Do I Need a Chimney Liner for a Gas Furnace?

Unfortunately, many people still don’t fully understand gas ventilation requirements, with the majority contented with annual inspections and regular repairs. This guide focuses on one particularly overlooked area – chimney liners. Does the gas furnace need a chimney liner? And, if so, what else do you need to know about gas furnace chimney liners? Let’s begin with the big question. 

Do I Need a Chimney Liner for a Gas Furnace?

Yes, a gas furnace needs a chimney liner for optimal performance and safety. In fact, most gas furnace manufacturers state expressly in owner manuals that you must install a stainless-steel chimney liner or other appropriate liners. 

What is a Chimney Liner for Gas Furnace?

A chimney liner is a special conduit installed inside the chimney to contain the combustion byproducts (exhaust gases, including moisture). The liner has two primary functions.

The first one is to safely and effectively direct the exhaust byproducts outside the house and into the atmosphere. Secondly, it also protects the chimney walls from heat and corrosion. 

Do I Need a Chimney Liner for a Gas Furnace? 

Yes, you need a liner for your gas furnace. Many people assume that you don’t necessarily need a liner for gas furnaces because of the inherent efficiency of gas furnaces. However, that’s not correct. Although gas furnaces are way more efficient than their wood-burning and coal-powered counterparts, they still produce significant amounts of exhaust fumes. 

Benefits of a Chimney Liner

The following are three main reasons you need a chimney liner for a gas furnace (even if your furnace is fairly efficient).

  1. Protect you from accidental fires 

While chimneys are designed with material that doesn’t catch fire, the standard chimney sits right against the rest of your home. For instance, the chimney sits between wooden structures in the attic and combustible roofing materials. Now, remember that the chimney can become very hot because flue gases are typically very hot, typically up to 1,400°F.

Support structures or combustible framing adjacent to the chimney can easily ignite when exposed to such high temperatures for an extended period. This is especially true in older chimneys with gaps between the building blocks. Chimney liners limit the heat transfer, thus going a long way to preventing such fires. 

2. Prevent damage to the furnace masonry 

Most chimney furnaces are constructed with brick or clay, while the rest are made from metal. Unfortunately, all three materials are easily damaged by combustion byproducts.

Take an example of brick chimneys, the most common type of chimney. Although they look solid, brick chimneys are highly vulnerable to extended moisture exposure and corrosive elements.

 Maybe you’ve even heard that bricklayers use certain acids to dissolve mortar from brick. The acids dissolve the alkaline chemicals in mortar, rendering masonry joints weak and vulnerable. A liner protects the chimney from direct exposure to flue gases, thus extending the life of the chimney. 

3. Give modern chimneys a correctly-sized flue

Finally, many modern furnaces are designed with a specific flue size in mind. As a general rule, the chimney diameter should match the flue collar on your furnace. So, a 6-inch chimney can only fit a 6-inch furnace. These requirements seek to maintain a moderate velocity of flue gases.

Too fast or slow flue gas velocity within the furnace poses significant risks, such as potential backdraft. As such, it’s implausible that your new furnace will seamlessly fit into an existing chimney. 

Installing a chimney liner gives you the chance to adjust the chimney flue to fit the new furnace, so you no longer have to worry about the consequences of an incorrectly sized chimney flue. 

Are Chimney Liners Required by Code?

Yes. The National Fire Protection (NFPA) sets out chimney lining requirements under Section 211, part 7.2. Section says that “masonry chimneys shall be lined.” Then section onwards gives guidelines on choosing a choosing lining. For instance, states that the selection of chimney lining shall be appropriate for the class of chimney services.

It also states that the liner choice should be guided by the type of appliance connected and “in accordance with the terms of the appliance listing and the manufacturer’s instructions.” Further down, the NFPA 211 7.2.2 guidelines are chimney liner construction requirements.

For instance, section states that “the lining shall extend for the entire height of the chimney to a level not less than two inches above the crown or wash.”

Meanwhile, 7.2.14 states that the “liner must be as near vertical as possible with a maximum of slope not greater than 30° from the vertical” position.

Types of Chimney Liners

You can choose from three main types of chimney liners, i.e., clay liners, cast-in-place liners, and metal liners. 

  • Clay liners

Clay liners are the most common type of furnace liners. They are inexpensive and easy to build. Moreover, clay liners perform excellently with well-maintained fireplaces and stoves. The only downside is that clay liners are susceptible to cracking.

Once the tile cracks, it creates a massive fire risk. This is a major concern. Worse still, repairing clay liners is very challenging. 

  • Cast-in-place liners

Cast-in-place liners are a slight variation from their clay-made counterparts. They are made from concrete-like material, poured in, and left to harden. This usually results in a highly effective liner with no cracks or leaks.

In addition, they can resist temperatures up to 2100°F, better than any other type of liner, and even improve the chimney’s structural integrity. The only major downside is that cast-in-place liners are permanent structures. So, you need to replace the entire liner if it breaks. 

  • Metal liners 

Finally, metal liners are the most popular solution when adding or replacing a chimney liner in your home. They are typically made from stainless steel though a few are made from aluminum, both known for excellent corrosion resistance.

