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Why Does My Chimney Smell When My A/C Is Running?

by | Jun 25, 2025 | Smoky Smell

It’s a familiar scenario for many Pittsburgh homeowners – summer rolls in, the air conditioner kicks on, and suddenly there’s a strange, musty odor drifting into the living room. The culprit? Often, it’s the chimney. While this may seem surprising during the off-season for fireplace use, chimney odors are a common summer complaint. The issue tends to become especially noticeable when central air conditioning systems are running. Top Cat Chimney Sweep and Service hears this concern often, and there’s usually more going on than just lingering fireplace soot. Understanding the relationship between your chimney and HVAC system is key to solving the problem.

If it’s time for your annual chimney inspection or cleaning, contact us today to book an appointment!

Is a Chimney Smell Always a Sign of a Bigger Problem?

It’s easy to dismiss chimney odors in the summer as harmless or simply “part of the season,” especially when the fireplace hasn’t been used in months. But if your chimney starts to give off a strong, musty, or smoky odor when the A/C is running, there’s likely more going on than just stale air. That smell usually indicates a disruption in airflow, and it’s often the air conditioner that unintentionally causes it.

When your central air conditioning system is pulling air through the home, it can create negative pressure that draws air down through the chimney flue. That downward airflow carries whatever’s inside the flue with it: soot, creosote, decaying organic matter, or moisture. The stronger the negative pressure, the more noticeable the odor tends to become, especially on hot, humid days.

While a smelly chimney isn’t always linked to a serious structural problem, it does point to conditions that should be corrected. Odors often result from things like creosote buildup, trapped moisture, animal nesting, or decaying debris – all of which can worsen if ignored. It’s not unusual for those smells to intensify as time goes on, going from occasional nuisance to persistent presence.

original infographic about chimney smells

While chimney odors don’t always signal major damage, they’re rarely meaningless. They indicate that the system isn’t sealing, venting, or maintaining airflow the way it should. Addressing the cause early on can prevent more extensive repairs or indoor air quality issues later.

How Do I Stop My Chimney From Smelling When the A/C Is Running?

There are several practical ways to address chimney odor that intensifies when your air conditioning is on. Solutions range from minor adjustments to full professional cleanings, depending on the root cause:

  • Schedule a chimney sweeping to remove creosote, soot, and debris buildup left over from past fireplace use.
  • Install or repair the chimney cap to prevent moisture, animals, and leaves from entering and creating odor-producing buildup.
  • Seal the damper tightly when the fireplace is not in use, especially in the summer months when negative air pressure is most likely to draw air down the flue.
  • Check for gaps in chimney flashing or crowns that might be letting in rainwater. Even a little moisture can mix with creosote and organic matter, intensifying odors.
  • Consider an energy-top damper or top-sealing damper that creates a tighter seal than traditional models, reducing the amount of airflow between the chimney and the house.
  • Use a deodorizing agent or chimney odor neutralizer, but only after identifying the source of the smell. Masking a problem won’t solve it long term.
  • Have the home’s ventilation system evaluated to check for imbalances that are pulling air downward through the flue instead of allowing it to vent upward naturally.

Combining these solutions can significantly reduce or eliminate chimney odor, even when your air conditioner is running full blast.

When Should I Call a Professional Chimney Sweep?

Sometimes, chimney odor is more than just a minor annoyance, it’s a sign that your chimney needs professional attention. Here are a few situations where it makes sense to bring in a certified chimney technician like Top Cat Chimney Sweep and Service:

  • If the smell persists even after cleaning or using deodorizing products.
  • When you haven’t had a chimney inspection or sweeping in over a year.
  • If there’s visible staining or moisture around the fireplace or flue.
  • When animals, nesting material, or debris are suspected inside the chimney.
  • If you notice smoke odors or burnt smells despite not using the fireplace recently.
  • When your damper or chimney cap appears damaged or no longer seals properly.

Calling a professional isn’t just about dealing with the odor, it’s about addressing the underlying airflow or structural issues that are allowing that odor to enter your home in the first place. A trained sweep can diagnose the cause quickly and recommend cost-effective, practical solutions that make a real difference.

Reach Out to the Experts

A chimney that smells during A/C season can be frustrating, but it’s also fixable. Top Cat Chimney Sweep and Service offers thorough summer inspections and cleanings for Pittsburgh homeowners looking to eliminate off-season chimney odors and get ahead of future problems. Contact Top Cat Chimney Sweep and Service today to schedule your appointment and get back to enjoying a fresh-smelling home.