Can I vent a portable air conditioner (not window AC) through my fireplace? I never use fireplace and it is metal insert. Portable AC I'm thinking about is 14,000 BTU if this helps. Thanks

Not a good idea. The residual heat from a portable A/C will not be enough to keep the vent/chimney drawing correctly and the discharge heat will accumulate. The manufacturer gives specific instructions on how to terminate the warm air discharge from the portable unit. Follow the directions for a trouble free install.

Do not do it!! You are looking at creating a deathtrap or bad bad living condition.