A balanced flue gas fireplace is supplied with oxygen via the flue. The exhaust gases are carried outside again via the same channel. No oxygen from the immediate area is needed. That is why the flames of a balanced flue fireplace are always behind glass. That is of course safer, but it offers even more advantages. The gas fireplace is a lot warmer than with an open system. This provides more convection heat, making the entire room pleasantly warm. In addition, the double-walled pipe ensures that the cold outside air is pre-heated. So you get more return from your gas fireplace.