Recognising one
A fast-burning chimney fire is loud: a roaring or rumbling like a low-flying aircraft, dense smoke, and sometimes flames or sparks visible at the termination.
Slow-burning chimney fires are quieter and more common. They may not be noticed at all, but they still reach temperatures that crack tile liners and damage adjacent framing.
What to do immediately
Get everyone out and call emergency services. If you can do so safely without delay, close the appliance air controls to starve the fire, but evacuating comes first.
Do not throw water into the firebox. The steam and thermal shock can crack masonry and worsen the structural damage considerably.
Afterwards
Never use the chimney again before a full inspection. Chimney fire temperatures crack clay tiles, distort metal liners and damage the mortar between firebricks, and those defects give the next fire a route into the structure.
A camera scan documents the damage in a form your insurer can work with, and the report determines whether the system needs relining or rebuilding.
Preventing the next one
Chimney fires burn creosote. Reducing creosote means dry wood, hot clean burns rather than smouldering, and annual sweeping — plus insulated relining where a cold flue is the underlying cause.
If a previous sweep reported glazed creosote that could not be removed, treat that as an open item rather than something resolved.