Service Manual

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Sequence of Operation
1. Thermostat calls for heat.
2. Combustion blower starts with 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. 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.
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.
8. Thermostat is satisfied.
9. Gas valve closes and burner is extinguished.
10. Blower post purge for 30 seconds at maximum RPM.
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