Some systems are so obvious that no special knowledge is needed. Severe corrosion of key components leaps to mind. Visible fungus and or mold throughout the system might call for replacement. I might think about it where exposed and deteriorated asbestos ducts are visible.

I suppose by some stretch of the imagination, all defects could be repaired. But it seems intuitive that when repair of the deficiencies would equal or exceed the cost of a replacement unit, replacement is justified. And it seems intuitive that when the housing for the condensing unit is so badly rusted that the cooling fins are sitting on the pad (or the ground) that replacement might be called for.

Some systems are so badly installed that they present serious risks to the structures, and most important, to its occupants. The ONLY option is replacement. Perhaps the equipment can be saved, but installation has to start again at square one.

Because I ALWAYS use the statement attached in the last post, most of the time my Clients do have a licensed HVAC person verify (or potentially refute) what I have said when issues are significant. And most of the time they, not the seller, pay the guy. I suggest that they do it that way and I offer to reimburse them if the HVAC licensee determines that I have exaggerated my findings and is willing to put it in writing. The only caveat to that offer is that they not use the company that has been servicing the system for the current owner. So far, I haven't had any reimbursement requests.

I'm working on ten years in this business and had 24 years experience in the restoration of older homes before starting this business. The oldest was 1870. So I have a certain level of confidence regarding when I recommend replacement rather than repairs of most components of a home.

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The only reason some people get lost in thought is because it's unfamiliar territory.
- Paul Fix