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e. When the call for heat is satisfied the thermo-
stat contacts open, the gas valve shuts off
gas flow, and the induced draft blower stops.
GENERAL-DIRECT IGNITION MODELS
Read safety information on front page of these
installation instructions before operating fur-
nace. DO NOT ATTEMPT TO OPERATE
FURNACE IF YOU SMELL GAS.
Operating instructions for
M1M — MODELS WITH DIRECT IGNITION:
a. STOP! Read the SAFETY INFORMATION.
b. Set the thermostat to the lowest setting.
c. Turn off all electric power to the appliance.
d. This appliance is equipped with an ignition
device which automatically lights the burner.
Do not try to light the burner by hand.
e. Honeywell - push in the gas control knob and
turn clockwise to “OFF.” Robertshaw - push
the gas control lever to “OFF.” NOTE: The
lever cannot be placed in the “OFF” position
unless it is pushed in slightly. DO NOT
FORCE (See Figures 30 & 31).
f. Wait ten (10) minutes to clear out any gas. If
you smell gas, STOP! and follow step “d” in
the SAFETY INFORMATION section. If you
do not smell gas, go to the next step.
g. Set the On-Off switch to the “ON” position.
h. Honeywell - turn knob on gas control counter
clockwise to “ON.” Robertshaw - push the
gas control lever to “ON.”
i. Turn on all electric power to the appliance.
j. Replace the furnace door.
k. Set the thermostat to “HEAT” and the desired
temperature setting. The furnace should light
after approximately 75 seconds. If the appli-
ance will not operate, follow the instructions
“To Turn Off Gas To Appliance” and call your
service technician or gas supplier.
In the event of any flashback or explosion,
immediately shut off the furnace and call your
service technician.
TO TURN OFF GAS APPLIANCE:
a. Set the thermostat to the lowest setting.
b. Turn off all electric power to the appliance
before servicing unit.
c. Set the ON-OFF switch to “OFF.”
d. Honeywell - push in the gas control knob and
turn clockwise to “OFF.” Robertshaw - push
the gas control lever to “OFF.”
e. Replace the furnace door.
SEQUENCE OF OPERATION FOR M1M
MODELS WITH DIRECT IGNITION
Direct ignition model furnaces do not have a
pilot. Ignition is accomplished automatically by
a silicon carbide hot surface ignitor. A control
module takes care of all timing functions. After
lighting, the control module uses the ignitor as
a flame sensor, shutting off gas should the flame
go out. There are no external relays or timing
devices.
Do not try to light this furnace manually.
The control module is not field serviceable.
a. On a call for heat, the thermostat contacts
close, supplying 24 VAC between terminals
“C” and “W” of the control module, which
starts combustion motor.
b. When the inducer starts, the air pressure
switch closes at -0.20”WC differential pres-
sure and energizes the control.
c. After a 45 second purge period, the ignitor is
energized for a 30 second warm-up period,
after which the gas valve opens.
d. The trial period for ignition is approximately 6
seconds, after which the gas valve either
Gas Control
Figure 31. Direct Ignition
Gas Valve - Robertshaw
Figure 30. Direct Ignition
Gas Valve - Honeywell
GAS CONTROL KNOB