A gas log (as well as balls, shapes, stones) set must always be operated with the glass doors
open for the following reasons:
Cooling air being drawn in from the room is needed to pass
over the control valve and components in order to prevent overheating and
premature, unwarranted failure;
Adequate air for proper combustion.
With the doors closed, air drawn in from floor level
registers in the doors may create a high speed rush of air across the burner,
resulting in wild flames and improper combustion. Opening the doors can equalize the air flow,
although you still may need to close these registers.
Closed doors create a barrier to heat entering the living
It is entirely appropriate to close the glass doors after
turning the set off in order to keep the heat in the room after use.
Glass doors are either a personal choice or a local code requirement, but are not a requirement for installing a gas log. Glass doors are decorative and functional. They permit you to close off the loss of warm air up the chimney without having to reach into the fireplace to close the damper. If you are in a region that requires the damper to be permanently blocked open or removed, then glass doors can provide tremendous energy savings.