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Chimney Doctors' Homeowner's Guide

Chimneys and Moisture Problems

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by Chimney Doctors

If your home was built over 50 years ago, you are most likely experiencing moisture problems in your chimney. Older furnaces required more chimney area due to their high rate of heat loss (through the chimney) and lower efficiency.

With today's high-efficiency furnaces, most of the heat goes into your home - not the chimney - producing a much cooler exhaust and leaving the chimney filled with cold air. The smaller amounts of heat which do go into the chimney are cooled by the surplus cold air and simply don't have the energy to make it to the top of the stack. Normal amounts of humidity and moisture in the hot air condense inside your chimney as the air cools, leaving water which can be as acidic as bleach. Over time, this acidic moisture can do serious damage to your chimney and masonry work - but that's not the worst of it. This acidic water also combines with possible wood, coal, or oil buildups in your chimney, creating a highly corrosive substance. This process can cause several costly and harmful situations including white and brown stains on walls and bricks, efflorescence (which is mostly salt crystal buildup), and crumbling mortar. Today's high-efficiency furnaces can create one to one-and-one-half gallons of this substance each hour.

The simplest and most cost effective solution is to place a stainless steel liner inside your existing chimney. This liner reduces the usable air of your chimney, helping the more efficient furnace exhaust to rise all the way to the top and exit your chimney. The stainless steel also acts as a barrier, keeping acidic moisture and substances away from your chimney and masonry work.

Moisture Penetration: Common Causes and Effects

Effects of Moisture
Crystal
Buildup
(Efflorescence)
Mortar Deterioration
Chipped / Flaking
Bricks
Cracked Bricks
Rising Moisture
Corrosion
Mold/
Algae Growth
Interior
Damage
Sources of Moisture
 

Acid Cleanings

x
x
.
.
.
x
x
x

Sandblasting

.
x
x
.
.
.
.
.

Plant Growth

x
x
.
.
.
.
.
.

Sealant / Caulk Deterioration

x
.
x
x
.
x
x
x

Missing/Clogged
Weepholes

x
.
x
x
x
x
x
x

Deteriorated
Mortar Joints

x
x
x
x
.
x
x
x

Broken / Loose Bricks

x
x
.
.
.
x
x
x

Missing Flashing

x
.
x
x
x
x
x
x

 

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