The opening is frequently too large for the flue when a fireplace smokes constantly but other reasons might be an incorrect terminal, oversized flue or badly formed throat. Constant or worsening smoking can signal an obstructed flue. If smokiness is only a problem when windows or doors are closed the cause is likely to be inadequate room ventilation, and where smoking occurs solely in certain wind directions, this indicates a poorly sited chimney terminal. Smoke issuing through a stack suggests cracking or defective joints.

Insufficient air for combustion can result not in only smokiness but the escape of dangerous carbon monoxide fumes into accommodation. If the ‘draw’ of the flue improves when a window is opened, provide a permanent method of increasing ventilation by fitting wall or floor grilles. Venting a wall grille through another space, such as a hallway, rather than directly to the outside, reduces draughts.

Some fireplaces, particularly inglenooks, were built with excessively large flues, creating poor updraught. A remedy is to extend the neck of a canopy or flue pipe well above the register plate, or provide a liner correctly sized for the fire type.

With a very tall chimney, flue gases may cool too quickly when a fire is initially lit or later dies down. The solution is to maintain a hotter fire, use an appliance with doors or fit an insulated flue lining.

Obstructions can arise due to birds’ nests, a dead bird, the deterioration of mortar linings (parging), collapse of mid-feathers or build up of soot and tar at awkward bends, long offsets running near to the horizontal or the bases of carelessly installed chimney pots or cowls. Sweeping or scraping may clear constrictions, although in extreme cases opening up of the chimney may be required. Tight-fitting soot doors can provide useful access points at vulnerable locations. Lining may be necessary.