Already in 2019, I have inspected three homes where a 8″ chimney cap was “installed” into a 10″ flue termination. The obvious is that the person installing these chimney caps knows exactly what is going to happen eventually, along with the water intrusion damage that will occur when it does happen. What isn’t obvious is that a chimney cap was incorrectly installed and even the active water intrusion can be hard to spot until it progresses.
This drone image shows a 8″ cap that was at one point installed into the 10″ flue termination. As you can see, the end result is wind removing the chimney cap, sometimes turning it into a giant metal frisbee (huge liability issue). When you are having your chimney cap replaced ask for proof of measurement. This issue could not be seen from ground level (it was blocked by the shroud) and the client was unaware of the damage it caused until the inspection. Luckily it was still in the early stages of deterioration but ultimately it will result in a shorter lifespan for this system.