Service Manual

9
9
Sequence of Operation
1. Thermostat calls for heat.
2. Combustion blower starts at a 3,000 RPM “soft” start light off.
3. Blower pre-purge period of approximately 15 seconds.
4. Ignition control board runs an internal verification safety check for approximately 15 seconds.
5. Trial for ignition (approximately 5 seconds per trial, 3 trials total):
a. Flame establishing period (3 seconds), gas valve opens, sparks from spark rod to burner surface to
ignite the gas.
b. Burner on, flame proving period (2 seconds). Requires a minimum of 1 microamp through the flame
sense rod to prove flame.
c. If the blocked vent safety switch contacts (normally closed) are open, then the ignition sequence will not
start and error code “67” (pressure switch failed to close/open or vent safety switch failed to close/open)
will flash once on the digital display. The unit will then go into pre-purge/“Hold” while the unit is waiting
for the issue to be corrected. If the issue continues to occur, the digital display with flash error code
“137” (pressure switch is open, or vent safety switch is open) while the unit is waiting to restart (5
minutes) the normal sequence of operation.
6. Once the flame signal is verified, the blower will remain at the “soft” start RPM for 5 seconds to stabilize
the flame.
7. Steady state operation – Burner continues to operate until:
a. The thermostat circuit opens, gas valve closes, and blower continues to operate for 30 second post-
purge period.
b. If the blocked vent safety switch contacts open (normally closed) while the burner is on, then the gas
valve closes, and the unit will retry a normal sequence of operation. If issue remains on restart, the unit
will go into recycle as described in 5c.
8. Thermostat is satisfied.
9. Gas valve closes and burner is extinguished.
10. Blower post purge for 30 seconds at maximum RPM.