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Friday, January 8, 2021

What is a Flue Liner and Why do You Need One?

Flue liners are an important part of any furnace, water heater, boiler, or fireplace system. Liners have been required by code since 1927, but have been in use since the 1870’s. In a nutshell, the purpose of a flue liner is to keep toxic flue gasses such as Carbon Monoxide, other aldehydes, and tar vapors inside the flue and allow them to exit the house. It was found early on that with masonry chimneys, the mortar joints tend to deteriorate over time and develop cracks and gaps that allow gasses to leak through them. So, liners were invented. Vitreous clay tile flue liners were created to line the interior chimney so that flue gasses could not escape the flue. This is extremely important for the health and safety of the occupants of a home.

However, over time, rainwater, moisture, and acidic flue gasses eat away at mortar joints between the liner sections. Clay tile flue liners are made in two-foot sections and installed ahead of the exterior masonry chase. They have a bed of mortar between each section. When these mortar joints erode, they create gaps between the liner sections – and we are back to square one with toxic gasses leaking into the house. 

A liner has been removed and one remains

When rehabbing a house, a contractor may find that there is no liner installed in a masonry chimney. This is more common in older homes with utility flues serving heating appliances and water heaters, but occasionally is found with fireplaces as well. It is extremely common, however, to find a flue liner with missing mortar joints. This can be remedied in most cases with a method using a ceramic joint filler, which is applied using a heavy-duty vibrating bell sponge while pouring the mix from above the flue liner. It involves special equipment and trained professionals to do the job. In cases with large gaps between liners this method cannot be used, and relining is necessary.

A second common issue with flue liners is incorrect sizing. If a liner is not correctly sized to the appliance, smoking or backup of toxic gasses can result. This often occurs when a high efficiency furnace is installed, leaving a water heater to vent on its own, which it cannot do in an oversized flue. A liner for the water heater will be necessary unless a replacement water heater that vents out the side is used instead. An incorrectly sized flue liner serving a fireplace will cause backup of smoke and toxic gasses into the home.

Stainless steel liner with a single flue tile on top
The third common issue with clay tile flue liners is damage from a chimney fire. As fireplace inspectors we see this on a daily basis, and in most instances, the homeowner had no idea that they had a chimney fire unless they caught it at the time, or a neighbor knocked on their door after they saw flames shooting out of the chimney. Most chimney fires are of short duration because as flammable creosote burns it expands to ten times its size and snuffs out the fire. But even a short duration chimney fire can cause a lot of damage to flue liners, usually leaving vertical and sometimes horizontal breaks behind. These breaks open to as much as ¾” when the appliance is used, once again allowing gasses to escape. The only solution is to reline the flue. Fortunately, homeowner’s insurance covers chimney fire damage since it is considered a “hostile” fire. If a home was purchased within 12 months prior to finding the damage the new owner may have recourse with the seller, who can file a claim on their insurance. Chimney repair can be expensive, and the last thing a new homeowner wants is a huge un-budgeted-for expense after a home is purchased. This is why an inspection of all flues in a home is so important prior to purchase. 99% of Home inspectors do not inspect chimneys, only a chimney sweep with the proper equipment can, and should, do a chimney inspection using a chimney camera system. If timing does not allow for an inspection, figure in extra cost in your rehab estimate based on the size of the chimney.

Clay tile flue liners being installed as masonry is built up
Relining involves removal of the cement cap and extraction of the clay tile flue liner, installation of a replacement stainless steel liner with insulation, and rebuilding the cement cap. Other demolition such as removal of a damper casing, firebrick, or a wall may be necessary. Smoke chambers are often in poor condition and need to be parge coated with insulating mortar.  It is important to use a liner with the same I.D. dimensions as the previous liner, or in some cases, larger, and the only way to do that is to take the old liner out first. In rare cases, the flue is oversized so installation of a new liner may be possible without extracting the old liner.

Codes must be followed with flue liners – Chapters 10 and 18 in the International Residential Code cover most of this information, and the National Fire Protection 211 Standard for Chimneys, Fireplaces, Vents, and Solid Fuel Appliances also applies since it is the industry standard. We also follow the NFPA 54 National Fuel Gas Code.

A stainless steel flue liner being prepped   

A professional chimney contractor should do this type of work since they are familiar with the codes and standards and have the equipment and expertise to do the job. In the greater Kansas City area and other jurisdictions across the U.S., a Class DM Master Mechanical (HVAC) Contractor License is needed in order to reline flues, and a permit is required for the work, with an inspection by the building codes inspector.


The cost to reline a flue can very greatly and is determined by the type, size, and length of liner needed and difficulty level of the job.


Marge Padgitt is a 35-year chimney industry veteran. She trains professional chimney technicians across the U.S. and is the author of The Chimney and Hearth Pro’s Resource Book, and Wood-Fired Heating and Cooking, along with many training DVDS and presentations. Contact her at, 816-461-3665, or

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