Have you got one of these?

Filmed at the SureFire Centre - Derby: Iain Stevens shows an example of a chimney cowl that is commonly in use but should NEVER be used on a live chimney as it greatly reduces the flue size. This makes it more difficult for the toxic gasses to escape causing a backup of harmful fumes and a much higher risk of chimney fire due to soot & tar buildup in the chimney. These cowls are ONLY for ventilation of flues that are not in use...