Worried about chimney downdrafts causing you problems? You're not alone, and we're here to help.
A cosy fire is a beautiful thing. You get a relaxing fire that heats your home and a beautiful atmosphere to boot. But those warm fuzzy feelings can quickly turn to frustration and stress when you're worried about a downdraft.
Chimney downdrafts can range from a minor nuisance that lowers your fire's efficiency and raises your heating bill, to a severe health concern if you have carbon monoxide coming back into your home. It can also render your fireplace unusable.
We've created a thorough guide to help you identify if your chimney suffers from a downdraft. If it does, we have a range of possible causes and proven solutions you can explore to banish your downdraft for good.
Chimney 101
To fully understand what causes downdrafts and how to fix them, you need to know how a chimney functions. So let's briefly talk about the key chimney components relevant to downdrafts.
Chimney - The exterior housing for the entire "chimney system". You likely recognise it as either the brick or concrete stack on your roof, the metal housing attached to the outside of your home, or the cavity built into your walls.
Flue - The hollow space inside your chimney. Your flue is the passage all air, smoke, and gases travel between your fire and the outside air.
Flue liner - The liner protects the masonry of your chimney from the corrosive by-products of combustion. And it usually improves the efficiency and safety of your chimney. Common materials include concrete, pumice, clay, and stainless steel.
Damper plate - A metal plate located in the "throat" of your flue, just out of sight above your fire. You can open or close the chimney damper plate like a valve to regulate how much air and smoke can travel up and down your chimney.
Chimney Cowl/Pot/Cap - Sits on top of the chimney pipe on your roof. A chimney pot helps to prevent moisture, wind, animals and debris from getting into your chimney.
Now, let's build on this with a quick explanation of how a chimney works.
How Does Your Chimney Work?
It pulls in fresh indoor air from the bottom and exhaust waste gases out the top.
It relies on air being drawn up and out in order for it to function correctly. The key to a healthy chimney is a constant and balanced airflow, also known as a chimney draft.
What happens when you light a fire?
The fire emits heat, which adds energy to the air in your flue. Heat rises. The hot air rises up your chimney and is drawn up and out of your home.
The rising warm air is quickly replaced by the cooler air coming into the fire from your room, which feeds the fire and keeps it burning brightly. This cycle keeps your room cosy and warm as well as all the air moving in the correct direction.
Or, at least, that's how it's supposed to work.
So What Are Chimney Downdrafts?
Chimney downdraft occurs when smoke and other combustibles from your fire no longer travel up and out of the chimney. With severe cases of chimney downdraft, it can fall back into your fireplace and billow out into your house.
Before we discuss what can cause downdraft and how to fix it - we need to be able to spot the problem first.
How To Tell If You Have Chimney Downdraft
Sometimes chimney downdraft is a bit more subtle than clouds of smoke coming into your living room. Here are some of the most common signs of downdraft many homeowners report.
Visible Plumes Of Smoke
This is the most obvious sign. You can see intermittent puffs of smoke emanating from your fire and entering the room. More than an annoyance, it's also a major health risk. Plus, it can ruin your floors, furniture and walls too.
Irritated Eyes, Nose & Throat
Smoke isn't always visible. Chimney downdraft can cause your fireplace or stove to waft small puffs of smoke into your room that you can't see. Instead, you may experience nose, throat, or eye irritation.
It can also impair breathing and is particularly dangerous if anyone in your home has asthma or difficulty breathing. Does anyone notice discomfort when you have a fire?
Can You Smell Something?
Do you have odour issues in the room when the chimney isn't in use? Most homeowners with downdraft issues frequently comment their home smells like wet wood or wet ashes.
Soot On Glass
Is there dark-coloured soot building up on the glass doors of your fireplace? Chimney downdraft can be the culprit. If you have an open fireplace burning solid fuel, you may even have soot buildup on the windows in your room.
Any "False" Alarms?
If your carbon monoxide or smoke alarm is triggered unexpectedly, you should treat it as a serious problem. Don't ignore any "false alarms". If you can't find any apparent reason why your carbon monoxide alarm is going off, you might have low levels of smoke seeping into your room.
