A.O. Smith TC-099 User Manual

AOS WPC - Tech Training

56 of 72

Ashland City, TN © 2007

Servicing should only be performed by a Qualified Service Agent

MCB senses the normally closed Blocked Flue 
(Exhaust) pressure switch contacts are open.

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. 

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.