What problem a cowl solves
Different cowl designs address different issues: a bird guard cowl stops nesting, a rain cap keeps water out, and an anti-downdraught cowl reduces smoke being pushed back into the room on windy days.
Choosing the right type
The right cowl depends on whether the flue is in active use for an open fire or stove, since ventilation requirements differ and an unsuitable cowl can actually restrict draw on a working chimney.
Fitting to the pot
Cowls are fitted securely to the top of the pot, sized correctly for the flue diameter, so they stay in place through Bristol's exposed, windy hillside locations rather than working loose.