We just bought a wood stove and won't have the chimney lined for a couple of weeks, can we still use the wood stove and expect the existing brick fireplace and flu to be able to exhaust the wood stove through the chimney?
I wouldn’t. If this is an insert, smoke will “leak” out around the insert and the fireplace opening.
If there’s a problem and it catches fire, the insurance company will void your claim because it wasn’t installed properly. (The Fire Department always wants to know the cause of the fire.) I’d get a Building Permit too. Then you’ll get an inspection and know it was installed properly.