41
I-EEDU (02-21) 150492-A
• Options AG7, AG8, and AG9—electronic modulation between 50% and 100% firing rate (see
DETAIL A):
• Depending on the heat requirements as established by the thermistor sensor, the burner modulates between
100% and 50% firing . The thermistor is a resistor that is temperature sensitive in that as the surrounding temperature
changes, the ohms resistance changes through the thermistor . This change is monitored by the solid state control
center (amplifier) that furnishes varying DC current to the modulating valve to adjust the gas input . The amplifier
is shipped separately for field mounting .
• Each modulating valve is basically a regulator with electrical means of raising and lowering the discharge
pressure . When no DC current is fed to this device, it functions as a gas pressure regulator, supplying 3 .5 IN WC
pressure to the main operating valve .
• Electronic modulation for heating controlled by a specially-designed room thermostat (60–85°F) is identified as
option AG7 .
• Electronic modulation control systems for makeup air applications controlled by a duct sensor (refer to
AG8, AG9, or AG15: Discharge Air Temperature Sensor Installation
section) and temperature selector (55–90°F)
are identified as either option AG8 or AG9 . The temperature selector setting for option AG8 is on the amplifier .
Option AG9 has a remote temperature selector . Both systems are available with an override thermostat .
• Option AG21—computer-controlled electronic modulation between 50% and 100% firing rate (see
, DETAIL B):
• With this option the furnace is equipped with a Maxitrol signal conditioner that 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 positions, the conditioner accepts
a 0–10V signal . The conditioner converts the signal to the 0–20 volt DC current required to control the modulating
valve .
• The conditioner, a conditioner relay, a transformer, a fuse box with cover, and hardware are shipped separately
for field installation . Follow the conditioner manufacturer’s instructions and the wiring diagram supplied with the unit .
Figure 32. Electronic Modulation Options
Pilot and Ignition Systems
• Gas-fired intermittent pilot is standard:
• The vertical pilot is located under the aeration panel on the control end of the burner tray and is accessible only
after the burner rack has been removed . Remove the pilot for maintenance or service, such as checking the wiring
and cleaning (refer to
Burner Air Adjustment (Propane Units Only)
section) . Pilot is 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 adjustment screw in the control valve body so that pilot flame length is approximately 1-1/4 inches .
• Intermittent spark ignition safety pilot system:
• There are two types of intermittent spark pilots: one type shuts off the pilot gas flow between the cycles and the
other not only shuts off the pilot gas flow between cycles but also has a lockout device that stops the gas flow to
the pilot if the pilot fails to light in 120 seconds .
• The lockout feature has a 1-hour retry or requires manual reset by interruption of the control circuit .
• Propane units require the spark ignition with lockout .
DETAIL B: OPTION AG21
DETAIL A: OPTIONS AG7, AG8, AND AG9
