What Is A Chimney Flue – Everything You Need To Know

The fireplace chimney has several parts. So many, it can be confusing. For it to work properly, all these parts need to be in excellent shape and in good working order.

They must also be serviced regularly. Above all, it helps if you understand the basic working mechanisms of these parts.

One such part is the chimney flue. A critical component of the chimney structure and the heating process, the chimney flue impacts both the chimney and fireplace heat output design. It also influences the damper type and the safety of the people in the home.

Read on to learn everything you need to know about the chimney flue, including why it works, the different flue styles, chimney flue liners, etc.

What is a Chimney Flue?

chimney flue diagramThe chimney flue is a duct, pipe, or opening in the chimney designed to remove exhaust gases from the fireplace, furnace, water heater, generator, or boiler to the outdoors.

Originally, the term flue meant the chimney itself. In the US, the flue is also sometimes referred to as a vent when used in boilers and breaching when used in water heaters and modern furnaces.

The flue is made from different materials, though the majority are constructed from ceramic, clay, or metal conduit.

It is installed at the center of the chimney and contains the combustion products, ultimately directed them outside the house, thus protecting the chimney walls from heat and corrosion.

How Does a Chimney Flue Work?

The fire down below in the fireplace needs three things to burn continuously – oxygen, fuel, and ignition. Fuel may come from gas, wood, pellets, etc.

Ignition is often achieved through a pilot mechanism or manual match lighting, depending on the fireplace type. The oxygen is where the chimney flue comes in handy.

The fire requires reliable airflow to burn and vent. In flue-less fire systems, the required airflow is derived from the home.

However, taking up oxygen from the home to burn the fire and thereafter venting combustion gases back into the home creates serious health and safety issues, including carbon monoxide poisoning.

The flue system removes the risk by allowing the fireplace to continually draw fresh air from outside to facilitate the burning process and removing any exhaust gases out of the home.

Flues operate by buoyancy, also known as the stack effect. Some chimneys also use “induced” flues that use a fan to initiate air and gas flow into and out of the flue, respectively. Building codes and other standards typically regulate the materials, design, and installation of the flue system.

Why Do You need a Chimney Flue?

As we’ve seen, you need the chimney flue for two reasons – to draw in fresh air for fuel combustion and remove exhaust gases.

  1. Draw fresh air: All heating systems require a certain amount of fresh air for combustion. The required air is often referred to as combustion air. Inadequate combustion air can lead to serious problems. Limited air supply may also affect the quality of the fire and the health of the flame.
  2. Remove exhaust gases: Any process that burns fossil fuels produces several by-products, including water and carbon dioxide. Some of these by-products can be unsafe. Where there isn’t a sufficient supply of fresh air, for instance, the burning process may produce carbon monoxide and sulfur dioxide, both of which are extremely dangerous. The flue system helps remove these gases out of the house.

Types of Chimney Flues

Flues are available in several shapes, styles, and materials, so you can pick one that best matches your needs. There are at least six basic types as follows;

  • The brick chimney: The classic brick chimney is suitable for all gas fires and stoves. It’s also ideal for most solid fuel appliances. It’s characterized by a chimney stack, with either a pot or gas terminal on the roof. These structures also feature a breather found inside the house or the outside wall for the fire.
  • Pre-fabricated flues: Pre-fab flues are best suited to gas fireplaces and stoves. They also offer good depth allowing you to choose from full-depth fires to the slimmest fires. The pre-fab flue is common in older properties where the existing chimney is lined by s steel flue or newer properties with steel flues built-in. It is recognized by a metal flue and terminal on the roof.
  • Pre-cast flues: These are found in most modern homes and are suited for a wide range of slimline gas fires. They are characterized by a ridge vent or metal flue terminal on the roof. Most pre-case flue systems are made from concrete or clay blocks and are rectangular. They have a very shallow depth.
  • Balanced flues: The balanced flue is a glass-fronted gas appliance that vents directly through an outside wall. It’s used where there’s no chimney or flue system available and doesn’t require electricity.
  • Flue-less systems: It’s also important to mention that some fireplace systems don’t have a flue. Instead, they use a catalytic converter that converts all harmful gases to carbon dioxide and water vapor. We’re, however, not interested in flue-less systems.

What’s the Difference Between a Balanced Flue and an Open Flue?

Balanced flues rely on the principle of the natural convection of heated air to expel combustion by-products and can work with open flames or glass-fronted fires. In contrast, unbalanced flues work in conjunction with glass-fronted fires only.

