Venting and draft — why flue gases rise. Episode three of the Gas Service Animated series.

Every cubic foot of gas an appliance burns leaves something behind: carbon dioxide and water vapor — and when combustion goes incomplete, carbon monoxide, a gas you can't see, smell, or taste. None of that can stay in the living space. The code is blunt about it: a venting system must convey all flue and vent gases to the outdoors. Venting is how the products of combustion get home safely.

So what pushes flue gas out? Nothing pushes — it floats. Hot flue gas is lighter than the cold air outside, so a column of hot gas inside a vent weighs less than an equal column of outdoor air. That weight difference is natural draft, and it's what pulls the flue products up and out. And qualitatively, the taller the vent and the hotter the gas, the stronger that draft becomes.

Draft-hood-equipped Category one appliances — natural-draft water heaters and furnaces — ride on that buoyancy, with one simple, brilliant part in between: the draft hood. It does three jobs. It lets the products of combustion escape freely up the vent. It admits dilution air — room air that joins and tempers the flue flow. And it decouples the burner from the chimney: when a gust drives a momentary downdraft, the relief opening spills it around the skirt instead of blasting it down onto the flame. One note: Category one also includes fan-assisted appliances that have no draft hood.

That open relief opening cuts both ways. Block the vent — or starve it — and the flue gases take the easy exit: spilling out at the draft hood, into the room, carrying carbon monoxide you cannot see or smell. The worst offender is a house under negative pressure — bath fans, dryers, and range hoods competing for air can overpower a natural draft and pull flue gases backward. A brief puff at cold start, before draft establishes, is expected — but sustained spillage at the draft hood is never normal. Treat sustained spillage as a carbon monoxide emergency and follow your company's procedures.

The hardware matters too. Type B double-wall vent is the standard for Category one appliances with draft hoods — the air gap keeps flue gas warm, and it's listed for temperatures up to four hundred eighty degrees. Single-wall metal pipe mostly serves as the connector — the run from the appliance to the vent — and it loses heat fast, so it needs far more clearance to combustibles. A masonry chimney can serve too, but only with an approved liner — and an exterior chimney runs cold, weakening draft and condensing flue gas, which is why the Chapter thirteen rules treat it separately.

The connector has rules of its own. It must slope upward toward the vent — a rise of at least a quarter inch for every foot of run — with no dips or sags where condensate can pool and flow can stall. Keep it as short and direct as practical, secure every joint, and hold your clearance from combustibles: single-wall pipe needs much more room than listed Type B. A lazy, sagging connector can undo a perfectly good vent.

When it's time to size the system, three dimensions drive everything. H — the total vent height. L — the lateral run. R — the connector rise. Height and lateral enter Table thirteen point one for a single appliance connected directly; connector rise feeds the common-vent connector tables. Out comes a diameter — the fan-assisted columns give minimum and maximum appliance input, while the natural-draft columns give a maximum only. The companion Gas Service Master Atlas Chapter thirteen study sheets in this folder walk those tables line by line.

On the job, the tech proves the system works. After startup, check for draft and spillage at the draft hood relief opening, following your company's test procedure — with the burner running, the building closed up, and the home's exhaust equipment operating. Inspect the vent and liner condition and confirm the termination clears the roof as required. Verify the connector slopes up, is supported, and holds its clearances. And every reading and procedure defers to NFPA 54 twenty twenty-four, the manufacturer's instructions, the AHJ, and your company's current practices.

Buoyancy makes draft — a hot, light column rising inside the vent. The draft hood dilutes, relieves, and decouples. Spillage means carbon monoxide risk — act on it. And height, lateral, and rise drive the Chapter thirteen sizing tables. Next episode: combustion air — where the fire gets its oxygen.