Additionally, metal chimneys can replace or complement clay or cast-in-place liners with ease and effortlessly fit any modern home.

However, beware that the best metal chimneys are substantially expensive. 

So, which is the best liner used for furnaces? 

The best liners today are metal liners. Stainless steel liners are the gold standard currently. Cast-in-place liners come second and clay liners last. 

How to Install a Chimney Liner for a Gas Furnace?

Now, let’s discuss how to install a chimney liner for your gas furnace. We’ll only discuss metal liners as cast-in-place, and clay liner installations are best left to the experts. 


  • Razor knife
  • Flathead screwdriver
  • Caulk gun
  • Pair of gloves
  • Safety glasses
  • Sawzall or hacksaw
  • Power drill 

Step-by-Step Installation Process 

  • Get the appropriate liner kit

The standard chimney liner kit comes with four essential components, i.e., the stainless steel or aluminum chimney liner, a connector, top plate, and rain cap. The most important thing here is to ensure you’re getting the right size liner. Both the diameter and the length of the liner must be correct. Otherwise, the installation may not work as intended. 

  • Flatten the liner 

Use the razor knife to remove the plastic wrap around the liner. Then straighten the liner on a flat surface. Do this on a soft surface or the grass so you don’t puncture the liner. 

  • Apply insulation 

Next, get the insulation material. You can use pour-in insulation or an insulation blanket. Most people use a blanket. First, measure the circumference of the bottom termination (or measure the diameter and multiply by 3.14 to find the circumference).

Then add one to the figure and cut the blanket to match the figure. When done, lay the blanket flat on the ground with the foiling facing down, center the liner on it, and wrap it around the liner. The one-inch overlap lets you seal the blanket with tape without exposing any part of the liner. 

  • Affix the insulation to the liner

Working up in 12-inch segments, use spray adhesive to affix the insulation to the liner. Next, use bits of foil tape to seal the seams, then, when done, run the seal along the entire seam from top to bottom. Next, roll the wire mesh that came with the kit around the insulator, making sure it covers the entire liner, including the bottom connector.

Finally, use hose clamps to hold it in place. 

  • Insert the chimney down the flue

You must be extremely careful here. An easy way to send the liner down quickly is to tie a rope around it, attaching it to the bottom connector, so it doesn’t come off. Then send the rope down so that someone can help you pull the liner down as you push it down from the roof. 

  • Position the connector and cut the excess liner

The first part of the installation is complete when the bottom connector reaches the appropriate height of the chimney. Next, use the snips to cut off the excess liner, leaving at least 4.0 inches past the crown. 

  • Seal the crown

With the liner centered, seal the chimney with silicone caulking. Then position the top plate over the liner and press it into the caulking to create a closed seal. Next, use tap con screws to fix it permanently by drilling into the holes. 

  • Finish up

Find the tee snout on the bottom of the linear and use your snips to cut a hole in the mesh and insulation where the snout connects to the tee body. From there, fasten the tee snout to the tee body using the pre-attached metal. Then wrap it around the tee body, insulate the connection with leftover silicon, and finish by connecting the tee snout to the heating appliance. 

That’s it. You can now test the liner!

How Much Does It Cost to Install Chimney Liner?

The average cost to install a chimney liner is $2,500. However, actual costs vary widely, from $625 for DIY installations with cheap materials such as aluminum up to $5,000 for professional installations using more expensive materials. 

Thermocrete liners are the most expensive to install per foot, costing $200 per foot, while clay/terra-cotta liners are the most affordable to install, costing an average of $10 per foot.

However, cast-in-place liners are the most expensive to install once you account for materials and labor. Materials alone can cost $5,000 to $7,000. 

How Long Does a Furnace Chimney Liner Last?

The average lifespan of a chimney liner is 15 to 20 years. You’ll probably need to get the entire line replaced after about two decades to make sure you’re up to code and avert the risk of fire. 


Do you need a chimney for a gas furnace? No. You don’t necessarily need a chimney for a gas furnace as you can vent gas furnaces directly through the roof or the wall. In fact, 90%+ AFUE gas furnaces should not be vented through the chimney.

How do you vent a furnace through the chimney? You can easily run direct vent systems from the furnace to the outside of the house through the chimney. 

Do you need a chimney liner for an oil furnace? Yes, you need a chimney liner for an oil furnace as oil furnaces, like their gas counterparts, produce toxic byproducts that can damage the chimney. 

Can a high-efficiency furnace vent through a chimney? No, a high-efficiency cannot vent through the chimney because high-efficiency furnaces produce denser exhaust fumes that don’t readily rise out through the roof. Additionally, high-efficiency furnaces produce a lot of condensation. 

How do you seal the furnace exhaust to the chimney? You can easily seal the furnace exhaust using silicone caulk. Inject the caulk into the gap between the vent and side of the house and use a stiff-bristled brush to skim the area. 


That’s all you need to know about chimney liners. We strongly recommend getting an HVAC professional’s input even if you intend to DIY the entire installation.