cowl

A cowl is a usually hood-shaped covering used to increase the draft of a chimney and prevent backflow. The cowl, usually made of galvanized iron, is fitted to the chimney pot to prevent wind blowing the smoke back down into the room below. Undoubtedly named after the resemblance of many designs to the cowl garment worn by monks, they have been in use for centuries. When using an open fire to heat a room the smoke rises through a flue to a chimney pot on the roof. Under normal conditions the warm air from the fire will rise up the chimney emitting the smoke with it and dispersing it at rooftop level where it is less of a nuisance. In strong winds the pressure of the wind may overwhelm the updraft and push the airflow in reverse down the flue. Smoke will then fill the room it is intended to heat posing a health and fire risk, causing discomfort and dirtying furnishings in its path. When raw coal rather than smokeless fuel is burnt, the amount of smoke may be considerable and measures to prevent backflow occurring are a necessity.