The boiler flue is a large pipe that travels from the boiler to an external wall of the house. It allows the exhaust gases and fumes created by the boiler's operation to escape outside the house instead of inside the home. Looking at your boiler, the flue is the large chimney-like pipe often coming from the top or off the back of your boiler.

A boiler flue is effectively a chimney. The flue brings air into the boiler for combustion and expels the products of that combustion back to the outside. There are typically two waste products that a properly functioning modern boiler creates; carbon dioxide and condensation, both by-products of how boilers heat water. A modern boiler condenses; this condensation runs back through the flue system and collects in the boiler.