The distinction between vented and vent-free units lies in their design and how they manage air circulation:
Vented Unit: A vented fireplace draws fresh air from the exterior of your home and expels the combustion fumes, including carbon monoxide, through a vent. While this ensures safe indoor air quality, some of the heat produced is lost through the vent.
Vent-Free Unit: In contrast, a vent-free fireplace is constructed differently to avoid emitting harmful gasses. It does not require fresh air from the exterior of your home. With a vent-free unit, all the heat produced stays within your home.