The most important thing you should know:Water ruins masonry. Therefore, it is to your chimney’s advantage to avoid as much moisture as possible. Exterior moisture such as rain and snow can be limited with proper construction. Interior moisture in the chimney cannot be limited. Furnaces generally produce one gallon’s worth of water every hour they run. Additionally, hot water heaters generally produce a half gallon’s worth of water every hour they run. The interior lining of your chimney is constantly being ruined by this constant flow of moisture. As time passes, the interior liner of your chimney is becoming less able to confine the products of combustion. Failure is often the end result.