In traditional, open-flue systems, air for combustion is supplied from the room where the gas or stove is installed. However, balanced flue systems require that the fireplace is sealed from the room in which it is installed and twin-wall pipe vents directly indoors.

Thus, air for combustion in the balanced flue system is drawn from outside, traditionally via the outer pipe. The inner pipe removes combustion gases.

The twin-wall piping may exit the house horizontally through the wall or vertically through the roof, depending on the fire or stove selected.

What’s a Flue Liner?

Also popular as the chimney liner, the flue liner is a flexible tube connected to your stove pipe to line the inside of the chimney. It runs up the entire length of the chimney.

It’s important to understand the difference between the flue liner, chimney flue, and chimney vent. The chimney flue serves as a passageway for smoke, gas, and other products associated with combustion. It plays an essential role in protecting other parts of the chimney.

The flue lining is mostly found in modern flue systems. As the name suggests, it’s a lining material that protects the flue and, by extension, the chimney. The lining needs replacement periodically as it’s bound to wear off. Remember, the flue itself is just an opening; it doesn’t wear out.

Finally, vents are usually pipes. They also help evacuate exhaust gases and bring in combustion air. Indeed, the vent may also pass through the chimney.

However, it’s a pipe. Moreover, vents are 100% designed for gas fireplaces as they can’t handle the intense heat from traditional wood-burning stoves.

Chimney flue linings are beneficial in three key ways;

  • Protects the house from heat transfer to combustibles
  • Protects chimney masonry from corrosive combustion by-products
  • Ensures the flue is of the correct size for efficiency reasons

What’s the Difference Between 316 and 904 Flue Liner?

Many people prefer stainless steel flue liners because of their superior quality and durability. Of stainless steel flue liners, the two options most popular are the 316 Grade and 904 Grade. Which is best for your needs?

To answer this question, you need to understand how combustion fire and gases affect the flue lining. Open fires produce a very high flue gas temperature, compared, for example, to wood stoves, which are more efficient and have lower flue gas temperatures.

However, the low temperature has a serious downside – increased condensation. Wood stoves produce a lot more condensation of flue gases against the side of the liner.

Sulfuric acid, for instance, condenses at about 150°C. This condensation is a huge problem as the acid can attack the flue lining. The corrosion is further accelerated by water, which is one of the by-products of combustion.

Although the 316-grade liner is good enough for open fires, it’s barely capable of handling the aggressive corrosion resulting from wood fires. The 904-grade liner stands up better to such conditions.

However, remember that the 904-grade flue liner is also more expensive (often twice as expensive) than 316-grade liners.

How Long Will a Flexible Flue Liner Last?

The average lifespan of a flue liner is 15-20 years. This means that you’ll need to replace the liner after every two decades at most to make sure that it’s up to code and not a serious fire risk.

However, it’s important to understand that certain factors can shorten or improve the life of your flue liner. Stainless steel liners, for instance, tend to last up to 20 years with goof maintenance, with cast-in-place liners lasting even longer – up to 50 years – with good maintenance. Clay tile liners are the least durable, typically lasting 5-15 years.

The other factor is the frequency and quality of maintenance. The fireplace system, including the flue liner, should be cleaned at least twice a year. Otherwise, the liner deteriorates much faster. If you want to clean your chimney, you can choose one from this chimney cleaning kit review.

How Long Does a Chimney Liner Last?

The chimney liners and flue liners are essentially the same things. It’s the lining found inside the opening in the middle of the chimney, which basically refers to the flue.

The only slight difference is that “chimney liner” is the traditional term and “flue liner” the more modern term. The chimney liner, therefore, mostly refers to the fireclay tile lining the flue opening.

This special tile is carefully laid inside the chimney to eliminate any ridges and spots that may cause the accumulation of dirt and combustion gases.

A significant downside of the traditional chimney liner is the difficulty in repairs if tiles get damaged deep inside the chimney. As mentioned above, the liners typically last 5-15 years.

How Much Does a Flue Liner Cost?

The average cost of a flue liner is $2,500. On the higher end, you can expect to pay $5,000 while cheaper alternatives cost as low as $625.

However, the actual amount you’ll pay depends on the type of liner, liner dimensions (diameter and height), the chosen material, and whether you need insulation. Another critical factor is the number of units connected to the flue system.

Regarding material specifically, cast-in-place liners are the most expensive, costing, on average, $250/foot. In second place are thermocrete liners that cost $200/foot. Stainless steel liners are next, costing $65/foot on average, while the cheapest material, clay, costs about $10/foot.

