A chimney cowl is a revolving metal ventilator that fits over a flue or chimney pot to encourage updrafts and prevent downdrafts. Most importantly, it prevents wind from blowing smoke back down a chimney and into the room beneath. A chimney cowl also prevents birds and squirrels from nesting in the chimney and often acts as a rain guard. Sometimes referred to as a flue cowl or a chimney cap, it is most frequently called a cowl because it’s shaped like a hood or bonnet, and resembles the cowl worn by a monk. Traditionally, it was manufactured in the same red-colored clay as chimney pots, but today it is usually made from galvanized iron and is available in a variety of styles.
When a fire burns in a fireplace, the smoke is usually propelled along with rising warm air through a flue to a chimney pot on the roof. In windy conditions, however, the force of the wind may be stronger than the updraft of warm air and smoke, and it can push the flow back down the chimney flue. This back draft might result in a poor fire, or it could propel smoke and toxic gases back into the house. A chimney cowl stabilizes fluctuating flue drafts, prevents downdrafts, and increases the draw up the flue, which maximizes fuel burning efficiency and ensures a healthy environment.
The most effective type of chimney cowl is the H-pot or H-style cowl. It has a distinctive advantage over most other downdraft caps because it stabilizes the draft rather than increases it, allowing the fireplace to operate more efficiently. Made from chimney pipes shaped like the letter H, it isolates combustion gases from the winds and turbulence that cause down drafts. The H-cap was originally most prominently used in marine applications because of its bulky appearance, but it has become popular recently thanks to its energy saving functionality.
Today, the term "chimney cowl" is used loosely and may refer to a terminal or vent that fits into an existing chimney pot. A solid fuel insert, like a push-in hood or a push-in top type, may also be called a chimney cowl. Chimneys that are no longer in service may use mushroom hood shaped or bonnet hoods that are also called cowls. These feature round or square base spigots that operate as a flue vents, but they are only safe in unused chimneys.