A furnace reaches high limit after service
A furnace receives routine service, including filter inspection and a blower-motor replacement. On the post-service run, the burner ignites and the fuel input is verified at its specified condition. The air-temperature rise is above the permitted range and the high limit opens while demand continues. The new motor has the same listed power as the original, but its rotation, speed connection and complete application compatibility have not yet been verified. The customer asks for a higher-limit setting to avoid interruptions. No equipment has yet been returned to normal service, and the approved wiring and performance information are available. The blower compartment remains accessible for the planned inspection and tests.
1. Which direction should the diagnosis take?
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With verified input and excessive rise, investigate delivered airflow, restrictions and the replacement motor’s full compatibility and setup. Power rating alone does not establish correct rotation, speed or air delivery. Confirm the actual cause using the approved measurements and instructions.
2. What is required before return to service?
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Correct the underlying fault without weakening the specified protective limit. Verify temperature rise, operating sequence and affected safety functions under the required conditions, then record the results and any restrictions. A successful ignition alone is not a completed post-service check.