Remember: Using an inappropriate style of chimney cap or one that is not correctly sized to your flue can create the risk of a chimney fire. For example, a round flue with more than one concentric pipe is designed to be cooled by air that circulates in the spaces between the pipes. The wrong type of chimney cap will inhibit that cooling effect. Another risk of an inappropriate style or size of chimney cap is your fireplace not drafting correctly, resulting in a smoky room when you light your fireplace.