The fireplace damper is that thing you forget to open when you set the first fire of the season. The billowing smoke in your living room is a cheerful reminder that cold weather is here again. Well, perhaps that doesn't happen every year. But if you've had it happen even once, you may have wondered why that damper is there at all. Why not just leave the fireplace open all year round?
The damper is there to keep you from spending hundreds of extra dollars on your heating and cooling bills. It is meant to provide an airtight seal just above the firebox at the throat of the chimney. This keeps the outside air outside and your conditioned or heated air in. Leaving the damper open can thwart this process even more effectively than leaving a window open all year. The chimney can produce a draft effect. That's what it's designed to do when you've got a fire going. You want the fumes and smoke being drawn up the chimney and out. But when there isn't a fire, cold air falls down the chimney and warm air rises up and out.
When starting a fire, you need to have the damper fully open. Some dampers can rust or stick a bit or become blocked by ash so that full opening is not possible. This needs to be corrected either with cleaning or with a new damper in a better design. The open damper will provide plenty of air to help you build a hot, smokeless fire. Once the fire is going and you can see glowing coals or embers, it's safe to close the damper part way to control the size and intensity of the blaze.
When you're done with the fire, you can close the damper all the way along with the glass doors, starving the fire of air. This helps ensure that the fire doesn't resurrect itself in the middle of the night in response to a light draft.
Most fireplace dampers seal tightly with a metal to metal form that protects well against leakage. However, as these dampers age, the metal can warp. Heat from the fire can cause it to curve slightly. Moisture can bring in some rust. And cold can cause the metal to contract just a tad. All these things contribute to a looser seal that will allow some cold air in and some warm air to escape.
Newer damper technologies have proved more effective at dealing with these difficulties. If your damper produces a noticeable draft even when closed, purchase a new efficient damper to take care of the problem. Even if you don't notice the draft, you may notice a spike in your heating bills. Stick your head up the fireplace and listen to see whether the damper is the culprit. A tightly sealed damper should be silent. A problem damper will let you hear some whistling or blowing noises. You may even feel a slight breeze. If you're not sure, open up a few windows to enhance the draft and try again. Having a chimney sweep come out at least once a year (the way you know you should) can save you this trouble. The sweep knows how to check your damper for airtightness and can advise you on a replacement.
For additional control, you can have a damper installed at the top of the chimney as well. This will remain open during fire season and can be closed when spring rolls around.