Gusts Of Hot Air
If you get hit with occasional gusts of hot air, you may have a downdraft issue caused by changes in air pressure. Yes, you want your fireplace to give off plenty of heat. But it shouldn't randomly have bursts of heat. Remember, your chimney should draw air into the fireplace, not push it back into your room.
Even if downdraft isn't causing drastic problems, it's still worth fixing. Chimney downdraft is unpleasant, raises your fuel bill and can pose a huge health risk.
So let's examine some common causes and potential solutions so you can enjoy your fireplace again.
Downdraft Causes and Solutions
So you've spotted at least one telltale sign of chimney downdraft. It's time to learn about common causes so you can figure out how to tame your unruly fire.
Typically chimney downdraft occurs due to two primary reasons:
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Your chimney height is too short to create an optimal stack effect
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There are tall buildings or trees/shrubbery nearby that interfere with the wind currents around your chimney.
Your Chimney Might Be Too Short
To have a proper draft consistently, a chimney must be tall enough to extend above your roofline. The air close to your roofline moves slower, has lower pressure, and is more turbulent - which means it can tumble around and hang about. And ultimately take a tour down short chimneys.
Windy areas, coastal environments, and mountainous or hilly regions often require taller chimneys to function correctly.
How to fix a short chimney?
Make it taller! How you go about this depends on the type of chimney you have. Sometimes you can add a length of pipe, although you may need to build up your chimney stack too.
Most chimneys should be at least three to four feet tall. And preferably at least 2 feet above the highest point on a sloped roof to prevent downdrafts.
Wind-Related Problems
Are there high buildings or trees near your chimney? They can affect wind currents and cause low-pressure or turbulent air to enter your chimney.
Downdrafts can also be caused by dynamic wind loading. This type of chimney downdraft refers to situations in which different air pressures build up around your house due to strong winds.
Let's look at an example. If a wind blowing from the west hits your home, high pressure builds on the west side of your house. Meanwhile, the sheltered east side of your home turns into a low-pressure zone. This build-up of low pressure creates a vacuum which will act on any openings in your home (such as open windows or doors).
The vacuum pulls household air out of your home, thus disrupting the cycle of air your chimney needs. Air pushes down your chimney to replace what was lost - which creates a chimney downdraft.
How can you prevent wind-related downdrafts?
You can reduce the impact of wind on your chimney with a chimney fan. It sits at the top of your chimney pipe and exhausts air up and out of your home. This not only helps deflect the wind from the top of your chimney, but it improves the draw of your chimney as well. It also helps to close any doors, windows and exhaust vents to cut off air leaks to the outside world.
Is Your Damper Plate Open Or Closed?
We're sure this one doesn't apply to you, but we've got to start with the basics!
If it's your first time using this type of fireplace, it's normal not to know what a chimney damper plate is. It's a metal plate at the bottom of your flue and acts as a door to either open or shut your chimney. Make sure it's open! Otherwise, the smoke can't go anywhere.
A Cold Flue
If smoke refuses to rise up the chimney, it may be because your flue is too cold. If you haven't used your fireplace in a while, a cold, dense air column may push smoke back into your home.
What can you do about it?
To overcome a cold flue, you can prime your chimney before lighting a fire. The most common method is to roll up some newspaper to create a torch, light it, and hold it up towards the damper to warm the air and flue. This improves your fire's initial draw and hopefully draws any smoke up and out of your home.
The Stack Effect
Warm air is lighter (less dense) and rises to the highest points in your home. It eventually escapes through any cracks or openings it can find. This creates a net positive pressure at the top of your house.
As a result, the lower levels of your home have a net negative pressure. To balance out the pressure difference, your home attempts to draw in cold air from outside at the lowest levels in your home. This is known as the stack effect (or chimney effect), and your chimney works the same way.
But what happens when the house can't draw in enough fresh air to replace the hot air that escapes from your chimney?
Modern homes are extremely well-insulated and designed to be airtight. There aren't enough gaps and cracks to pull more air into your house.
