A chimney cowl is a bonnet-shaped cover that's placed over the top of a chimney. Cowls are generally made of galvanised iron, aluminium or steel. The primary goal of installing a chimney cowl is to prevent downdrafts, but they're also made to keep other undesirable elements from entering the home through your chimney.
Chimney cowls are designed to provide ventilation for smoke and fumes in functioning chimneys, although they're also utilised in unused chimneys as caps to prevent birds, squirrels, bats and other nuisance animals from building nests in the chimney. Cowls also act as rain guards to prevent moisture from getting into your chimney, and are sometimes designed with a wire-mesh barrier to stop sparks from escaping onto your roof and to provide a barrier against insects.
The appropriate cowl for your chimney depends on whether the chimney is in use, and if it is, the source of fuel it uses. Ventilation cowls, for example, prevent condensation and are usually intended for unused chimneys. Gas and solid-fuel cowls vent the smoke from gas and solid-fuel fires such as those of coal or wood. Oil cowls are for use with oil fires, and multiuse cowls can be used with most types of fuel. Except for multiuse cowls, each has a specific type of fuel it's designed to be used with. A cowl designed for use with oil-fuelled fires, for example, shouldn't be used to cover a gas-burning chimney.
A stationary chimney cowl is basically a cap that's easily fitted over your chimney. It's bolted or strapped in place, and although it's used on active, functioning chimneys, it's also ideal for covering chimneys that aren't being used.
Revolving cowls resemble a turbine vent in design, and prevent drafts from coming down into your chimney. This prevents the smoke and fumes of the fire from entering your home. The rotation is powered by wind or the heat within the chimney, and results in a sucking action that draws the smoke and fumes up and out of the chimney.
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