I don't have a link for fact info. I have looked into it and read various things. The efficiency difference from what I have read is minimal, a few % 1-3,4. I've never seen anything that states some high rate.
I don't believe that efficiency output is the real issue or concern, either in the field or by the manufacturer's from what I have read. I have to agree with Bob that the real issues area safety and proper operation.
There are usually two points I discuss with the client:
1- starving the unit for air and poor operation; You know when they build the furnace into those nice tight little closets with solid doors.
2- creating negative pressure in the house as Bob discussed

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Accurate Inspections & Consulting, Inc.
847/736-9975
Markus Keller