Chimney odors can be a common and recurring problem for many homeowners, especially during periods of warm or humid weather. The first thing to realize about chimney odors is that they are always present to some degree, but the natural draft during cold weather carries the odor up the chimney so you don't notice it. During warm weather the natural draft in the chimney can be reversed so that the airflow is into the home instead of out the top of the chimney. The odors will usually be greater when it is raining or very humid outdoors. This is probably because the moisture amplifies the odor. Just as a wet dog will usually have a greater odor, the porous brickwork will absorb moisture and retain the odors for a longer period of time.
Several steps can be taken to eliminate or reduce chimney odors. Listed below, in order, are the steps you should take to cure a "stinking chimney".
Have the chimney cleaned to remove as much of the odor causing creosote as possible. Glazed creosote is a special problem here because it is the hardest type of creosote to remove and also produces the worst odors due to its high moisture content.
Install a chimney cap if you do not already have one. This will keep the chimney interior dry but will not keep the humidity out.
Use a special deodorant to combat chimney odors. Your chimney sweep should be to able to furnish this.
If the first three steps have not cured the problem, try sealing the entrance to the flue at the chimney top. This will help prevent airflow in the chimney. Be sure to remove this seal before using the chimney!