Revolving chimney cowls tend to be the most misunderstood product. They will keep out birds, vermin, leaves etc, and will keep out most of the rain.
Revolving cowls are guaranteed to eliminate downdraught in your chimney stack. When the wind blows, they revolve, and wind can't enter the stack.
Important:- Revolving chimney cowls will not solve draw issues with your chimney stack. Whilst it's true to say that they may create a very tiny draw when revolving, it's not anywhere near enough to solve updraught problems. Please read the warnings against each product.