Why does a direct vent fireplace need the fixed piece of glass?
The chimney vent pipe is a co-axial (pipe within a pipe) system, where the outer walled pipe brings in fresh air to feed the fire and the exhaust goes out the inner wall of the same pipe. Everything comes in from the outside and everything goes back out to the outside. You do not use or lose any room air to feed the fire and there is no exhaust coming into the home because all direct vent gas units have a fixed piece of glass in front of the fire in order for them to function and provide additional radiant heat. With this fixed piece of glass and the two-way vent pipe, no room air is used for combustion which greatly increases the efficiency...up to 88%!.