A fireplace glass door is installed on a fireplace to keep the room warm longer when the flames die down. Whether a wood burning fireplace or a gas burning fireplace, heat radiated into the room escapes through the chimney. One does not notice this while the fire is burning brightly, because of the heat from the flames that radiates into the room. However, when the fire starts to die down, the hearth becomes cold quickly as the heat escapes through the fireplace opening. To stop this from happening, a fireplace glass doors blocks the opening, keeping the warm air from leaving the room.
It gets cold in the room, and it's really frustrating. I have a wood burning fireplace, and I'm probably one of the last people out there to have one. Many are switching over to gas due to the money it saves, and the positive effect on the environment as logs aren't being burned. I just bought my first fireplace blower and lit up my first fire last week. Man, does it ever make a difference! I can't believe I've gone all these years without one.