Prefabricated chimneys for heating systems, and for fireplaces, are equipped with rain caps to keep rain water from entering the chimney flue.
Look for a flat or curved plate at the very top of the chimney, the rain cap can be viewed from the ground. It is important for the home owner to periodically verify the integrity of the rain cap, especially after heavy rains and winds because weakened rain caps can often fail under these conditions. If a rain cap becomes dislodged, rain water can enter the flue and then run down into the heating system, or fireplace, and cause damage or system mal-function. During a rain, look for water and rust in and about the chimney flue located at the heating system, or fireplace; this is a sure sign that something is wrong.
In addition, a dislodged rain cap can sometimes cause a blockage in the flue which resticts the natural flow of toxic combustion gasses which contain carbon monoxide produced by the heating system, or fireplace; if the flow of flue gasses is restricted, toxic carbon monoxide may enter the house.