Short answer: annually.
It’s common for people to only book a chimney sweep when they have a new appliance fitted or when a problem develops. Chimneys can go un-swept for years. Unsurprisingly, this means that it’s also common for chimney sweeps to find problems in the chimneys they are called to!
It’s not just about the about the sweeping – when a chimney sweep inspects your chimney they will also perform a smoke test to check its integrity. They will look for damage and wear and tear issues that can affect the chimney’s performance and even make them dangerous to use.
If chimneys are completely blocked with birds’ nests or broken material from the roof this will prevent flue gasses from escaping. A lack of updraught resulting from a partial blockage will reduce the chimney’s ability to draw smoke up from the fire. Dislodged material can also damage the flue lining, so a chimney sweep will also check to see if you will need chimney re-lining.
A build-up of tar and soot can reduce the efficiency of the chimney, and over time these deposits will break down the chimney lining and seep into internal or external walls, causing staining. These build-ups are flammable – they can ignite and cause chimney fires which will undermine the integrity of the chimney and can necessitate costly building work.
Other serious problems can be identified by regular chimney sweeping. For instance if there is smoke leaking into the home with a risk of carbon monoxide poisoning (any combustion produces this, not just gas).
In short, booking a chimney sweep annually will reduce wear and tear on your chimney and flue, sort out any blockages or efficiency issues, and identify any problems with the chimney or chimney lining early on, before they turn into costly repairs.