Two main types
1. Insert fireplaces
which are for conversion from existing fireplace with burning logs to gas burning fireplace. The exhaust will be done thro chimney using liners
2. direct vent fireplaces, which requires a framing so that the fireplace can be installed into the frame, and the exhaust goes to the outiside wall. both combustion air comes from outside and exhaust go to outside, thus providing with a clean heated air.