I'll try to explain...
When a factory built fireplace is designed and then safety tested, the chimney system is part of the test. That same system is then sold with the fireplace because all clearances (such as hearth, mantel) are established.
Thanks, BKVP! That makes sense but looking at the Jøtul C 550 CB Rockland insert pre-fabricated chimney / liner instructions, it still doesn't appear to me that many chimney and liner variables are very well covered (as is also the case with a masonry chimney). There are still many degrees of freedom that fall within the approved requirements that appear to be quite relevant to me, but I guess the testing and installation criteria can be "looser" with an insert that's found to produce cooler exhaust than a stove.
The air to fuel ratio for a fireplace is vastly different than a sealed unit, like your Jotul.
The stack temperatures can be much higher. It's 99.9% unlikely that a stove or fireplace manufacturer set up in the lab the alterations you have contemplated in order to establish the same required clearances.
99.9% at least! I have little doubt that the F 55 was not tested with a chimney that had long expired from the marketplace by the time the F 55 was introduced.
Even though stove temperatures can be much higher than an insert, when I look at the "HomeSaver
Factory-Built Liner Adaptor Supplemental Instructions" they imply that free standing wood stoves
could, at least potentially, be approved with an AC factory-built chimney and liner. Of course that doesn't mean that any wood stoves have actually been approved in practice:
If a hearth stove will be connected to the liner, determine which type of tee will be used (see the liner installation instructions for specific recommended tees). If an insert will be connected to the liner, you may use a HomeSaver connector/adaptor to make the connection between the flex and the insert if it has a 6″ inside dimension flue collar.
If you home were to experience a loss due to a fire, then there is a 100% certainty that a fire inspector will determine your install was not legal.
Lastly, there is ZERO assurance the builder that installed the factory built fireplace venting system did so without taking any shortcuts.
Agreed here too... AND zero assurance that the chimney has never been subject to a chimney fire. These sorts of concerns would be applicable to even any previous owner code-acceptable HT/UL3 installations too. By the time these kinds of history unknowns are addressed, a new chimney is probably often the best option.
Your propane heater will most likely never experience a chimney fire. However if a wood stove/liner did (and that most definitely can happen) you'll find out if it was installed correctly.
You're on the best path...a new location.
I'm still rejecting tearing out the overhang. It turns out that there's no drywall behind the stone except at the edges. I'm not sure what's supporting it--maybe rebar. After tearing out a sizable amount of stone, replacement drywall would need to be installed and the ceiling completed and textured. If I'm going to need to do drywall work, I'd rather limit it and also avoid tearing out a sizeable amount of stone, which I find to be attractive. On the other hand, ceiling work should be very limited with a new chimney installation.