Factory-built chimneys and gas vents are designed so that any liquid running down the inside is conducted past the joint between sections and is not scooped into the joint. If the sections are installed upside-down, as may happen when a base section is not used and there is no other indication of which end should be up, acid will accumulate in the joints and will hasten corrosion. Ordinary mortar contains lime which is particularly susceptible to acid attack and its use for joints between tile liners may lead to premature joint failure. This would permit air to infiltrate into the flue passage to cool the gases and increase condensation in the flue. Acids could also leak through the joint to the outside surface of the liner and attack the mortar of the masonry casing resulting in structural failure of the chimney.
Condensation increases with lower flue gas temperatures.
A chimney exposed for its full height to the outdoors will lose more heat, have lower vent gas temperatures and consequently more condensation than the same chimney located inside. Therefore an exterior chimney would be expected to deteriorate sooner. The required area of flue for a chimney can be calculated or may be specified in some building codes. The flue area must be large enough to exhaust all the vent gases when the lowest theoretical draft occurs, usually during mild weather. It must also be sized to allow for possible adverse effects of wind.