Free Sample
The Complete Handbook of Vents & Flues
A practical reference for HVAC professionals, facilities managers, and serious DIYers
by Alumigogo Books
Chapter 1: Understanding Vents & Flues
A vent or flue is a pipe that carries hot, moist combustion gases from heating equipment—a furnace, boiler, water heater, or fireplace—to the outdoors. That simple definition masks a surprising amount of physics and many opportunities for failure.
In practice, a vent or flue must do several things simultaneously. It must pull exhaust up and out of the equipment fast enough that combustion products don't back up into the home. It must carry away moisture without trapping it in the duct where it condenses and corrodes the pipe or drips back into the equipment. It must stay clear of obstructions—ice, soot, debris, nests—that block flow. It must not leak combustion gases into the living space or attic. It must survive temperature swings and chemical attack from acidic condensate. And it must do all of this reliably, year after year, in basements, crawlspaces, attics, and exterior walls where it faces moisture, temperature extremes, and sometimes neglect.
When a vent or flue works, it's invisible. You don't think about it. When it fails, the consequences range from inconvenient to dangerous: a furnace that won't fire because it can't draft, a basement that smells like exhaust, a wall or ceiling stained with rust from leaking condensate, an equipment shutdown due to a pressure switch that detects blocked flow, or—in the worst case—carbon monoxide seeping into living space because exhaust is backing up rather than venting.
Understanding how and why vents and flues work is the foundation for