Comment by bluGill
16 hours ago
Maybe I misunderstand your description. I'm referring to the fresh air inlet for the fire, not additional HVAC pipes. It is common for attempts to put a fresh air inlet in a fireplace to instead have smoke go out that fresh air inlet.
My understanding is that the venting dylan604 is describing relate to airflow around the firebox / chimney, rather than in or out of the firebox.
An incorrectly-placed firebox intake, or even a poorly-drafting open-hearth fireplace, can indeed dump smoke (and carbon monoxide) into the living / heated space.