Both of my childhood homes featured fireplaces. If you've had one, you know how terrific they can be—great places to bask on cold winter nights, an easy opportunity to toast marshmallows, picturesque, undeniably a pleasure in the house that has one.
However, fireplaces have chimneys...and chimneys can be problems when it comes to home energy efficiency. For one thing, many people do not take care to close their chimney flue when there is no fire. This means warm air has an easy exit from your home; you have a big hole in your roof, after all, and warm air will take advantage of that. Ditto cold air in the summertime, when you're probably far from thinking about winter nights by the fire.
Be sure to close that flue.
Second, be sure that your hearth is sealed against cracks. Sometimes the mortar holding together the bricks will develop hairline fractures, and these might be enough to lose heat. Take precautions with the screens you use as well, to ensure that glass doors shut tightly when the fireplace is not in use.
Now that it's winter, it's the time to enjoy that fireplace. But take a little care, so that you don't end up heating the outdoors when the fire isn't burning. Visit the Stay Warm, Save Money site for other ways you can reduce heat loss from the fireplace.