Your fireplace always has an odor, but normal draft takes the smell up the flue to the outside. If you smell your fireplace, outside air is being sucked into your house.
As a general rule, a chimney sweeping is in order if you haven’t done it recently. Any build up of soot in the chimney will increase odor problems. Sweeping is just the first step and will not remove the imbedded creosote that has penetrated the masonry of the fireplace. Creosote, especially when wet, has a very powerful, obnoxious odor.
Water penetration and negative pressure in the house are the main causes of fireplace odor. So the next step is to insure that there is an adequate rain cover (cap) on the top of the flue and that the exterior of the masonry is somewhat “waterproof.”
Sweeping and capping the chimney (and waterproofing if necessary) may help to solve some of the odor problems, but it is doing nothing to solve what is really happening when you smell your fireplace. Something in the house or room is causing negative pressure and sucking outside air into your home (even through the closed damper and glass doors).
This often occurs during a wet, hot season following major and minor weatherization or venting changes and improvements. New furnaces, sky lights, kitchen and bathroom exhaust fans, windows, dryers, and new roofing are often enough to tip the interior pressure from a positive to a negative.
All the air being forced or exhausted out of the home has to come in to the house from somewhere as make-up or replacement air. This replacement air always re-enters the house through the “path of least resistance” just like water or electricity. It’s easier to come into the house down a 12” x 12” flue that’s already cooler than the home, than through the normal openings in the house. This condition is correctable, and is getting much more preventable as we all try to make our homes more energy efficient.