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Because the and post purge operating states and at lower speeds sensing tubes are T fitted together it is not con- during all other operating states. High blower prover clusive which BPS is at fault. IE: a leaking diaswitch contacts must close during the pre and post phragm on the Low BPS could prevent the High purge states ONLY and open during all other BPS from closing its contacts. Therefore - operating states. replace both the High and Low BPS. OR Normally Open Low Blower Prover pressure switch contacts have opened during any operating state when the blower should be running. IE: This can be caused due to blower failure during the heating cycle or if the pressure sensed by the switch rises above the switches activation pressure for any reason. The normally open contacts on both Blower Prover If the actual pressure observed in the pressure test above did not drop to at least -4.8" W.C. during the pre purge period when the blower is running at 5000 rpm - see page 13. Or did not drop to at least -0.12" W.C.or lower at any time while the blower was running during the test the BPS switches are not defective. Continue: switches close on a fall in pressure - the pressure • Closely inspect all fittings and flanges in the gas sensed must be in a vacuum. train between the outlet of the boiler’s 24 VAC gas valve and the Venturi assembly. See page Operational Note: 12. Reseal/repair any leaking connections - ensure all gaskets are properly seating. See blower prover switch information on page 24 • Disassemble the Venturi from the inlet connecand the Sequence of Operation on page 15. tion at the blower and the intake air connection - ensure the large “O” ring gaskets are not damaged and seating properly. Closely inspect the cone shaped restrictor inside the Venturi for any signs of damage or wear. If the restrictor is damaged or worn replace the Venturi. AOS WPC - Tech Training 55 of 72 Ashland City, TN © 2007 Servicing should only be performed by a Qualified Service Agent VF BOILER SERVICE MANUAL 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 terminals 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 digital 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 system is installed per requirements in the Instruction Manual that came with the boiler -call the A. O. Smith technical information center for further 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 PERFORMING THIS TEST. FAILURE TO DO SO CAN RESULT IN PROPERTY DAMAGE AND/ OR PERSONAL INJURY. AOS WPC - Tech Training 56 of 72 Ashland City, TN © 2007 Servicing should only be performed by a Qualified Service Agent VF BOILER SERVICE MANUAL ERROR MESSAGES (CONT) DISPLAYED MESSAGE CONDITION/INDICATES CHECK/REPAIR “Flame Stg1” • Ensure the supply gas shut off valve to the boiler and the gas train shut...

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