Dangers of a Cracked Chimney Flue


Your chimney’s flue, also known as the liner, is one of the most important parts of your whole chimney system. A cracked chimney flue can cause an urgent safety hazard, and you’ll need to repair it before you use it again. 

Luckily, with annual chimney inspections and regular maintenance, it’s fairly simple to avoid chimney cracks. 

Today, I’m going to tell you how chimney flues crack and what happens if you have a cracked chimney flue.


What Do Chimney Flues Do?

To understand why cracked chimney flues are dangerous, it’s important to know what the flue does. 

Simply put, it helps move the gases up and out of your house.

Before the 1940s, chimney liners and flues weren’t very common. But since then, flues have been required in order to prevent house fires. 

The purpose of the fireplace is to remove smoke and byproduct gases, like carbon monoxide, from the air in your home. All the smoke and gases move up and out of your home through the chimney flue. 

The chimney flue serves four primary functions: heat protection, protection from corrosion, protection from toxic gases, and ensures optimal efficiency.


Heat Protection

Your chimney’s liner prevents combustibles in  your home from being exposed to the hot temperatures in your chimney. 

The woodwork in your house can catch fire in 3.5 hours when using your fireplace with an unlined chimney. When your chimney begins to deteriorate and has a crack, the same effect occurs. 

It’ll take longer than 3.5 hours, but if you use your fireplace when the wood is exposed, then you put you, your family, and your house at risk.

Protection from Corrosion

Smoke and creosote are naturally occurring byproducts of using your fireplace. Despite being natural, they are highly acidic. If you have a crack in your chimney liner, the flue gases can penetrate brick and mortar. 

Mortar joints can deteriorate easily, which can then expose your fireplace and chimney to moisture and cause a whole new list of problems.

Protection from Toxic Gases

As I mentioned before, your chimney’s job is to move the smoke and gases up and out of the home. If you don’t have a sound chimney liner, the gases can leak back into your home through the cracks. 

If the gases leak back into your home, you can then risk carbon monoxide poisoning, which is colorless, odorless, non-irritating, and tasteless. So you could pass out from carbon monoxide poisoning before you even know what hit you. 

Or the poisoning could help you see ghosts!

Ensures Optimal Efficiency

Modern flues are custom fit to the appliance or fireplace that you have. They need to be custom so that they can move the right amount of gases to keep your fireplace burning to help you stay warm. 

An appropriately sized flue should be one inch wide for every 10 square inches of the fireplace opening. This 10:1 ratio allows air to flow at the perfect velocity through your flue. 

If your flue is too big, too much air is sucked up through your chimney and you will lose heat. But if your flue is too small, then air cannot exit the chimney fast enough. That means you could have a smoke-filled room.

How does My Flue Get Cracked

After years of use, your flue can simply begin to deteriorate. The problem usually starts with excessive creosote build-up. 

The acid from the creosote encourages deterioration on each of the different flue liners.



Clay tile liners are the most common type of flue liner in homes across the United States. Most brand new homes actually have a clay tile liner. 

This is because clay liners do a great job of protecting the home for about 25 years and are relatively inexpensive. 

But, over time clay tiles deteriorate and repairs aren’t a cost-effective option since a partial tear-down of the chimney is almost always needed. 

Clay tiles cannot distribute heat evenly in the event of a chimney fire. So, as a result, the clay might struggle to contain all the liquid byproducts from combustion and will begin to crack.

Cast in Place

Cast in place liners are built from cement that allows gases and toxins to escape the chimney freely. Cast in place flues can accommodate all types of fuels (wood and gas) and they provide structural reinforcement to older chimneys. 

Metal Liners

We like to recommend metal liners, much like many of the other chimney companies we know. Metal liners typically come in either aluminum or stainless steel. 

Aluminum is a much cheaper option for relining, but it’s only feasible as a flue material when it’s used with medium-efficiency gas heating appliances. 

Stainless steel is extremely durable and comes with a lifetime warranty.

What Happens if My Flue is Cracked?

You might not even know that your chimney’s flue is cracked. In fact, a lot of times it’s hard to see up the chimney even with a flashlight. 

That’s why at Patriot Chimney, we always use a camera to inspect the inside of your chimney. We’re not in the business of “eyeballing” important safety measures.

What Happens if My Flue is Cracked?

The easiest way to make sure you are safe from chimney cracks is to have your chimney inspected and swept at least once a year. 

Of course, there are a few reasons why you’d want an inspection and a sweep more than that, like if you burn more than the normal amount. Otherwise, once a year it should be fine. 

Eventually, you may need to have your liner replaced, just by the laws of nature. In that case, you should make sure that you hire a CSIA professional (ideally the same CSIA professional that sweeps and inspects annually) to install your liner.

Schedule an Appointment

If you haven’t gotten an inspection, this year, now might be the time! 

Patriot Chimney has a few CSIA Certified Chimney Sweeps that you can trust with your home and family. During your service, you can expect your technician to inspect your chimney with a video camera so we can give a full top-to-bottom, inside out inspection. 

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


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You’ll be so thrilled that your family is safe that you tell all your friends! You’ll be glad you did. 

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