Hi all, new member here so thanks in advance for your input. We moved into a new house with three has fireplaces. They are all gas powered and vent through two chimneys, one on either side of the house. I noticed some sort of vent on the outside of the chimney that services the family room. After getting my head into and behind the gas log set for the family room I found something that appears to be a vent and can twist it counter-clockwise to line up holes and allow outside air through the opening and into the firebox. There is also a standard damper that allows exhaust gasses up the chimney. So my questions are: 1. What is the name of this device in the back of the firebox that appears to be a vent? 2. When I have a fire in this fireplace what should I do with each of the venting devices (both open, one open and other closed)? I believe the rear vent (for lack of the correct term) is meant to allow outside/cold air to be combusted rather than simplly combusting the already warm air in the house. Can someone confirm this and tell me how to best use the device with the best efficiency? We're looking at having it replaced with a sealed high-efficiency unit but until that happens I want to be sure I'm using what I have correctly.