Section Xi: Service And Troubleshooting Guide - Bradford White UCG100H199 Installation & Operation Manual

SECTION XI: SERVICE AND TROUBLESHOOTING GUIDE

1. Thermostat calls for heat.
2. Combustion blower starts at 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.
c. If the blocked vent safety switch contacts (normally closed) are open, then the ignition sequence will
not start and error code "67" (Interlock OFF) 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" (Interlock failed to close) 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. After this 5 second period the blower will increase RPM to full power.
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. As the temperature of the water increases towards the set point of the thermostat, the RPM of the blower
will reduce ("soft" off) until the thermostat set point is reached. This reduction of the blower's RPM will vary
with the rate of change between the thermostat temperature and set point. For example: The faster the
temperature climbs towards the thermostat set point, the larger the reduction of blower RPM will take
place. The lowest RPM setting the blower will drive to is the "soft" start light of level of approximately 3,000
RPM.
9. Thermostat is satisfied.
10. Gas valve closes and burner is extinguished.
11. Blower post purge for 30 seconds at maximum RPM.
Sequence of Operations
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