Flue

A flue is a duct, pipe, or opening in a chimney for conveying exhaust gases from a fireplace, furnace, water heater, boiler, or generator to the outdoors.

As combustion products contain carbon monoxide and other dangerous compounds, proper "draft", and admission of replacement air is imperative.

They often have the disadvantageous effect of releasing useful household heat to the atmosphere when not properly set—the very opposite of why the fire was lit in the first place.

This flue gas circulation avoids the considerable heat loss to the chimney and outside air in conventional systems.

The heat from the flue gases is absorbed quickly by the bricks and then released slowly to the house rather than the chimney.