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Why Does Your Fireplace Smell When It Rains?

All fireplaces have some odors, but you will usually encounter only mild smells for the most part. These odors can sometimes become more pronounced and often unbearable when it rains. So, why does your fireplace smell when it rains and is there anything you can do to prevent the unpleasant odors?

Your fireplace smells when it rains because the moisture-laden backdraft through the flue or chimney worsens and spreads the odors of soot, creosote, and other buildups. Also, you may have mold growth and decaying organic matter in the chimney, flue, or fireplace. 

All of us love the smell of the first rain after a dry spell, and we even have a name for the aroma: petrichor. But we all know that the fireplace smell when it rains is a far cry from the lovely earthy scent. Read on to understand the reasons why your fireplace smells when it rains and the fixes.

Why Your Fireplace Smells When It Rains

Your fireplace smells when it rains due to the following interrelated factors:

  • A fireplace and its flue or chimney have pre-existing odors.
  • Negative air pressure in your house causes a backdraft.
  • The backdraft causes outdoor air to flow into the fireplace.
  • This reverse airflow causes the odors to pervade your house.
  • Rain or moisture aggravates the odors, and so does high humidity. 

In effect, you have an immensely unpleasant concoction of unbearable odors emanating from a fireplace when it rains. What’s even worse is the reality that these odors will continue to spread in your house as long as the indoor air pressure is negative or low. 

Now, there are several culprits in this quagmire. You need to be familiar with each factor so that you can choose the most appropriate remedy. So, let’s start with the unavoidable soot and creosote problem. 

The Odors of Soot, Creosote, and Polycyclic Aromatic Hydrocarbons

Your fireplace has residual soot and creosote unless you have just swept and cleaned the entire system. The soot and creosote buildup in the fireplace and flue or chimney have distinct odors. These odors are stronger when the soot and creosote are exposed to moisture. 

When you use a fireplace, the flue or chimney allows the air loaded with exhaust gasses to flow up and out of the house. Thus, the soot and creosote odors don’t invade your indoor spaces. 

However, the backdraft during rain reverses the airflow. So, the odors don’t escape through the flue. Instead, moist air during or after rain carries these odors down the chimney to the fireplace and spreads the unpleasant scents in your house.

Now, you may have a relatively clean fireplace without any evident soot or creosote buildup. Yet, the fireplace flue or chimney may have creosote and soot throughout the brick & mortar, clay, or terracotta structure. 

In the context of odors, creosote smells worse than soot. Creosote is used as a preservative for wood. So, burning wood in your fireplace will naturally lead to creosote as a by-product, which will gradually accumulate inside the flue or chimney. If the interiors of your fireplace chimney or flue are porous, such as clay, creosote can penetrate the material and worsen the odor problem. 

Also, creosote comprises polycyclic aromatic hydrocarbons, a class of chemicals that smells like asphalt and car tires. Furthermore, burning wood, gas, or coal in a fireplace produces polycyclic aromatic hydrocarbons. Thus, you’ll have a significant buildup of these chemicals in a fireplace. 

All these unpleasant odors flow out with the exhaust gasses when your fireplace is operational in the winter. Imagine the reverse happening when it rains due to the backdraft through the flue or chimney, and you will have a lucid idea of why your fireplace smells so bad.

The Smells of Moisture, Mold, and Bacteria in Damp or Humid Air

Rain poses more than one problem for your fireplace. First, some rainwater may run down your fireplace flue or chimney unless it is impeccably sealed. Second, high relative humidity when it rains and immediately after increases the moisture content inside the fireplace system. 

Thus, the musty smell of moisture is inevitable. Also, the moisture seeping into the fireplace or its flue facilitates mold growth. Furthermore, damp or humid air traps the molecules causing the distinct odors, and the smells will linger much longer than when it is relatively dry. 

Warm and humid conditions trap and spread odor more effectively than dry and cold air. Also, a significant spike in relative humidity and warmer temperatures are ideal thriving conditions for smelly bacteria. Therefore, you have not one or two but a plethora of issues to deal with.

Your fireplace already has residual soot, creosote, and polycyclic aromatic hydrocarbons. These odors worsen due to rainwater and linger longer as high relative humidity traps the molecules in the air. Additionally, the fireplace has mold growth and smelly bacteria thriving in the condition.

The Unpleasant Scents of Clogged and Decaying Organic Matter

Residential fireplace flues or chimneys are often hideouts for many small animals and nesting grounds for insects, pests, rodents, bees, and birds. Not all critters may find their way out of the cavity. Also, some critters may leave behind substantial organic matter. 

Decaying organic matter produces various types of odors, none of which is pleasant. Plus, rain and the accompanying high humidity aggravate these smells by facilitating the growth of mold and several smelly bacteria. Additionally, decaying matter is a feast for innumerable microbes.

Even if your fireplace doesn’t have dead critters, there could be leaves and other organic matter clogging the flue or chimney. Such organic matter decays, too. And all the unpleasant odors will flow into your living spaces as the humid air backdrafts through the fireplace when it rains. 

Fireplace Backdraft Due to Negative Air Pressure in the House

The severity of any odor in a fireplace mainly depends on its condition, i.e., soot, creosote, etc. The secondary factors are mold, bacteria, and decomposing organic matter. However, all these odors wouldn’t pervade your home if the stale and exhaust air flowed out of the flue.

A lit or burning fireplace exhausts the flue gasses naturally as hot air rises. This outflow leads to negative air pressure inside the house unless a fireplace has an inlet duct or vent. So, a room draws air from the adjoining spaces through any leaks in the doors, windows, and insulation. 

