A chimney is one of those parts of a house that quietly does its job until it doesn't. Most serious flue problems — a cracked liner, a blocked terminal, a failed flashing — announce themselves early with small, easy-to-spot clues. An hour spent checking yours each autumn, before the heating season begins in earnest, is time well spent. You do not need to climb anything or take anything apart. You need a torch, a pair of binoculars, a notepad, and a healthy suspicion of anything that looks or smells wrong.

Start With a Walk Around the Outside

Stand well back from the house, ideally at the end of the garden, and study the stack with binoculars. Daylight and distance show up problems that are invisible from the hearth.

• Leaning or twisted stacks, which often mean movement in the brickwork or a failing mortar bed.
• Crumbling mortar joints, spalled brick faces, or bricks that have lost their corners after a hard frost.
• Flaunching — the sloping mortar shoulder around the base of the pots. If it is cracked, water is running straight down the flue.
• Cracked, chipped, leaning or missing chimney pots. A pot that moves in the wind needs attention before it comes down.
• Bird guards and cowls that have rusted, come loose or been pushed out of shape.

Never climb onto the roof to investigate. Note what you have seen, take a photograph if it helps, and pass it to a professional.

Flashings, Pots and the Loft Space

Where the chimney passes through the roof covering, you should see lead flashing stepped into the brick joints, an apron at the front and back, and a neat mortar or lead fillet. It should look dressed and tight, not smeared on and hopeful.

• Lifted edges, splits, or lead that has pulled away from the brickwork.
• Cracks or gaps where the flashing meets the stack, and mortar fillets that crumble when you press them lightly.
• Moss and debris collecting behind the flashing, which holds water against the lead.

Then go into the loft. With the lights off, look for pinpricks of daylight around the chimney. With the lights on, look for dark damp streaks on rafters, water marks on the chimney breast itself, and damp or discoloured insulation around the base. A flashing that has failed for months will usually have left a signature here.

Inside the Flue, Fireplace and Chimney Breast

Open the stove or fireplace and inspect it with a torch. Soot is expected; the amount and the condition of it tell you a lot.

• More than roughly 3mm of soot on the flue walls, or soft soot falling down in flakes.
• Shiny, tarry, black glaze — creosote — which is a fire risk and cannot be brushed away easily.
• Cracked or crumbling firebricks, missing mortar in the fireback, or visible gaps in the flue joints.
• Water stains, tide marks, or white powdery salts on the chimney breast, which point to water getting in from above.
• A sooty or musty smell in upstairs rooms, especially in damp weather.

If you use an open fire, check the hearth, the register plate and the surrounding plaster for heat damage.

Test the Draw and the Room Ventilation

A flue should pull steadily. Light a smoke pellet, or a tightly rolled tube of newspaper, and hold it near the opening of a cold appliance. Smoke should be drawn up cleanly and not spill into the room. A piece of tissue held at the fire opening should be pulled gently towards the flue.

Then repeat the test with a bathroom or kitchen extractor fan running and the room door closed. If smoke drifts into the room instead of up the chimney, the flue is struggling to compete with the fan for air. Check that any permanent ventilation — an air brick or vent — is clear and not painted over. Solid fuel appliances need a reliable supply of combustion air.

Alarms, Cowls and Stove Hardware

Fit a carbon monoxide alarm in the same room as any solid fuel, oil or gas appliance, and test it monthly. It should sit at roughly head height, not directly above the stove. Smoke alarms on every floor are not optional.

• Check the door rope seal on your stove by closing it on a strip of paper — it should grip all the way round.
• Look for cracks in the glass, crumbling fire cement, a warped baffle plate, and air controls that no longer move freely.
• Clear ash regularly, but leave a thin bed if the manufacturer recommends it.

When to Call a Professional

Any of the findings above is a good reason to book a registered chimney sweep. As a rule of thumb, gas and oil flues need sweeping once a year, wood-burning flues twice a year while in use, smokeless coal quarterly, and bituminous coal twice a year. Ask for a certificate of sweeping and keep it with your household papers — insurers often ask for it after a chimney fire.

If a sweep finds cracks, gaps or a flue that has been compromised, get advice from a registered solid fuel or heating engineer about relining with a suitable flexible liner. Gas appliance work must be carried out by a Gas Safe registered engineer. A home CCTV flue inspection is also worth the modest cost if you have moved into an older property with no sweeping history.