Match the conclusion to the method
Ask whether the technician observed burner operation, inspected accessible exchanger surfaces, followed the model's applicable diagnostic procedure, or performed other operating checks. Each method answers a different question. A flame observation can be part of an evaluation; it should not silently become “every hidden surface is intact.”
The right procedure depends on the furnace. We recommend a qualified assessment that can explain what was inspected, what could not be seen, and what further check is warranted by the actual concern. Do not ask for a generic internet test or attempt pressure, smoke, or combustion tests at home.
For a second opinion, share the first report and say: “I want to understand what supports the heat-exchanger finding.” Ask the evaluator to document the applicable method and any limits. If no damage is found, that explanation is useful too. You do not need someone to manufacture a replacement recommendation to make a visit worthwhile.
The next step is a documented condition assessment. “The flame looked fine” can describe one observation. It should not be the whole answer to a heat-exchanger safety question.