What is a room-sealed boiler, or more correctly known as a forced draught fan flue or TYPE "C" boiler?

A room-sealed or forced draught fan flue boiler is an appliance in which the combustion circuit (drawing of combustion air, combustion chamber, heat exchanger and discharge of products of combustion) is sealed off from the room in which the appliance itself has been installed (UNI 10642: 2005 p3.3). This type of system is also known as a Type C boiler.

Usually, a room-sealed or forced draught fan flue boiler uses purpose-designed Flue/suction ducts (UNI 7129/3/08).