
25D. Computer Controlled Electronic
Modulation between 20-28% and 100%
Firing Rate (U.S. Patent 6,109,255),
Option AG40
- Model RPB only; not
available on Size 350; available with natural
gas only
With this option the furnace is equipped with a Maxitrol signal condi-
tioner (see illustration in Paragraph 25B) which accepts an input signal
of either 4-20 milliamps or 0-10 volts from a customer-supplied control
device such as a computer. With the dip switches on the conditioner in
the "on" positions, the conditioner accepts a 4-20 milliamp signal. In
the "off" positions, the conditioner accepts a 0-10V signal. The condi-
tioner converts the signal to the 0 to 20 volt DC current required to
control the modulating valve.
The heater functions and is equipped in the same way as described in
Paragraph 25C (Option AG39) except that with computer control the
temperatures are selected through the software and there is no tempera-
ture selector or duct sensor.
25B. Computer Controlled Electronic Modu-
lation between 50% and 100% Firing
Rate (Option AG21)
With this option the furnace is equipped with a Maxitrol signal condi-
tioner which operates much the same way as the amplifier above to
control the regulator valve. The conditioner accepts an input signal of
either 4-20 milliamps or 0-10 volts from a customer-supplied control
device such as a computer. With the dip switches on the conditioner in
the "on" positions, the conditioner accepts
a 4-20 milliamp signal. In the "off" posi-
tions, the conditioner accepts a 0-10V sig-
nal. The conditioner converts the signal to
the 0 to 20 volt DC current required to
control the modulating valve. Temperature
selection is through field-provided com-
puter software.
25C. Electronic Modulation between 20% -
28% and 100% Firing Rate (U.S. Patent
6,109,255), Option AG39
- Model RPB only;
not available on Size 350; available with
natural gas only
Depending on the size, furnaces equipped with electronic modulation
Option AG39 have a 20-28% turndown ratio. The furnace will ignite at
any input rate in the available range and will maintain average thermal
efficiencies equal to or greater than the thermal efficiency at full fire.
Maximum
MBH
Inlet Pressure
Gas Supply
Model
Turndown
Input
to Modulating
Pressure
%
Range
Valve (factory set) Required
RPB 125
20
25-125
3.9" w.c.
5" w.c.
RPB 150
27
40.3-150
3.7" w.c.
5" w.c.
RPB 175
23
40.3-175
3.7" w.c.
5" w.c.
RPB 200
26
51.8-200
3.9" w.c.
5" w.c.
RPB 225
23
51.8-225
3.9" w.c.
5" w.c.
RPB 250
28
69-250
4.0" w.c.
5" w.c.
RPB 300
23
69-300
4.0" w.c.
5" w.c.
RPB 400
25
100-400
4.4" w.c.
6" w.c.
The gas train includes a single-stage gas valve, a modulating valve, and
two gas pressure switches. The burner rack is equipped with one flash
carryover and a regulated gas lighter tube system. The carryover lighter
tube receives its gas supply through the regulator, simultaneously with
the gas to the burner. Control of the system is through a Maxitrol
#A1092 amplifier with a corresponding remote temperature dial
(Maxitrol #TD92).
FIGURE 24 - Signal
Conditioner used in
Options
AG21
and
AG40
Carryover
Regulator
Modulating
Valve
Single-
Stage Gas
P rimary Gas
Flow Pressure
Switch
White Label
1.1 w.c.
Gas Flow
Pressure Switch
White Label
1.1 w.c.
NOTE:
Arrangement may
vary slightly depending on
gas valve; components are
the same.
outlet pressure setting higher than when it is used on a standard gas
manifold. The pilot tubing connects to the pilot port on the single-stage
gas valve. When the valve receives a call for heat from the amplifier and
pilot is established, gas flow from the single-stage valve goes to both the
modulating valve and the regulated lighter tube system.
When the sig-
nal from the amplifier to the modulating valve, requires less-
than-high fire operation,
the modulating valve functions to lessen the
gas flow to the burner to reduce the input rate to that required to
maintain the desired temperature. When the input rate is reduced enough
to decrease the gas pressure to 1.1" w.c., the primary gas pressure
switch in the manifold activates the gear motor that controls the bypass
damper in the venter/combustion air system. The bypass damper opens
diverting some of the incoming air directly into the flue duct, reducing
airflow through the burner. Safety switches monitor the position of the
bypass damper. When the gas pressure increases above 1.1" w.c., the
bypass damper closes. See the manifold arrangement in
FIGURE 25
.
Combustion Air Pressure Switch Setting
This uniquely designed modulation system requires combustion air pres-
sure settings different from the standard system. The approximate set-
tings for the combustion air proving switch at sea-level operation are:
Model RPB
Startup
Equilibrium
Factory
with AG39, AG40
Cold
at Full Rate
Setting
125,150,175,200,225 -1.3"w.c.±0.2 -1.05"w.c.±0.1 -1.0"w.c.±0.2
250,300,400
-1.2"w.c.±0.2 -0.95"w.c.±0.1 -.75"w.c.±0.5
Sensor Location
For the convenience of the installer, the duct temperature sensor is
factory installed in the cabinet leg (See
FIGURE 23
). Although the
sensor has a mixing tube, at this distance from the discharge it does not
receive a true mix, so the temperature read by the sensor will be slightly
higher than the actual air entering the ductwork. The system will pro-
vide comfort level heat if the selector is set slightly lower to compensate
for this reading. The offset temperature will vary with the application.
If a direct correlation of these two temperatures is required, move the
duct sensor to a location in the ductwork about 10-12 feet from the
furnace discharge.
Wiring and Service
For wiring, consult the wiring diagram attached to the furnace. All wires
in the electrical box connecting the modulation controls must be rated
for 150°C.
This is a unique system which includes custom-built components and
custom settings. If service is required, follow the general troubleshoot-
ing guide on page 43 and the special troubleshooting guides on page 28.
FIGURE 25 - Option
AG39 and AG40
Manifold
Arrangement
Description of Operation of Option AG39
The gas supply (see pressure requirements in the table above) connects
to the single-stage gas valve. To compensate for additional pressure loss
through the modulating valve, the single-stage gas valve has a custom
