What Causes Chimney Odors?
Smoky, musty, or foul smells from the fireplace have specific causes and solutions.
Reviewed by John Freeman, chimney and fireplace professional with 20+ years of field experience.
Last updated
Short Answer
Chimney odors usually come from one of four sources: creosote deposits, moisture, animals or debris in the flue, or air being pulled down the chimney into the house. A smoky, tar-like smell that is strongest in humid summer weather is typically creosote. The Chimney Safety Institute of America (CSIA) says the smell is usually worse in summer when humidity is high and the air conditioner is running, and that the underlying problem is air drawn down the chimney by pressure problems in the house.
What the Smell Points To
| Smell | Likely source | What helps |
|---|---|---|
| Smoky or tar-like, strongest in humid weather or with the air conditioner on | Creosote deposits in the flue, with air being drawn down the chimney (CSIA) | Sweeping helps, but CSIA says it usually does not solve the problem completely. Make-up air elsewhere in the house and a tight-sealing top-mounted damper reduce the airflow down the chimney. CSIA also mentions commercial chimney deodorants and baking soda or kitty litter set in the firebox. |
| Damp or musty | Moisture entering through the cap, crown, flashing, or masonry, or condensation in the flue | Find and fix the water source first. Cleaning does not remove a smell that a leak keeps renewing. |
| Rotten, sour, or like droppings | An animal, a nest, or droppings in the flue | Have the flue checked before burning. The US Consumer Product Safety Commission (CPSC) says nests built over spring and summer can block vents and send exhaust into the home. |
| Burnt plastic or chemical | Something was burned that should not be. EPA lists household trash, plastics, foam, painted or pressure-treated wood, and plywood among materials not to burn. | Burn only dry, seasoned wood. A sweep removes what has already built up. |
| Smoke in the room as well as a smell | A draft problem or a blockage | Stop using the fireplace until it is checked. EPA says that if you smell smoke in your home, something is wrong. |
Why Summer Is Worse
CSIA calls the combination of high summer humidity and a running air conditioner the perfect storm for bringing out creosote odor. It says the real problem is air being drawn down the chimney, which it describes as a symptom of overall pressure problems in the house. That is also why a good sweeping helps but, in CSIA's words, will not usually solve the problem completely: removing the deposits does not change the airflow that carries what is left into the room.
Animals, Nests, and Debris
CPSC says birds, other animals, and insects can build nests in vents over spring and summer, so a smell that first appears when you light the fireplace again in the fall is worth checking for a nest or debris. CSIA lists debris-clogged passageways and animal or bird nests among the blockages that can push toxic gases back into a home, which is why it treats them as a safety problem and not just a nuisance. Clearing the flue is the first step. A cap with a screen helps keep animals out afterward.
An Odor Is Not a Carbon Monoxide Test
CPSC describes carbon monoxide as colorless and odorless, so a smell tells you nothing about whether it is present. CPSC recommends carbon monoxide alarms on every level of the home and outside sleeping areas. A chimney that smells and also lets smoke into the room should not be used until it has been inspected.
What to Do Next
Match the smell to the table, then start with the cause rather than a deodorizer. If you are in Southern Oregon, describe the smell and when it is strongest in a free photo review, or book a $99 inspection to find the source. We check for creosote, moisture, nests, and damper sealing, and explain what we find before recommending any work.
Sources
- Chimney Safety Institute of America: Homeowner FAQs
- Chimney Safety Institute of America: Avoiding Carbon Monoxide Hazards
- US Consumer Product Safety Commission: Annual Furnace Inspection to Prevent CO Deaths (2005)
- US Consumer Product Safety Commission: Carbon Monoxide Information Center
- US EPA Burn Wise: Best Wood-Burning Practices
- US EPA Burn Wise: Learn Before You Burn Wood
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