Bonnet 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 beneath. No doubt named for 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 taking the smoke with it and dispersing it at rooftop level where it is less of a nuisance.

In strong winds the force 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.

When raw coal rather than smokeless fuel is burnt, which was the case for centuries before clean air legislation took effect, then the amount of smoke is considerable and measures to prevent backflow occurring are a necessity.

Another secondary function is to prevent birds and squirrels from nesting in the chimney. They often also act as a rain guard to keep rain from going down the chimney. A metal wire mesh is sometimes added as a spark arrestor. Wooden cowls were used on oasts to prevent the ingress of rain into kilns, and create a flow of air through the kiln.