Often a set of glass doors will have some adjustable air vents at the bottom. If so, that's an invitation to use the fireplace with the doors closed. The main purpose of glass doors is to restrict the flow of heated room air up the flue, with the understanding that every cubic foot of heated warm air that goes up the flue is necessarily replaced with a cubic foot of cold air that's drawn inside the house from outside.

Operating the fireplace with the doors closed redduces the losses of that heated room air. The air shutters at the bottom provide a small amount of fresh air for the wood to burn.

Even more important is to leave the glass doors closed and the air shutters closed in between uses of the fireplace, to minimize the loss of heated room air when the fireplace is not being used.

I am not familiar with that fireplace - but you should look on line to see if you can download the instructions. If it is a direct vent wood burner -like mine is- then you should have the doors closed. Otherwise, the fireplace is sucking warm air out of your house. But read the manual - make sure that there is an external vent for bringing oxygen in needed for combusution. Otherwise - you will create a lot of smoke if you close the doors and the fire doesn't have enough air for good combustion.

My woodburner (a Freplace Xtrodinare 44) is designed to have the doors closed when burning. It has 2 external air intake vents plus it is a direct vent and takes air in through the outter vent inside the flue (the inner vent is for exhaust). I have to be very careful when I open the doors since there is a pressure change between inside the unit where the wood is burning and the rooms in the house. When I open the door just a crack, the fire breaths more air and burns more rapidly until the presue is equalized. I throw in a few more logs and then close the doors.

I also have an additonal blower on the unit that takes fresh cold air from outside and circulates it around the fireplace to heat it up - and that warm air is then blown into the family room. This fan is on a thermostat and only comes on when the fireplace is really hot. That is the trick to making this work as a backup heater when the power goes out - that fan must be on a backup generator so that I can push warmed air around the house.

I love this thing - I only wish I put a second wood burner in as a backup heat source. I have 2 other gas fireplaces - but you cannot beat the wood burner for the looks, the amount of heat it can generate, and the realtively low cost to run it.

The fireplace is going to take heated air through whatever source in can get. Closing the doors at most will restrict the airflow, and lead to a lesser fire. Unless you have the fireplace set up with an outside air supply, it's inevitable that you'll use heated air for combustion.

all that said, I leave them open. More heat comes into the room that way. I close them at two times--(1) starting the fire, which means higher velocity airflow through the vents at the bottom, which helps "fan" the flames to start and prevents smoke from coming into the room and (2) at the end of the night when the fire is burning down, so that I don't get ash blowback into the room once the fire is out.