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All About Chimney Flues

To maintain the chimney flues that safely channel smoke and fumes from fuel-burning appliances to the outside, they should be inspected and cleaned on a regular basis.

A snow covered blue house with smoke coming from the chimney iStock

If you have a fireplace or wood stove in your home, then you also have some sort of chimney that funnels smoke outside. Chimneys are a relatively recent invention—if you had lived before the 12th century, the smoke from an inside fire would most likely have been vented through a hole in your roof.

What is a Chimney Flue?

Chimneys are built to promote the upward progress of smoke, and among their many parts is the flue—a smooth secondary layer on the chimney’s inner walls that eases the smoke’s passage, protects the masonry from deteriorating effects of acids in smoke and prevents any smoke or gases from entering the house through cracks in the masonry.

In the case of a chimney fire (more on that in a bit), flues also help stop flames from spreading to the rest of the house. A typical chimney has a dedicated flue for each fireplace, wood stove, or furnace, sometimes all contained in a single chimney stack.

What are Flues Made Of?

Flues can be made from terracotta clay, concrete, or stainless or galvanized steel. Clay and concrete flue sections (called tiles) are square or rectangular in cross-section; clay tiles are usually 24 inches long, while concrete tiles are shorter. As a new masonry chimney is built, flue tiles are grouted in a stack at the center of the chimney.

Stainless steel flues are made up of sections of rigid double-walled tubing that can range in diameter from four to twelve inches. Stainless flues, often associated with wood stoves, are also commonly used in new construction as the de facto chimney (often housed above the roof in a wood-framed chimney-like structure) venting a furnace.

Chimney Flue Safety

Code regulations concerning chimneys and flues are fairly stringent. The flue must be sized by the appliance (fireplace, furnace, boiler, etc.) that it’s venting; flue materials must be able to withstand a minimum temperature of 1800 degrees F; and the flue must be without cracks, gaps, or perforations along its entire length.

Specific clearances around the flue or chimney must be maintained, and a chimney or flue must rise a certain distance from the roof, usually ten feet high, measured horizontally from the chimney top to the nearest roof section.

Inspections for chimney flues

It’s also recommended that your flue be inspected once a year, and if necessary, cleaned by a professional chimney sweep. A well-maintained flue is efficient and most importantly, doesn’t allow the accumulation of creosote, a dark, oily byproduct of fossil fuels that builds up on the inner surfaces of flues.

Not only does creosote accumulate and interfere with the flue’s efficiency, but it is also combustible and causes chimney fires. Under the right conditions, a spark or burning ember launched from the fire can travel up the flue and ignite the creosote. The resulting fire can send flames or burning debris out from the chimney top, posing a potential threat to the roof or adjacent surfaces.

A chimney fire can also damage the grout or mortar joints in the flue, and in the worst-case scenario, will penetrate the flue and spread to the wood framing that surrounds the chimney. Your chimney fire has now turned into a house fire.

Ways to Clean Chimney Flues

There are things you can do to prevent the buildup of creosote. If you have a fireplace or woodstove, burn well-seasoned, dry wood—green wood with a higher moisture content produces smoke that is lower in temperature and contains more volatiles that condenses into creosote on the flue walls.

Make sure the fire has an adequate airflow—closing dampers or glass fire screen doors for a prolonged period can have an effect that’s similar to burning green wood. And of course, have the chimney cleaned on a regular basis.

Cleaning with a chimney brush

You can also buy a chimney brush and clean the chimney yourself. Chimney brushes can be found at most home centers, and they’re meant to be paired with a kit of fiberglass rod extensions used to push the brush into the flue. Wire brushes are for masonry chimneys, while polypropylene brushes are used for metal flues; brushes are also available in various shapes (round, square, diagonal) and sizes.

  • You’ll need to measure your flue and choose the brush that’s about ¼ inch larger than the flue. An estimate of the length of the flue will tell you how many rod sections you’ll need.
  • Cleaning from the top down is an accepted method, but it only works if you don’t mind standing on your roof.
  • Experts recommend closing off the fireplace(s) with plastic sheeting and duct tape before the event and laying drop cloths over adjacent furnishings.

Cleaning from the inside

Alternatively, you can push the brush up the flue from the inside. First, shovel any ashes left in the hearth into a bucket. Spread out a drop cloth around the hearth to protect rugs and furniture. It’s a good idea to tape a drop cloth or plastic sheet around the fireplace opening, leaving a gap for the brush but minimizing the amount of soot that can fall into the room.

  • After opening the damper, connect the brush to the first section of the rod and push it up into the flue.
  • Connect the next sections of the rod until the brush is at the top of the flue, push and pull the brush a few times; pull it lower, disconnect one section of the rod, brush again, and repeat until the brush has reached the firebox.
  • After removing the brush, use a vacuum or brush to clean off the smoke shelf that’s just above the damper, and then clean up any soot that’s fallen into the fireplace itself.
  • Don’t forget to add a dust mask or respirator to your chimney sweep outfit when sweeping the flue. Creosote is considered a carcinogen.

How to Repair Chimney Flue Tiles

You may also find, after your chimney inspection, that some flue tiles are cracked or damaged. To restore the flue, one option is to have a new flue liner installed. Made of rigid or flexible stainless-steel tubing, these replacements are installed from the top of the chimney and can be used to vent conventional fireplaces, woodstoves, and any other appliance that burns fuel. The advantage to these flues is that they can be easily replaced.

A more permanent option is a poured concrete replacement flue. Ideal for reinforcing the structure of older chimneys, a concrete liner is installed by first inserting a long, heavy-duty balloon called a former down into the chimney, applying a temporary seal to the bottom of the old flue, and pouring concrete down around the former.

While you can buy a stainless DYI kit for $486 (about $20/ft.), your purchase assumes a certain level of expertise. If you want someone else to replace your chimney liner, it’ll cost about $100/ft. for a stainless-steel liner, or about $250/ft. for a poured concrete liner.