Originally Posted by
ToiletDuck
You are not exercising the privilages of your commercial certificate to be a CFI. This is why you can do it with a 3rd class medical.
No, you can do it with a 3rd class medical because the FAR 61.23 (b) (5) allows you to do with without a medical. It's called and
exemption.
Flight instruction is the only job you can get paid to do and it not be considered commercial flying. If it was you would be in violation of the limits placed on a commercial certificate. You would be "holding out" which can even be described as "general knowledge you are for hire". Call your local FSDO to ask.
You can "hold out" and do all sorts of flying. Ferry flights, Aerial photography, sightseeing flights, etc. They are all commercial operations (ie they would require a commercial license to do) but the exemptions for them, as well as instructing, are contained in Part 119 - CERTIFICATION: AIR CARRIERS AND COMMERCIAL OPERATORS.