We were scared to close the flue pipe after an incident we had happen twice to us. I started a fire one night while my husband was gone. I went to bed then was woken by the sound of my alarm saying "carbon monoxide detected". At the end of the hall you could see smoke coming up the stairs. The entire downstairs was full of smoke. This happened a second time while we were out of the house and came home to the burn on the face of the fire place. We have been hesitant to mess with it since.