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I-SCE (09-18) PN207697R9
OPTIONAL ELECTRONIC MODULATION—CONTINUED
Electronic Modulation Between 50% and 100% Firing Rate (Option AG7, AG8, or AG9)
Electronic modulation for heating that is controlled by a specially-designed room thermostat (60–85°F) is identified
as option AG7. Electronic modulation for makeup air application that is controlled by a duct sensor and temperature
selector (55–90°F) is identified as option AG8 or AG9. The temperature selector setting for option AG8 is on the
amplifier (see Figure 27). Option AG9 has a remote temperature selector. Both systems are available with an
override thermostat.
Computer-Controlled Electronic Modulation Between 50% and 100% Firing Rate (Option AG21)
With option AG21, the furnace is equipped with a Maxitrol signal conditioner (see Figure 27) 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 0–10V 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.
Figure 27. Maxitrol Amplifier and Signal Conditioner
Electronic Modulation Between 20–28% and 100% Firing Rate (Option AG39)
NOTE: Option AG39 (US Patent 6,109,255) is available only with natural gas and is not available
on size 350.
Depending on its size, a furnace equipped with option AG39 has 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.
The gas train includes a single-stage gas valve, a modulating valve, and two gas pressure switches (see Figure
28). 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 amplifier (see Figure 27) with a corresponding remote temperature dial.
Note: Arrangement may vary slightly
depending on gas valve; components
are the same.
Figure 28. Option AG39 Manifold Arrangement
AMPLIFIER SIGNAL CONDITIONER
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