The 300mm - 460mm telescopic horizontal flue kit from Navien is a component used to install a horizontal flue system for a boiler. The term "telescopic" refers to the adjustable length of the flue pipe, allowing it to extend or retract to fit different installation requirements within the specified range of 300mm to 460mm.

In summary, a 300mm - 460mm telescopic horizontal flue kit provides a flexible, secure, and efficient solution for venting exhaust gases from the boiler through a horizontal wall installation.