Wow, Lots of conversation. Let me explain my intentions about chase inspections. This is my experience doing this work on subrogation cases concerning prefab chases. I started to write a response but it went on too too long. I've done this work since 1994 and the last in California in 2013, so I have some history doing it. If you are interested in learning how I do them and my answer to your questions concerning entry holes, getting around inside the chase, getting dimensions, lighting issues, borescopes, firestops etc.
1) Yes this in my opinion is a level 3 inspection
2) Intended Purpose: If your intentions are replacement of the existing unit, it would be helpful for a chimney contractor to “see the interior” before setting a saw-z-all to it.( electrical wires, gas piping, water pipes etc.)
3) What is real: All of us understand that prefab chases especially track homes are done by the lowest bidder, therefore corners are likely been cut. So if your gut is telling you something doesn't look or feel right, you have the opportunity to offer a selective service to evaluate the interior without destruction. ( Especially if chase covers are sealed well)
4) Experience: I have done 45 such inspections in the state of NJ. several years ago and went through the side shoulder, through vinyl siding, and in some cases through the shingles on the roof near the chase. I can tell you there were many things covered up that caused hidden water issues within these homes along with a worker stuffing their lunch bag and hoagie wrapper right behind the firebox so it was in direct contact with the unit and the back OSB wall!!
5) Experience: I also have experience with Randy Brooks in California (45 units) where many of the photos were taken so the areas for gas lines outside air and critter presence can be seen. Check with Randy on his take about doing them.
6) Dimensions: As far as taking dimensions, the camera is under 1.5” so a 1.5” hole saw will allow the camera to pass. Plus with the use of a gooseneck rod you can bend it so reaching around chimney pipe can be done. Since the camera is 1.5” I use that as my measuring device. If the camera can go through an opening between combustible and pipe with wiggle room then in most cases the chimney is 1.75” to 2” away from any combustibles. The same with fireboxes. Plus in most cases the mfr. has instructions on the side of the unit, especially sidewall clearances.
7) Firestops: Should you find firestops ( past experience tells me not too many) then your inspection can only go that far and another entry point might be a consideration. Again it will go back to how bad do you need to see?? Since this is a level 3 there is some leeway on this. That’s why I made an inspection checklist so you can tell the customer what you saw and what you didn’t see.
8) Operations: I place the unit on the roof or chase top and drill the hole into the chase. Then attach the camera to one or two rods and lower the camera cable and rod into the chase. When I reach the top of the firebox I’m looking directly down. There is enough light from the LEDs to illuminate the area. I check the sides of the firebox and clearances, insulation ( or Lack of) outside air, gas line entry and other supports. When you need to look at 90 degrees, I push the camera into the firebox and bend the gooseneck. Now looking for clearances around chimney pipe can be done along with close up of mantel support or creative use of 2x4 construction.
9) Borescopes: I have one from Dewalt that works through a ¾ to 1” hole. Works great but my light is limited to only a few inches around the head, so positioning the borescope can take a while. It has a place but don’t expect it to illuminate a chase!!
10) Sealing: Usually RTV on the chase around the hole, then a 6x6 alum plate with six screws, then cover the edge with RTV silicon. On wood siding under vinyl, same 6x6 sheet of alum. With six screws. In masonry, stuff hole with some ceramic blanket then mortar over top. ( Not sold on that one but was the best I could come up with at the time.) On roofs I use a wonder bar to pry up a shingle tab and drill the hole. When done RTV around the hole and slide plate under tab, put a dab of RTV on the underside edge of the raised shingle and press down. Also there is a company that makes a 1.5” plug out of chrome steel which looked interesting also. Plug hole and add RTV around plug-done.