Service Manual

Sequence of Operation

1. Thermostat calls for heat.

2. Combustion blower starts at a reduced rpm for a “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. Spark establishing period (3 seconds), gas valve opens, sparks from spark rod to ground

rod, igniting the fuel air mixture at the burner surface.

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 pressure 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 pressure 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.