Reasons for fitting a Cowl include:

• To prevent birds from nesting or other pests entering the chimney
• To reduce or prevent downdraught where air is being forced down the chimney
• To prevent rain entering the flue which can cause damage, damp and staining around the fireplace
• To improve the draw of the appliance. Some cowls are designed to increase the draw of the appliance

Reasons for fitting a Cap include:

• To reduce heat loss through the flue
• To prevent water / rain ingress which can cause damage, damp and staining 
• To prevent birds from nesting or other pests entering the chimney

(Capping a chimney pot can only be done on a disused flue)

Reasons for fitting a bird guard:

• To prevent birds from nesting or other pests entering the chimney