How Do Chimneys Work? 


There’s nothing like sitting in front of a fireplace on a cold night. You might not think about your chimney that much, but keeping it in good working order is a critical part of ensuring you’ll have that fire in the fireplace on cold nights. 

And it’s not magic. It’s not so much that having a smokestack above an opening is enough to have a properly built chimney, either. It’s physics. It’s science that helps make your chimney move the smoke and gases from your fireplace up and out of your house. 

Chimneys are designed to remove smoke and byproduct gases, like carbon monoxide, from the air. When you think of a chimney, you probably think of a fireplace first. I do too, actually. And I’d be willing to be that most people in our chimney company would as well. But the truth is that any heat source that burns fuel requires a chimney. For example, if you have a gas furnace, then it has a chimney. 

What’s convenient is that your furnace’s chimney works in the same exact way as your fireplace’s chimney!

Your House is a System

Even though you can’t see it, the air in your house is in constant motion. Air flows to the upper parts of your house, leaks out through openings, and is replaced by air in the lower parts of the house. Many different factors can contribute to your home’s airflow, such as stack effect, wind loading, interior mechanical systems, and fuel-burning appliances. 

Stack Effect

You know the old saying: warm air rises. That’s exactly what’s happening here. But it’s more accurate to say that the colder air is pushing the warm air up. That’s because the cooler air at the bottom is denser and heavier than the warm air. It’s like a water hose, where the water from the faucet is pushing the water in the hose out. 

As the warm air is being pushed up, the trapped air forces its way out, even if it has to force through the smallest openings, such as light fixtures and window frames. All while the colder, outside air is trying to push its way in. 

Wind Loading

Wind loading is the effect on your interior house pressures caused by the wind. Whenever the wind blows on a building, it creates high pressure on the side that it hits and low pressure on the other side. Any open windows or doors on the side the wind hits will help to pressurize the house, helping and increasing the chimney draft. On the other hand, openings on the other side, downwind, will depressurize the house and increase the likeliness of back-drafting from your chimney. Backdrafting reverses airflow and causes the smoke to come into your house instead of up the chimney. 

Interior Mechanical Systems

This is just a fancy way of bundling your dryers, kitchen fans, bathroom fans, attic fans, or central vacuums. All of these systems can create depressurization by pushing a lot of air out of your house. This causes negative pressure where your fireplace is located, making it difficult for natural draft chimneys to function as it should. 

Fuel Burning Appliances

This could be your furnace, water heater, fireplace, or woodstove. These need a lot of air for combustion. Unless they were built to draw air from outside, like direct vent appliances, operating them will reduce the inside air pressure.

Five Primary Factors that Affect Chimney Draft

#1. Air Pressure

Air pressure is the weight of the atmospheric pressure exerted onto the earth. When the air pressure is balanced in your home, then the air that’s flowing into the home is doing so at the same rate as air is flowing out of your home. 

The stack effect allows lighter, warm air to rise up and out of your chimney and replaces that air with cooler, heavier air in the lower parts of your home. All this air comes in through very small openings in your house. 

It’s not always bad, though. If your home sealed up too tight, like many modern homes are, that creates negative air pressure. When this happens your fire may not burn or your furnace may not work as it should. 

If you’re wondering about your chimney’s airflow, then you can give Patriot Chimney or any other CSIA certified chimney company with a Chimney Physics designation a call and we’ll be able to check it out for you. 

#2. Flue

Your flue liner can increase your chimney’s efficiency and performance. An appropriately sized flue should be one inch wide for every 10 square inches of the fireplace opening. The 10:1 ratio allows air to travel at the perfect velocity through your flue. 

If your flue is too big, too much air will be sucked up through your chimney and you will lose heat. On the other hand, if the flue is too small, then air cannot exit the chimney fast enough, so you’ll likely find yourself in a smoke-filled room. 

#3. Chimney Height

I already mentioned that taller chimneys produce a stronger draft. But what does that mean? 

  • Your chimney should be at least 15 feet tall from the base of the fireplace to the top of the chimney. 
  • The top of your chimney should be at least 3 feet higher than the point where it penetrates the roof
  • If your chimney is within 10 feet from another building or obstruction, then the height should extend to be at least 2 feet taller than the other building. 

Having a taller chimney has two main purposes: 

  1. It reduces wind-induced downdraft – this can create a situation where smoke from a shared chimney stack is blown down a nearby chimney that may not even be in use at the same time. Modern houses are more airtight and that has caused more low-pressure areas in rooms, which can cause a vacuum that draws the fumes down the flue. Of course, this can lead to dangerous fumes entering the room. 
  2. It minimizes the likelihood of hot embers landing on the roof – your roof could contain combustible material, like dried leaves or sticks, that can catch fire if it comes into contact with an ember. For this reason, it’s extremely important to keep a chimney with a sufficient draft. 

#4. Obstructions & Damage

To create a sufficient draft, your chimney needs to be free from obstructions and damage. If you don’t have a chimney cap, then the tight safe space and added warmth from your home are extremely inviting for birds, squirrels, raccoons, and other critters. 

Also, creosote buildup can hinder the airflow. Creosote is a naturally occurring byproduct of combustion that sticks to the walls of your chimney’s flue. It’s like a sticky, sooty material that can thicken and harden and build upon itself. This makes the opening narrow and causes the same effect as if your flue liner isn’t big enough for the opening. This is why it’s important to have your chimney inspected and swept annually. 

#5. Heat Inside the Chimney

The gases in the chimney have to stay very hot or they will not rise through the chimney. You can have a small length of about 1 meter of uninsulated flue pipe directly from the stove, but the entire length of the chimney above must be insulated. This is usually by a thick masonry around an insulated ceramic liner or using prefabricated insulated components.

Draft & Flow

When a fireplace chimney is full of hot air, it pulls the air through the firebox and up through the chimney. This phenomenon is your chimney’s draft. Remember the stack effect, where I said that the chimney is like a hose? 

Well, using that same metaphor, increasing the draft in your chimney is a lot like turning the faucet more on the hose, allowing more water to flow out. Except, with your chimney, the faucet is just burning your fire hotter. This is because hotter air is lighter – that’s why it’s pushed out by the cooler air. Since it’s lighter, it has more pull

Another way to turn the faucet, so to speak, and increase your draft in your chimney is to increase the height of your chimney. A larger chimney can pull more air like a larger hose can handle more water.

Schedule Your Inspection

Is your chimney’s draft working well enough to keep the hot air flowing up? What about in a few months? If you’re not sure, then it might be time to get an inspection. 

At Patriot Chimney, we include a video inspection for all chimneys we sweep! Schedule an inspection and a sweep today to see why hundreds of customers in Southwestern Virginia have trusted us to take care of their chimney. 

We’re licensed, insured, certified, and guarantee you’ll be happy with your service. 


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