How Much Does It Cost to Reline a Chimney?

how to reline a chimneyIt costs between $2,500 and $7,000 to realign a chimney. Ultimately, the cost is determined by the type of line you’d want to be installed and the chimney’s dimensions. It’s always best to let a professional replace your chimney liner.


Installing a Chimney Liner (Step by Step)

First off, if you’re going to attempt a DIY, you should only consider stainless steel chimney liners. The other materials are too complex for DIY installations.

Before you begin the installation, determine the appropriate size of the chimney liner. The chimney liner diameter you’ll need equals the size of the exhaust hole in your chimney. The length is essentially the height of the chimney. Always consider ordering a slightly longer liner to give room for error.

Next, you need to assemble the necessary tools. You need a razor knife, flat head screwdriver, caulk gun, safety glasses, and a pair of working gloves. You may also need a grinder, hacksaw, and power drill.

The chimney liner kit comes with the main components needed for the installation. These include the chimney liner itself, connector, top plate, and rain cap.

The Actual Installation

  1. Unpack the liner: Using the razor knife, remove the plastic wrapping in which the liner is coiled. Then, straighten out the liner.
  2. Insert the liner into the vertical tee: The vertical tee is the one with the cut on the side. Insert the liner into the top of the female end and tighten the clamp at the top of the vertical part around the liner using the flat-head screwdriver.
  3. Insert chimney liner down the flue: The part that goes down the chimney first is the vertical part of the tee connection attached to the liner. Lower the liner down into the chimney until you reach the thimble (hole on the side of the chimney). Rotate/twist the liner whenever you’re stuck.
  4. Finish tee connection installation: Locate the horizontal part of the tee connection and insert it into the thimble, hose clam end first then, snake the vertical part through the horizontal hose clamp until the lips of the tee are inside the horizontal part. Look inside the horizontal part of the flue to find a worm screw and gently tighten this screw.
  5. Connect the stove pipe: Most stove pipes come with three screws per connection point. Use your drill and drive the screws into the horizontal part of the tee end via the stove’s exhaust collar.
  6. Install the top plate: Use the caulk gun to run a bead of silicone along the top edge of the clay flue, then thread the chimney liner through the round collar of the top plate. Finally, center the top plate on the clay flue and use the flat head screwdriver to secure it to the clay flue side.
  7. Finish up: Cut off the excess liner using the grinder or hacksaw, then connect the installed liner to the top plate. Then, install the chimney rain cap.

Chimney Flues Frequently Asked Questions

What’s the Difference Between a Flue and a Chimney?

The chimney is the visible outside structure usually made of brick, slate, or rock. Meanwhile, the flue is the internal shaft that runs from the fireplace or stove below and vents combustion gases at the top of the chimney. The flue may or may not extrude from the top of the chimney and is often made of stainless steel.

What’s the Difference Between a Damper and a Flue?

The flue is the open middle of the chimney structure that directs combustion gases from the fireplace or stove out of the house.

A damper is a lever system found inside the flue that opens or closes to let out combustion gases or block downdraft as appropriate. Typically, the damper remains open during the heating season and closed during the summer season.

How Much Does it Cost to Fix a Fireplace Flue?

The cost of fixing a fireplace flue ranges from about $150 to $800. The majority of homeowners pay between $163 and $820 for general flue repairs, regardless of the type of flue. However, expect to pay anywhere between $90 and $2,000 for high-end chimneys. 

How to Open a Chimney Flue

You can open the chimney flue by pushing or lifting a rod all the way to open the damper. If the unit uses rotary control, turn it all the way clockwise. Finally, for some models, especially top-mount dampers, you need to pull a chain.

Is it Okay to Leave the Flue Open?

No. The flue should only be open during the heating season when you need to expel combustion gases. Otherwise, keep it closed to keep at bay twigs, branches, stray animals, and downdraft from outside the house.

When Should You Close the Flue?

During the summer, when you’re not using the fireplace. Leaving the flue open during this period leaves a passageway for downdraft, litter, and raccoons, among other unwanted elements.

How Do You Close the Chimney Flue?

You close the chimney flue by pulling a rod to open the damper, turning the control knob anti-clockwise, or, in the case of top-mount dampers, releasing the secured chain.

Is it Normal for Water to Drip in the Chimney?

No. If you notice water dripping down the chimney when, in your knowledge, everything inside the chimney is in top shape, then there’s a problem. You need to schedule an inspection or waterproof your chimney.


The chimney flue is an integral part of the chimney and fireplace system in general. It provides fresh air for efficient burning and helps remove combustion gases from home.

If you don’t have one, you should strongly consider getting it installed. Above all, always go with high-quality flue systems and ensure regular maintenance to extend the flue’s life.