Type B vents and connectors. Episode eight of the Gas Service Animated series.

Type B vent is double wall pipe built for gas appliance flues. A metal inner liner carries the flue gases, an outer metal wall surrounds it, and the air space sealed between them acts as insulation. That air gap keeps the flue gases warm for better draft and less condensation, while the outer wall stays cool enough for a listed clearance of just one inch to combustible material. Type B is listed for flue temperatures up to the four hundred eighty degree Fahrenheit class, which covers draft hood and fan assisted gas appliances.

So where does Type B belong? It serves Category one gas appliances, the natural draft and fan assisted units that vent under non positive pressure, like most furnaces and water heaters. It is never connected to appliances that burn solid or liquid fuel, and never to incinerators; the code even requires a label saying so where a gas vent runs inside a chimney. And it is not for Category three or Category four service, where positive pressure and condensing flue gases demand special listed venting with sealed joints per the appliance manufacturer.

A connector is the pipe that links an appliance to the vent or chimney. Single wall metal pipe is allowed as a connector in many cases, but it runs hotter on the outside, so it needs six inches of clearance to combustible material, compared to one inch for a Type B connector. Single wall can serve only runs that stay in the open, directly from the appliance space; it cannot be buried in a concealed wall, floor, or ceiling. And every connector must remain accessible over its entire length for inspection, cleaning, and replacement.

Now the installation rules. Keep the connector as short and direct as practical, and give it a continuous upward slope toward the vent of at least a quarter inch per foot, with no dips or sags where condensate could pool. Secure every joint with sheet metal screws or the fastening method in the listing, and join branches only with a manufactured tee or wye fitting. Manually operated dampers are not permitted in a vent connector. And where a connector enters a chimney flue, it enters above the extreme bottom, so debris at the base cannot block it.

The vent itself needs a skeleton. Listed supports, straps, and plates carry the weight so joints cannot pull apart, and each section locks to the next with the full engagement the manufacturer's instructions call for. Where the vent passes through a floor or ceiling, firestop spacers keep the listed clearance and block fire spread between levels. The rule is simple: the listing and the manufacturer's instructions govern every inch of the run.

At the roof, the vent must terminate high enough to breathe. A Type B vent twelve inches or smaller, at least eight feet from a vertical wall, may use its listed cap and the code's roof slope table: a flat or gentle roof needs at least one foot above the penetration, and steeper roofs need progressively more. Larger vents, or vents close to a wall, must instead rise at least two feet above anything within ten feet horizontally. The listed cap matters too: it resists downdraft, keeps rain and debris out, and is what earns those minimum heights in the first place.

Here is the failure gallery. A rusted through connector leaks flue gas before it ever reaches the vent. A sagged run collects condensate and chokes the draft. Missing screws let joints separate with a bump. Single wall pipe buried in a wall cavity is a hidden fire and corrosion hazard. And ordinary tape holding a joint together is not a listed fastener; it is a red flag for the whole installation.

So what does the tech verify? Identify the material first: Type B, single wall, or a chimney liner, because the rules differ for each. Then check clearances to combustibles, confirm the upward slope and solid support, and test that every joint is fastened and fully engaged. Finish outside: the cap should be listed, intact, clear of debris, and at the required height. When in doubt, the code tables, the listing, and your company's procedures make the call.

Type B is double wall insulation for the flue: warm gases inside, cool wall outside. Connectors run short, sloped, secured, and visible. Terminations ride high on a listed cap. And anything rusted, sagging, taped, or buried gets written up. Next episode: sediment traps and shutoff valves.