To be effective, fireplace dampers need to be tight fitting and fully intact. A lot of times with all the smoke, heat, and flames that come in contact with a fireplace damper, they deteriorate to the point of being non-functional. When that happens, you need to replace your damper. While you could call in an interior fireplace expert, you can likely do the replacement yourself.
Clean out your fireplace. Use a steel garbage can for this chore, and with your leather gloves on, throw out all you pieces of charred wood into the can. The fireplace grate will need to be moved as well.
Locate the damper. You will need to stand where the wood and grate were and shine your flashlight up the chimney or attach an automotive drop-light to beam light up the chimney as well. The damper will be up the chimney, generally a foot or so above the opening of the actual fireplace.
Remove the damper. There will be a rod that is attached to the actual damper and it will be held on by 4 bolts. Using your adjustable wrench, unscrew these 4 bolts in a counter clockwise direction. Turn the damper sideways, and it will slide right off the rod.
Check your dampers against each other. Your new damper needs to be the same diameter as the old damper, and if you have not gotten one yet, take the old one to the hardware store and make a comparison to get the right one.
Attach the new fireplace damper. First you will slide it on the rod, then screw the four bolts back into place.
Reassemble your fireplace. Replace the grate that holds the wood, restock the fireplace, and you are ready to start enjoying your new fireplace damper.