Now think of all the home appliances that exhaust indoor air outside. Your dryer, air-conditioner, furnace and hot water heater all have an exhaust vent. You have indoor exhaust fans in your kitchen and bathrooms too. The situation only worsens in winter when you keep all your doors and windows closed.
Have you ever tried to open an exterior door and felt a suction pull of air trying to hold it closed? You just experienced a negative air pressure situation in which the building attempted to pull replacement air back inside.
So where does the air come from?
It often comes from the only opening it can find - and it rushes down your chimney.
What can you do about it?
The easiest solution is to shut off exhaust fans (reduce negative pressure) and open a window near your fireplace to help it operate properly.
Another solution is to install an air supply vent at the back of the firebox. This supplies cold air from outside, balances indoor-outdoor air pressure and encourages combustion by-products to escape via the flue.
The stack effect is at its strongest when there is a large temperature difference between the air inside your home and outside. If the outside air is almost as warm as the air in your home, there may not be enough of a pressure differential to pull replacement air into your home to get a good draw in your fireplace. A fan-powered air supply vent helps in this situation.
Your Fireplace Opening Might Be Too Big
If the opening of your fireplace is more than ten times the circumference of your flue, the ratio is off for proper ventilation. And you'll have a smoky fireplace. Maybe the previous homeowner installed a flue liner? While flue liners are great, they reduce the diameter of your flue. If your fireplace opening is rather large, your flue may now be too small to handle the volume of smoke produced.
What can you do?
The best solution is often to install a Smoke-Guard, a metal strip that makes the opening of your fireplace smaller. A smaller opening means less air flows into your fire. It burns lower and slower, which means a lower volume of smoke headed for the flue.
You Could Have A Dirty or Clogged Chimney
It's not uncommon for soot to build up in the chimney, which can affect fire performance. Soot layers physically restrict flue gases and reduce ventilation space. Deposits of just 1 cm can reduce your chimney's efficiency by 15-30%. They also raise the risk of a chimney fire. If you run your finger on the inside of your chimney and it comes away black, you may have a potential problem on your hands.
Foreign objects can accumulate in your chimney, too. Especially with strong winds. Insects and animals, such as birds, hornets, and wasps, may also use your chimney to seek refuge during the summer months when it's not in use.
How to fix a dirty or clogged chimney?
You can grab a flashlight and shine it up your flue to check if you have any unwelcome objects to clean out. Look at your smoke shelf; its job is to catch fallen debris. You should always regularly clean your chimney. Professional cleaning is recommended at least once a year to ensure peak performance and safety.
Is Your Chimney Damaged?
Your chimney or liner might also be damaged. Anything that lowers your flue's insulation and warmth impairs your chimney's natural 'draw'. And can cause smoke and combustibles to fall back into the fire and out into your room.
What to do about Chimney damage?
You should take care to inspect as much of the chimney as you can. Do it yourself, or hire a chimney sweep. A professional can tell you if any parts of your chimney need to be replaced.
Don't Burn Poor Quality Wood
Poor quality, damp wood also contributes to chimney downdraft. Why? Because wet wood doesn't burn as well. This leads to lower temperatures which could potentially disrupt your chimney's draw. If your fire doesn't give off enough heat, the smoke and combustibles won't rise far enough up your chimney flue to escape. Instead, they can fall back down into your fireplace and living room.
Solution?
Use high-quality wood to get your fire lit. It gives off more heat which leads to a better draft. Ideal firewood is usually seasoned for around 12 months. You want a log with less than 20% moisture for low smoke levels. But it can't be bone dry, either. Too damp or too dry firewood smokes more than most flues can handle. If your firewood sizzles as it burns, it's likely too wet.
When In Doubt - Call A Pro
Chimney downdraft can be a really simple fix, or it can be harder to figure out what's going on and what you should do about it. Your best option is always to call a reputable chimney sweep and let them investigate for you.
If you aren't sure where to look, we recommend HETAS-approved providers for solid fuel fires. And Gas Safe Registered Engineers for all your gas fireplace needs. You can also check out the National Association of Chimney Sweeps.
That concludes our guide. We hope you found what you were after and get your chimney sorted pronto!
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