How do you calculate what size of chimney liner you need?

  • A chimney liner prevents contact between combusting gases and your home’s structures, reducing the chance of a fie, or deterioration of bricks and mortar. It also offers a smooth unobstructed path for flue gases, raising the efficiency of your fireplace.
  • Chimney liners are now a standard in recent building codes
  • Different fuel fireplaces require different sized liners
  • Gas fireplaces require a single skin, 326-grade liner with a diameter matching that of the connector collar
  • Multi-fuel burners require twin skin, 904-grade liners with the diameter determined by the appliance’s heat output

Most homeowners don’t think about replacing their appliances until they are faulty, which applies to the chimney liner. And even then, they may not know how to pick the correct size.

It can be a daunting task to size up and install one, but you don’t have to worry! This article will discuss what a chimney liner is, why you need one, and how to pick the correct diameter and length for your appliance.

What is a chimney liner?

A chimney liner, also referred to as a flue liner, is a layer of protection that helps prevent combustion gases from coming into contact with your home’s structure. It also protects bricks and mortar of your vent from the corrosive effects of these gases. Over time, it can crack or damage, so it’s essential to inspect it regularly and replace it when necessary.

It also helps improve the efficiency of your fireplace or stove by providing a smooth, unobstructed path for hot gases to travel up and out of the vent. In fact, many building codes now require that all new fireplaces and wood stoves be equipped with one.

Why do you need one

A properly functioning flue liner is essential for your home’s safety and fireplace or stove efficiency. If it is damaged or missing, harmful gases can enter your home, putting you and your family at risk. In addition, if faulty, it can cause your stove to work less efficiently, as the hot gases will have a harder time making their way up.

How to pick the correct diameter for your appliance

The type of appliance in your space helps determine the diameter of the liner needed. For example, a gas fireplace will require a different size than a wood stove. You should check the manual instructions guide or seek professional help before making your purchase.

1. Gas appliances

Gas appliances require gas-specific single skin liners or 316 grade that help prevent the leaking of harmful carbon monoxide. They are single-ply, lightweight, and less expensive.

Determining the correct diameter

The first step in picking the right size chimney liner is to determine the diameter of your flue. You should simply measure the inside of your chimney. Many have diameters ranging from 5’’-6″.

First, determine your gas fireplace’s BTU (British Thermal Units) output. You can check the manufacturer’s manual guide to be sure. The size of your connector collar should match the flue liner diameter you need. For example, if your connector collar is 5″, you need a 5″ diameter liner.

Many appliances come with a sizing guide, and you should follow the manufacturer’s sizing recommendations by matching the internal diameter with the outlet of your gas burner.

2. Multi-fuel burners

Multi-fuel burners use solid fuels like coal and wood and require twin skin liners or 904 grade. Their twin skin design makes them handle higher temperatures from the solid fuel.

Determining the correct diameter

The diameter will be determined by the heat output of your appliance. For example, if you have a wood stove with 20Kw output, you will need at least 6″ inches in diameter. Higher heat outputs require larger ones as big as 7-9″. DEFRA-approved appliance needs 5″.

How to pick the length of your appliance

The next step is to determine the correct length, depending on the height of your vent and the type of appliance you are using. For example, if you have a wood-burning stove with a flue that extends 15 feet above the roofline, you will need at least 17 feet long liner.

To determine the length:

  1. Measure the distance from the top of your appliance to the top of your vent. This distance will give you the total height of your chimney.
  2. Next, subtract the height of any chimney components above the level of your appliance. This includes things like the chimney cap and rain pan.
  3. Now, add two feet to this measurement to account that it will need to extend past the top of the chimney. Now you have the minimum length you need.
  4. It is always better to err on the side of caution and choose one that is slightly longer than you need. You are now assured that your appliance is adequately vented.

Tips to help in calculating the correct measurements

Now that you know how to calculate the size you need, you should understand a few tips when determining the size of the ducts. Those tips include:

  • Make sure you know the BTU output of your appliance.
  • Get accurate measurements and find the appropriate flue size based on the BTU output of your appliance.
  • Once you have determined the required size, you can select a liner with the same diameter or slightly larger.
  • Never use 316 grade for multi-fuel burners or 904 grade for gas burners.
  • Make sure that the one you choose is not too small, as this could create a safety hazard.
  • Seek help from a professional chimney sweep to help you determine the height of your vent pipe.
  • Your appliance might have a small collar than the required flue liner. In such a case, purchase a connector collar that you can connect easily to it.
  • Do not connect a flue that is too short. If it is too short, do not attempt to connect two separate ones as it could bring about dangerous problems in the future.


We all want to keep our loved ones safe from harmful carbon monoxide emitted by solid fumes, so choosing the correct size chimney liner for your appliance is essential. This will ensure that your appliance is adequately vented and prevent safety hazards.

Remember to follow the tips in this article to help you determine the right size for your needs. If you are still unsure, seek help from a professional chimney sweep.

I hope that the article has helped guide you to choose the right size chimney liner for your home.



Eugene Duke Pic

Hi, my name’s Eugene Duke and I love sitting by my fireplace reading a book and sipping on an adult beverage. Do you have a fireplace in your house? I’ll help you figure out the best type and style of fireplace for your home.


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