16
Figure 30. Direct Ignition Gas Valve - Honeywell
d. When the gas valve is energized it steps open at a
reduced flow and opens fully after approximately 14
seconds.
e. When the call for heat is satisfied the thermostat 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 furnace. 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 appliance 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
-0.30”WC differential pressure 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 remains open if flame is
sensed, or closes if flame is not sensed.
e. If flame is not sensed, the entire sequence is repeated
four more times before “lockout” occurs. To reset, wait 30
seconds and then interrupt the 24 VAC power by turning
the room thermostat below room temperature, then
returning it to the original set point.
f. If flame is not established on the fifth trial for ignition (initial
try + 4 re-tries), the control de-energizes the gas valve,
flashes “4” on the Status LED, and lockouts out heat
operation for 1 hour.
g. If a flame is present, the control energizes the main blower
on heat speed 30 seconds after the gas valve opens.
h. When call for heat is satisfied the thermostat contacts
open, the gas valve shuts off gas flow and the combustion
blower remains on for a 30 second post-purge period.
i. The main blower is de-energized after a 120 second
blower off delay.
Gas Control
Figure 31. Direct Ignition Gas Valve - Robertshaw
GAS CONTROL KNOB
