Concentric venting consists of a "pipe within a pipe". Flue gas exits the building through the inner pipe and combustion air is drawn into the boiler through the space between the inner and outer pipe.

Each Crown concentric vent component consists of an inner pipe of polypropylene and the outer pipe of steel. Integral gaskets on each concentric fitting provide a gas tight seal.

"Vent Size" denotes nominal inner and outer pipe diameters in millimeters. For example 60/100 concentric components have a 60mm diameter inner pipe and a 100mm diameter outer pipe.