How to clean a chimney

Find out how to clean a chimney to keep them working safely and efficiently

How to clean a chimney
(Image credit: Amanda Turner)

If you light a fire every winter, you need to know how to clean a chimney. Cosy and comforting, an original fireplace alight with burning coals or logs is the ultimate period feature. But keeping the fireplace burning involves more than just filling the grate with coal or logs. Chimneys require regular attention to work safely and efficiently and, even if they are never used, some routine maintenance will be inevitable.

The simplest and safest way to tackle chimney cleaning is to have the chimney swept by an experienced chimney sweep at least once a year and more often if fires are lit very regularly. Address problems with a smoking chimney promptly, along with any doubts about its structural stability — it will work out cheaper in the long run.

Read on to find out more. If you are after any more cleaning tips and hacks, go and check out our dedicated hub page too. 

Why have your chimney swept?

The fumes from a fire pass up the flue, which is contained within the chimney. These flue gases are potentially deadly and it is important to remember that any form of combustion may result in carbon monoxide, an insidious gas that has no smell. In addition, poorly maintained chimneys can lead to house fires, especially in thatched properties and where roof joists or other timbers have been built into the structure of the chimney — something that is not unusual in old buildings.

Find out more about repairing chimneys in our guide.

Common chimney defects

spurling fireplace sitting room

If smoke rushes into a room from an open fireplace, there may be a number of causes

(Image credit: Jody Stewart)

A fire should burn well with smoke and fumes carried away via the flue. Chimneys must be swept at least once a year by a professional chimney sweep and more often if fires are lit regularly. Good chimney sweeps will advise on potential problems.

What to look for:

  • Smoky fires
    There are several reasons why smoke may gush back into the room instead of rising up the flue. A lack of air is the most common cause. Open fires need at least six changes of air in the room per hour to burn well. To check if poor air starvation is the problem, try opening a door or window – if the smoke clears then the solution is to boost air supply, for example by installing vents or underfloor ducts. Smoky fires can also be caused by chimneys getting blocked by debris or nests, hence the importance of regular sweeping. Smoke can sometimes be blown back down by downdraughts where a chimney is too short or overshadowed by surrounding high buildings, trees or hills. Short stacks can be built up or extended by fitting special cowls or pot extenders.
  • Fire burns poorly
    Where a fire struggles to burn, it may be choking on exhaust gases that are not fully dispersed. This may be due to a poor airflow or because of a downdraught. Problems with indoor air supply are common in houses where draughts have been totally sealed up so there’s just not enough air being sucked into the fireplace. The solutions are as described above for smoky fires.
  • Smoke and fumes leaking into other rooms
    Air leaks through defective brick joints can allow toxic smoke and fumes to seep into rooms. Leaks can be traced using smoke pellets. Test for leaks using a smoke pellet bought from a fireplace shop or plumbers’ merchant. If possible, temporarily seal the top of the flue, although scaffolding will generally be required for access. Light the pellet in the hearth and then examine the entire length of the chimney, including the loft space, for smoke leaking out through the masonry.The solution is to have the flue professionally lined.
  • Chimney fires
    Over time, excessive soot and tar can build up on ledges inside a flue, eventually igniting. Flues need to be swept at least once a year to remove combustible soot deposits and blockages, especially if burning green unseasoned wood or peat, which are particularly aggressive fuels. Sweeps should be NACS or HETAS approved.
  • Recurring sooty smells.
  • Birds entering the house
    Unprotected chimney pots can allow bird access and nesting. Twigs in the hearth from a bird’s nest can cause problems. The solution is to fit a special protective bird guard.
  • Plaster, brick, stone or soot debris coming down the chimney.
  • Damp patches or staining appearing on chimney breasts.

Chimney maintenance checklist

stone fireplace with lit fire and tea set

Although strong, cast iron can crack when stressed, especially when subjected to intense heat. Blacksmiths are sometimes able to help with repairs, as are fireplace shops and salvage yards

(Image credit: Jeremy Phillips)
  • Have chimneys swept regularly by an experienced sweep.
  • Ensure the chimney is safe and in good working order when first used.
  • Check carbon monoxide and smoke alarms frequently. Fit both if they are not already installed.
  • Take corrosion into consideration: chimneys are affected by erosion from the inside due to the corrosive effect of flue gases.
  • Always burn dry, well-seasoned wood, otherwise tar deposits might collect within the flue.
  • Check for debris falling down a chimney, including mortar and bricks, may be a sign of problems so always ask a sweep whether there is any evidence of this when they are working. 
  • Test for leaks: with any chimney, but especially one that has not been used for some time, it is worth testing it for leaks using a smoke pellet. This is lit in the hearth and is always more effective where it is possible to block off the top of the chimney to contain the smoke within the flue. Check for smoke leaking through the walls of the chimney in the rooms above and in the roof space. Where problems are suspected specialists can survey chimneys internally with a camera and it may be necessary to have the chimney lined.
  • In the roof space check for dampness around the chimney stack.
  • Regularly clean the fireplace, clearing ash and debris from the grates.
  • Fill any cracks and gaps that appear in the fireback and surround.

Neglect often takes its toll on many fireplaces and grates, but with a little care and effort they can be brought back to life

(Image credit: Brent Darby)

You can find out more about how to repair a chimney in our guide but here are the basics:

  • Inspect chimneys externally as they take the full force of the weather. Look for loose chimney pots, eroded mortar on the stack and defective flashing where chimney and roof meet. To make repairs, employ experienced bricklayers or roofers who understand old buildings and use appropriate lime mortars and techniques; scaffolding will probably be needed.
  • Contact a surveyor: even if a chimney leans it may still be perfectly stable but if you are in any doubt, it is advisable to consult a structural engineer or surveyor. In some cases it is possible to add a tie rod and strap connected to the roof to enhance stability.
  • Know your chimney: chimneys from inglenook fireplaces are often topped by flagstones raised on bricks to provide a rain cap. Various terminals or inserts are available for chimney pots to improve performance or stop rain entering the flue. Bird guards can also be fitted to prevent them from nesting inside.
  • If the roof is thatched, spark arrestors are generally installed to stop cinders escaping. It is essential to ensure that these are properly maintained and cleaned otherwise they may increase the risk of fire.

Installing flue liners

A look at repairing and maintaining different roofs

Carry out regular external checks of chimneys using binoculars

(Image credit: Roger Hunt)

It’s essential that old flues are correctly lined, to prevent dangerous leaks developing. Flexible single-skinned stainless-steel liners are widely used for gas fires, and for oil and gas-fired boilers. Hardier double-skinned versions are required for solid fuels and for stoves.

Stove flues need to be properly insulated as they generate extreme heat, a common cause of thatch fires. Liners are usually installed by being pulled down the old flue by an attached rope, with the space around the liner then back-filled with loose insulation to prevent condensation.

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