28
RP-HRPD-IOM (07-22) 132210-B
CONTROLS—CONTINUED
Optional Electronic Modulation—Continued
Electronic Modulation Between 20–28% and 100% Firing Rate (Options AG39 and AG41)—Continued
•
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 signal 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 which is necessary to maintain the desired temperature . When
the input rate is reduced enough to decrease the gas pressure to 1 .1 IN WC, 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 to divert some of the incoming air directly into the flue duct to reduce air flow through the
burner . Safety switches monitor the position of the bypass damper . When the gas pressure rises to >1 .1 IN WC,
the bypass damper closes .
• Sensor:
The duct temperature sensor and mixing tube are shipped loose for field installation in the dicharge duct .
Refer to the
Discharge Air Temperature Sensor Installation
section for instructions on locating the sensor in
the ductwork .
•
This uniquely-designed modulation system requires combustion air pressure settings different from the standard
system . Refer to
for the approximate combustion air proving switch settings at sea-level operation .
Electronic Modulation Between 20–28% and 100% Firing Rate (Options AG40 and AG42)
NOTE: Option AG40 is available only with model RP natural gas units and is not available on unit
size 350. Option AG42 is available only with model HRPD natural gas units and is not available on
unit size 700.
•
Depending on its size, a furnace equipped with option AG40 or AG42 has a 20–28% turndown ratio .
•
With option AG40 or AG42, the furnace is equipped with a Maxitrol signal conditioner (see
) that receives
an input signal of either 4–20 milliamps or 0–10V from a customer-supplied control device such as a computer .
•
With the dip switches on the conditioner positioned to ON, the conditioner accepts a 4–20 milliamp signal . With
the dip switches on the conditioner positioned to OFF, the conditioner accepts a 0–10V signal . The conditioner
converts the signal to the 0–20V DC current required to control the modulating valve . The heater functions and
is equipped in the same way as option AG39 or AG41 except that with computer control, the temperatures are
selected through the software and there is no temperature selector or duct sensor .
• Sensor:
The sensor is field-supplied .
•
Refer to
for options AG40 and AG41 pressure requirements and to
for troubleshooting options
AG40 and AG41 .
•
This uniquely-designed modulation system requires combustion air pressure settings different from the standard
system . Refer to
for the approximate combustion air proving switch settings at sea-level operation .
Pilot and Ignition Systems
⚠ WARNING ⚠
Due to high voltage on the pilot spark wire and pilot electrode, do not touch when energized.
• Pilot:
The horizontal pilot is located in the control end of the burner rack (see
) and is accessible after
the control compartment panel has been removed . All pilots are target type with lint-free feature . Pilot gas pressure
should be the same as supply line pressure (refer to
section) . If required, adjust the pilot
flame length to approximately 1-1/4 inch using pilot adjustment screw in control valve body .
Flash Carryover
Main
Burner
Spark Pilot
Figure 18. Typical Burner Rack with Spark Pilot
