I see the rollout switches inline with the gas valve, but it is surprising that the gas valve gets 24 volts without a proven pressure switch.
. The furnace control checks for an open main limit (this
limit is normally closed). If the limit is open, the furnace
will remain inoperable until the limit is closed. During an
open limit, the circulating air blower will be energized. The
status light will blink four (4) times.
. The room thermostat reacts to a demand for heat.
. The control will then check to insure that the vent pressure
switch is open. If, at this point, the vent pressure switch is
closed, the control will blink two (2) times and will remain
inoperable until this situation is corrected.
. The venter blower is energized.
. The vent pressure switch will close when it detects a
pressure in excess of its setting. If the pressure switch
fails to close, the status light will flash three (3) times. The
sequence cannot continue until the pressure switch closes.
. After a pre-purge of about fifteen (15) seconds, the
electronic ignition device will be energized.
. The flame rollout switches are then checked to assure
they are in the closed position.
. After a slight delay, the gas valve will open if the flame
rollout switches are closed.
. The burners will ignite and the flame sensor will detect the
presence of flame. The ignition device will deenergize. If
the sensor does not detect the burner flame, the gas valve
will close and the ignition cycle will be repeated for a total
of three attempts. If, after the third attempt, the presence of
flame is not detected, the furnace will go into a lockout
condition for one (1) hour. It will then repeat the ignition
cycle. This one (1) hour lockout and retry will occur
. Thirty (30) seconds after the main valve is energized the
circulating air blower will be activated.
. The furnace will remain in operation until the demand for
heat is satisfied.
. Once the demand is satisfied the venter will shut off, and
the circulating air blower will shut off after the field
selectable time off is attained.
. The furnace will remain dormant until the next demand for