Now, even when the fireplace is running, there is a possibility of the flue drawing outside air due to the negative pressure inside a house. And this problem gets worse when the fireplace is not lit. The air inside the flue isn’t hot, so it doesn’t rise naturally to flow out of the chimney. 

Modern homes are insulated, and most properties have additional weatherstripping. Thus, there is less outdoor air flowing into the house through leaks and natural ventilation. 

Plus, some household appliances contribute to and worsen the negative air pressure, such as a dryer, kitchen hood, bathroom exhaust fan, etc. This negative air pressure naturally leads to a backdraft. In fact, the indoor spaces draw air through the most convenient and swiftest routes.

A fireplace chimney is a much larger opening than cracks in weatherstripping or insulation and leaks around doors and windows or other such gaps. Hence, outdoor air flows in through the fireplace flue to neutralize the pressure difference. 

This natural phenomenon worsens when it rains. The atmospheric pressure at a macro level is already low when it is about to rain. If a house has a negative air pressure in such a condition, the natural backdraft will be much stronger than usual. 

Therefore, the fireplace and its flue or chimney become a reverse-exhaust duct when you have negative air pressure inside your house during rain. 

The backdraft reduces as the outdoor and indoor air pressures stabilize. However, the humid air would have already trapped, carried, and spread the odors in your house by that time. And the moisture in the air enables the odor-causing molecules to linger in your home. 

How To Prevent Your Fireplace From Smelling When It Rains

Some people light candles inside the fireplace to warm the air so that it rises naturally and flows out of the chimney. However, such an option may not prevent a strong backdraft when it rains if the negative air pressure inside your house is significant. 

Some homeowners also use vinegar to counter the unpleasant odors coming from the fireplace. While this could be a temporary solution, vinegar doesn’t solve the causal problems, whether you use it in the fireplace or around it to absorb the odors. You should look for solutions that can effectively prevent your fireplace from smelling every time it rains.  

So, consider the following remedies. You may need more than one solution, depending on the fireplace, type of flue or chimney, and overall condition of the system. Furthermore, make sure to account for the negative air pressure in your house.

Opt for a Chimney Sweep and Routine Fireplace Maintenance

The most effective way to eliminate soot and creosote from a fireplace flue is a chimney sweep. Ideally, you should opt for a chimney sweep as preventive maintenance. You should also have a fireplace maintenance routine. 

Soot and creosote buildup doesn’t happen overnight. However, the rigidity of creosote buildup increases over time. Creosote is a tar-like substance that is easier to get rid of when it is yet to set into the interior surface of a fireplace and its flue. 

You must be proactive with a chimney sweep if the flue’s liner or surface is porous. Similarly, the fireplace needs preventive maintenance and routine cleaning. The routine or frequency depends on how extensively you use the fireplace. 

Get a Chimney Damper, Cap, or Chase Cover

Your fireplace probably has a flue damper, but it isn’t always sufficient or effective in preventing a backdraft. Hence, consider installing a chimney cap. The damper and chimney cap should prevent a backdraft, but remember to remove the latter before using the fireplace.

In some cases, installing a chimney chase cover is the best solution. A chase cover protects the entire protruding structure of a fireplace chimney, not only the flue’s outlet. So, the structure will have a weather shield, especially against rain and snow. 

Also, the fireplace and chimney can function with the chase cover on because its design allows flue gasses to flow out. In other words, the chase cover will deflect rainwater and snow, keep moisture out of the system, and prevent critters from finding their way into the chimney.

Install a Nonporous Flue or Chimney Liner 

A chimney chase cover has many benefits, but it doesn’t have any role in the soot and creosote buildup inside the flue. So, you should consider a nonporous flue or chimney liner if you have a porous material right now. 

You can install a steel liner that won’t allow creosote to penetrate its surface. Also, sweeping a chimney that has a steel liner is easier and more effective at eliminating most residual buildup. 

Facilitate Positive Ventilation in Your House

Last but not least, you should do something about the negative air pressure in your house. Backdraft or negative air pressure isn’t a problem unique to fireplaces or chimneys. Even HVAC systems have this issue. 

Many houses don’t have a fresh air intake duct or vent. A combustion air duct is necessary for a furnace. Likewise, an inlet vent or duct for fresh air is essential for a fireplace. Otherwise, the constant exhaust of flue gasses and the indoor air will lead to negative pressure, causing a backdraft.

The negative air pressure problem is not limited to winters or the colder months when you use a fireplace. 

A centralized air conditioning system has no natural or positive ventilation in a house. So, the loss of indoor air through the dryer vent, kitchen hood, and bathroom exhaust fans will cause negative air pressure. Thus, your fireplace and flue or chimney might facilitate a backdraft. 

You can open a door or some windows as a temporary remedy to eliminate the fireplace smells when it rains. However, such options aren’t sustainable when you operate a heating or cooling appliance. 

You also need positive ventilation to effectively prevent or counter negative air pressure. You won’t succeed in averting a backdraft without ventilation, whether through the fireplace or elsewhere. 

Conclusion

There are four main reasons why your fireplace smells when it rains:

  • Soot, creosote, and polycyclic aromatic hydrocarbons in the fireplace, flue, or chimney.
  • High relative humidity and moisture-enabled mold growth in the flue and fireplace.
  • Microbial growth, including smelly bacteria, and decomposing organic matter.
  • Negative air pressure in the house, causing a backdraft through the fireplace.

Author

  • Jake Alexander

    Jake is a freelance writer from Pennsylvania who enjoys writing about science and sports. When he's not writing for Temperature Master, he can be found watching the NFL or playing basketball with his friends.

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