15-25 years seems to be the expected lifespan for an HVAC, if regularly serviced.
I suspect that, like many things, the lifespan of those manufactured in the past ten years will be less than those made 25 years ago.
If that is true, then it will have slewed the average such that an "old" one could well last 25+ years whereas its replacement might last under fifteen years.
So you could end up having to replace it again before the original would have failed had you kept it.
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