First and foremost, pay attention to your “firestarter”. You should try and use seasoned wood, newspaper or kindling to avoid the bad smell. Certain products will leave a waxy residue that will smell bad when left uncleaned. Make sure you check underneath the hearth refractory, on top of the fireplace, and on top of the mantel. If you are using a used gas fireplace, you should clean behind the upper and lower louvers.