This video shows an example of an incorrectly installed flue pipe and explains why it is dangerous...

Iain explains why it is important to have your Aga, Rayburn (or similar) Flue regularly swept. Although there may be very little soot, sometimes none at all, a lot of fluff/dust/pet hair etc. gets drawn into the flue and collects around the edges of the flue and liner. If left unchecked this can significantly reduce the draw and therefore the performance of the appliance. Also it is worth noting that your Gas or Oil service engineer will usually only check for draw with a smoke match and not inspect or sweep the flue. The flue may pass the test but failure could still be imminent. This means that investigation of the flue will only happen once the problem is so bad that the chimney/flue has stopped working effectively.

It's amazing (and a little frightening!) how many incorrect chimney cowls and terminals are fitted. The more houses I visit the more I realise the seriousness of the problem. This really needs adressing!