Note: The illustrated photos are of chimneys in the Chicago land area and  are in their conditions as a result of lack of maintenance, weather damage and/or a flue which is improperly lined.

"Prevention is KEY"

The NFPA's Code 3.3.80 states that an appliance that operates with a positive vent static pressure and with a vent gas tempature that may use excessive condensate production in vent.

Translation:

Your Furnace/Boiler Heating System if not properly lined, will create enough moisture or condensation to cause structural failure as in the chimneys you see pictured here. Others were in poor condition due to age weather elements or neglect by property owner(s).