Efflorescence on chimneys (See the above blog Efflorescence) can also be caused by condensing exhaust gases of gas and oil fired appliances.  This "condensate" is slightly acidic and when this moisture moves through the brick structure much damage to the chimney can occur.  In this case the efflorescence that forms is indicative of more than "cosmetic" issues and costly repairs may be necessary.  When this type of efflorescence is seen it usually means that the chimney is not properly lined for the furnace or other gas appliances.  Even from the street, when the chimney has more than one flue, one can usually tell which flue is the furnace flue.  In the following picture of a chimney with 2 flues, the whitish efflorescence is indicative of an unlined flue for the gas furnace.  

     The chimney on this home had been destroyed by the gas furnace venting into the unlined chimney for 40 years.  

    If the discoloration is yellowish colored, like the picture on the left, we can get our first hint that the home is heated by an oil fired furnace (note how it is primarily concentrated along the left flue).  Sometimes the yellow efflorescence is visible on the portion of the chimney in the attic and is indicative of either past or present heating of the home by oil---- this might be our first clue indicating a buried oil tank on the property.

Charles Buell

www.buellinspections.com