If a flue pipe is missing its cap, the flue should be inspected and then a new cap installed. Flue caps have a wire mesh around the sides that keeps sparks from bouncing out onto a roof. If the mesh is damaged, the cap should also be replaced.
Flues can be simple pipes leading from furnaces or installed as a liner inside brick fireplaces. Flue caps keep rain from getting into the flue. Water can rust the inside. If that flue runs down a chimney, water, combined with the gases emitted from a fire, can eat away at the mortar and brickwork. Mold can form and deliver an unpleasant odor.
Birds, raccoons and squirrels love to nest in nice, warm chimneys. Sometimes they can't get out and die. The natural decomposition process produces odors, attracting flies and other insects. A professional chimney sweep is usually needed to get rid of the problem. A flue cap prevents the animals from ever getting in.
Flues help prevent downdrafts. Air coming from a fireplace or furnace is warm, which normally travels up the flue and out. Without a flue cap, cold air gets sucked into the flue, pushing the warm air back toward the heat source. The air cools, causing condensation to form on the sides of the flue. Condensation is just as destructive to a flue as rainwater.