If the exhaust system is in damp operation, the temperature on the inner wall of the exhaust pipe may be below the dew point. The water vapour contained in the exhaust gas condenses in the flue pipe and is led out of the system via a condensate drain, or returned to the fireplace in the case of condensing technology. See also encyclopedia article exhaust system, condensate resistance classes according to DIN V 18160-1.
Source: bauwion
