Things You'll Need
Tarp or plastic sheeting
Rust penetrating oil
A flue damper is a metal plate inside a fireplace chimney. It is shut when the fireplace is unlit to prevent cold air from entering the home. When the fireplace is lit, the damper must be open to release the smoke. A lever located somewhere easy to reach near the face of the fireplace is used to operate the damper. Over time dampers can stick, rust, corrode or fall out of alignment. Signs that the damper is not working properly are that it does not open or shut at all or it does not open or shut fully, causing smoke to fill the house when the fireplace is lit or cold air to blow into the house when no fire is burning.
Remove all wood and ash from the fireplace. Sweep up any residue with a whisk broom and discard in a metal container. Lay tarp or plastic sheeting around the hearth and over the floor in front of the fireplace to protect them.
Use a flashlight to locate the damper in the flue. You'll find it 1 to 2 feet above the fireplace opening. Operate the damper lever to determine what the problem may be. If the damper refuses to open or shut, push it open by hand. When the movable joints are exposed, spray them with a household lubricant. Operate the lever again to see if it works. If it still does not open, or if it does not fit firmly in place when shut, proceed to the next step.
Remove the bolts that hold the damper in place by turning them counterclockwise with a wrench. You may need to spray the bolts with a rust penetrating lubricant to free them before you can turn them. Once the bolts are off, the damper will move. Turn it far enough to one side to slide out the metal rod that holds it in place. With the rod out, the damper should drop from the flue. Inspect the damper. It may have a heavy buildup of creosote on it that is getting in the way of a tight fit. If so, scrape it clean with a metal scraper. If the damper appears bent or otherwise permanently damaged, you must replace it.
Bring the old damper to a fireplace supply store or large hardware supply store. Purchase a new damper the same size as the old one. You may also wish to purchase new bolts in the same size as the old ones if the old ones show signs of warping or stripping.
Purchase a damper that is approximately the right size, if a damper the exact size as your old one is not available. To get an inexactly-sized damper to fit may require quite a bit of work and expense and could involve tearing out some of the fireplace masonry. It may be less expensive to choose a top-sealing chimney damper. One type is placed on the top of the chimney flue like a hat and the other is fitted just inside the chimney flue's rim. These relatively new style of dampers have the added benefit of keeping birds and other small animals out of the chimney flue. In both cases, you operate them using a lever near the face of the fireplace.
Wear gloves, old clothes, a hair covering and cloth over your nose and mouth because ash will fall down onto you as you work inside the chimney flue, especially if you must remove the old damper.