Water leaking into a chimney base (C) Daniel Friedman Water Leak & Frost Damage to Chimneys & Flues

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This article describes water leaks and moisture that damage masonry and metal chimneys and the heating appliances that are connected to them. We list the common sources of water and leaks and we include warnings for inspectors who need to examine equipment as well as the chimney itself when water or moisture leaks are detected.

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Moisture Problems that Damage Chimneys

Water leaking into a chimney base (C) Daniel Friedman

Our photo (page top) shows water pouring into a basement, coming out of the chimney cleanout during a heavy rainstorm. Readers of this article should also see CHIMNEY COLLAPSE Risks, Repairs.

These articles on chimneys and chimney safety provide detailed suggestions describing how to perform a thorough visual inspection of chimneys for safety and other defects. Chimney inspection methods and chimney repair methods are also discussed.

Our photograph at left shows a stunning waterfall flowing out of a "sealed" chimney cleanout in a basement. Lots of roof spillage or surface runoff was entering the base of this chimney.

[Click to enlarge any image]

Moisture is the major cause of chimney corrosion and disintegration in both masonry and metal chimneys. The flue gases are acidic in nature and if allowed to condense and saturate the masonry or joints in metal flues the destructive results will soon be apparent.

Mechanical problems in the construction or settlement after construction and over firing or flue fires will also contribute to the problems.

External moisture enters the chimney through cracked caps, porous masonry, poor mortar joints and improperly designed and installed roof flashings. Internal moisture (condensation) collects in cracked or separated flue tiles, blocked flues and chimney caps.

Masonry chimneys subjected to moisture damage can have efflorescent salt stains, spalled bricks, eroded mortar joints, flaked cracks in the ceramic flue liner and cracked caps.

Metal components of a vent system can have rust and white acid stains at joints, corrosion holes along the bottom of horizontal connectors and corroded chimney cleanout doors at the base of the flue.

Moisture enters a chimney structure from several locations:

  • Missing chimney cap lets wind blown rain and snow enter the flue
  • Damaged chimney crown lets water enter the chimney structure
  • Leaks at the chimney sides let wind-blown rain enter the chimney structure
  • Leaks at chimney flashing let water enter the chimney structure
  • Ground and surface runoff or roof spillage by a building often enter the base of a chimney that extends below ground.

Water leaks into a Chimney can Damage the Chimney and the Appliances Connected to It

The result of leaks into a chimney can be unsafe heating equipment as well as costly damage to the chimney, the appliances connected to it, and to the building itself.

  • Frost or water damage can crack or break a masonry clay chimney tile resulting in fire or flue gas hazards at the building
  • Broken chimney flue liner parts can obstruct the chimney flue resulting in dangerous, potentially fatal carbon monoxide gas hazards
  • Water entering the heating appliance such as a furnace, boiler, water heater or woodstove can cause rust damage to the heater components or can damage electrical controls on the appliance, leading to costly repairs
  • Water entering a wood-framed chimney chase can also invite insects, rot, or mold problems in the building
  • Water entering a metal chimney or flue can cause corrosion or rust which perforate the chimney or flue vent connector leading to equipment failure, improper operation, and dangerous flue gas leaks in the building. Even a stainless steel manufactured chimney is not immune to rust and corrosion when it faces the combination of water in the flue interior (or between the layers of metal of a multi-wall chimney) and corrosive acids that form as soot and creosote are dissolved by the water in the flue.

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