A.O. Smith BTX 100 Service Handbook page 32

Sensing Tubes
The three pressure switches monitor pressure through plastic sensing tubes from three different sensing
ports on the water heater, see Figure 32 below.
The Blower Prover switch monitors pressure from the Combustion Blower's outlet, see Figure 14 on page 15.
The Blocked Intake Air switch monitors pressure from the Combustion Blower's intake air connection fitting,
see Figure 1 on page 5. The Blocked Exhaust switch monitors pressure from the exhaust elbow on the bottom
of the water heater; there is an aluminum connecting tube mounted on the side of the water heater between
the Blocked Exhaust switch at the top and the exhaust elbow at the bottom. Two short pieces of plastic
sensing tube are used at either end of this aluminum tube to connect to the pressure switch and the exhaust
elbow, see Figure 3 on page 8.
These plastic sensing tubes must be routed to the correct sensing ports on the water heater. Sensing tube
routing should always be checked whenever Blower Prover Open, Blower Prover Failure, Blocked Air
Intake or Blocked Exhaust is displayed on the Control System LCd, see Figure 31 on page 29. These "Fault
Conditions" can be caused by incorrect sensing tube routing.
BLOWER PROVER
BLOCKED EXHAUST
30
PRESSURE SWITCH SENSING TUBE CONNECTIONS
Servicing should only be performed by a Qualified Service Agent
BLOCKED INTAKE AIR
BLOCKED EXHAUST
Figure 32