A chimney is a structure for venting hot flue gases or smoke from a boiler, stove, furnace or fireplace to the outside atmosphere. This is important because the gases released could be dangerous to health. Chimneys are typically vertical, or as near as possible to vertical, to ensure that the gases flow smoothly, drawing air into the combustion in what is known as the stack, or chimney, effect. The space inside a chimney is called a flue.
A characteristic problem of chimneys is they develop deposits on the flue walls when used with wood as a fuel. These deposits can interfere with the airflow and more importantly, they are flammable and can cause dangerous chimney fires if they ignite in the chimney. Thus, it is recommended - and in some countries even mandatory - that chimneys be inspected annually and cleaned on a regular basis to prevent these problems.