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VF BOILER SERVICE MANUAL
AOS WPC - Tech Training 56 of 72 Ashland City, TN © 2007
Servicing should only be performed by a Qualified Service Agent
ERROR MESSAGES (CONT)
DISPLAYED MESSAGE
CONDITION/INDICATES
CHECK/REPAIR
“Blocked Flue”
MCB senses the normally closed Blocked Flue
(Exhaust) pressure switch contacts are open.
Important Service Reminder:
When performing any troubleshooting steps outlined
in this service manual always consider the wiring
and connectors between components. Perform a
close visual inspection of all wiring and connectors
to a given component before replacement.
Ensure wires were stripped before being crimped in
a wire connector, ensure wires are crimped tightly in
their connectors, ensure connection pins in sockets
and plugs are not damaged or worn, ensure plugs
and sockets are mating properly and providing good
contact.
Failure to perform this critical step or failing to
perform this step thoroughly often results in
needless down time, unnecessary parts
replacement, and customer dissatisfaction.
• Turn off power and disconnect the wires to the
Blocked Flue (Exhaust) Switch, see page 24 for
location. Check for continuity between the termi-
nals of switch with an ohm meter. If there IS
NOT continuity through the switch contacts -
replace the switch (this is a normally closed
switch).
• Check all wiring between the Blocked Flue
Switch and the MCB J5 socket pins 5 & 6 (page
28). Closely inspect the J5 socket/plug - repair/
replace anything worn or damaged.
• Disconnect the sensing tube from the Blocked
Flue Switch. Take a pressure reading with a dig-
ital manometer from this end of the sensing tube
with the boiler firing at 100%.
Turn the boiler on and put the boiler in the Max
mode to ensure 100% firing rate (see page 17)
If the pressure reading taken is at or above
+1.0” W.C. - check for excessive equivalent feet
or excessive number of elbows in the exhaust
vent, (see the boiler’s Instruction Manual for
venting requirements), check for/eliminate any
restrictions in the exhaust vent. If the vent sys-
tem is installed per requirements in the Instruc-
tion Manual that came with the boiler - call the
A. O. Smith technical information center for fur-
ther assistance at 800 527-1953
• If the pressure reading taken is below +1.0”
W.C. - turn power off to the boiler.
TEMPORARILY DISCONNECT THE TWO
WIRES TO THE BLOCKED FLUE SWITCH
Turn the boiler on and put the boiler in the Max
mode to ensure 100% firing rate (see page 17)
Check for continuity between the terminals of
switch with an ohm meter while firing at 100%. If
contacts are opening during operation with
pressure below +1.0" W.C. - replace the switch.
RECONNECT THE WIRES TO THE BLOCKED
FLUE SWITCH IMMEDIATELY AFTER PER-
FORMING THIS TEST. FAILURE TO DO SO
CAN RESULT IN PROPERTY DAMAGE AND/
OR PERSONAL INJURY.