Reznor SCE Unit Installation Manual - Manual (Page 26)

Form RZ-NA I-SCE, Page 26
21. Optional
Electronic
Modulation
The type and capability of the electronic modulation system, depends on the option selected.
Electronic modulation options are identified by a suffix to the Serial No. printed on the heater
rating plate. AG7 is identified as MV-1; AG8 is identified as MV-3; AG9 is identified as MV-4; AG21
is identified as MV-A; AG39 is identified as MP-1; and AG40 is identified as MP-2.
Installation NOTE: Sizes 350 and 400 with electronic modulation require a minimum of 6" w.c.
natural gas supply pressure.
21A. Electronic Modulation between 50% and 100% Firing Rate
(Options AG7, AG8, AG9)
Depending on the heat requirements as established by the thermistor sensor, the burner modu-
lates 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 ther-
mistor. This change is monitored by the solid state control center (amplifier) which furnishes
varying DC current to the modulating valve to adjust the gas input.
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" w.c. pressure to the main operating valve.
Refer to the wiring diagram supplied with the furnace for proper wiring connections. Electronic
modulation for heating controlled by a specially designed room thermostat (60°-85°F) is identi-
fied as Option AG7. Electronic modulation control systems for makeup air applications controlled
by a duct sensor and temperature selector (55-90°F) are identified as either Option AG8 or Option
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.
21B. Computer Controlled Electronic Modulation between 50%
and 100% Firing Rate (Option AG21)
With this option the furnace is equipped with a Maxitrol signal conditioner 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" positions, the conditioner accepts a 0-10V signal. The
conditioner converts the signal to the 0 to 20 volt DC current required to control the modulating
valve.
FIGURE 26 - Maxitrol
Signal Conditioner used
in both Option AG21 and
Option AG40
Model SCE
with AG39 or
AG40
Maximum
Turndown
MBH Input
Range
Inlet Pressure to
Modulating Valve
(factory set)
Gas Supply
Pressure Required
SCE 125 20% 25 - 125 3.9" w.c. 5" w.c.
SCE 150 27% 40.3 - 150 3.7" w.c. 5" w.c.
SCE 175 23% 40.3 - 175 3.7" w.c. 5" w.c.
SCE 200 26% 51.8 - 200 3.9" w.c. 5" w.c.
SCE 225 23% 51.8 - 225 3.9" w.c. 5" w.c.
SCE 250 28% 69 - 250 4.0" w.c. 5" w.c.
SCE 300 23% 69 - 300 4.0" w.c. 5" w.c.
SCE 400 25% 100 - 400 4.4" w.c. 6" w.c.
20. Optional 2-
Stage
Operation -
Makeup Air
Application
(cont'd)
FIGURE 25 - (A) Remote
Temperature Selector,
(B) Stage-Adder Module,
and (C) Optional Display
Module for Ductstat in
Two-Stage Makeup Air
Control Options (Option
AG15 or AG16)
(A) (B) (C)
21C. Electronic
Modulation
between 20 to
28% and 100%
Firing Rate
(U.S. Patent
6,109,255),
Option AG39 -
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.