These types of terminals are designed to give ventilation to unused chimneys only.
They are not allowed to be fitted to chimneys that have working gas, oil or solid fuel fires or boilers as they slow down the fumes trying to escape the chimney and could cause smoke and carbon monoxide to enter occupied rooms, yet because of their cost many uneducated builders or cowboys still fit them to live appliances. If you have one of these fitted to your chimney get it removed at once and DO NOT USE IT.
![](https://oliverschimneysweep.co.uk/wp-content/uploads/2021/04/Cowls-and-terminals.png)