| The house is too tight. The fireplace uses lots of air. Must be sucking it in the drain vents. I added a 4'' air intake to my fireplace. I took the fire brick out of the back and used a grit tooth hacksaw blade to cut a hole in it, a hole saw on the metal housing and a hammer on the outside block. Stuffed in a length of 4'' stove pipe. I filled the gap in the firebrick with furnace cement, and used mortar to fill the gap in the block. I have a cap for the outside, and a metal plate that slides in convenient groves in the back of the fire brick I can use as a damper. Hadn't had the problem with the sewer gas, but have nicer fires now.
A core drill will make a neat hole in almost anything. | 
Linear Mode
