Modern appliances, whether oil, gas or wood-fired, are much more efficient and most of the heat they produce stays within the building. As a result, there is often insufficient heat in the flue and condensation and moisture will result. If there is not a water-tight liner in the flue, this moisture will bleed through the masonry as shown in the photos below.
Moisture Stainingchim-admin2011-04-12T17:12:44-04:00