Single-Wall, Common Vents, and Masonry. Episode eighty-five of the Gas Service Animated series.

Same appliance, same height, same lateral — different metal, different number. Venting capacity is a thermal story, and every table in chapter thirteen is just that story told for a different construction. Today: what single wall costs, how two appliances share one vent, and where masonry chimneys stop being vents and start being liabilities.

Table 13.1(b) — Type B vent fed through a single-wall metal connector. At fifteen feet, lateral two, three-inch: the Type B table said forty-eight max, eleven min. Single wall says forty-seven max — and the FAN Min leaps from eleven to thirty-eight. One thousand off the ceiling, three and a half times the floor: the uninsulated connector bleeds flue heat, so it carries slightly less and demands far more input just to stay warm and draft. And single wall lives by extra rules: length capped at seventy-five percent of the vent height, six inches of clearance to combustibles, and never as the vent itself in a dwelling — connectors only.

Connector craft, while we are here: every connector rises toward the vent at least a quarter inch per foot — no dips, no sags, no flat runs collecting condensate. Joints fastened — screws or listed connections. And when two connectors meet one vent, the smaller one enters at the highest level the headroom allows. Small and hot rides on top.

Common venting — two appliances, one vertical vent — is three table lookups, and the method is written into the structure of the 13.2 tables themselves. Lookup one and two: each appliance's connector, in the vent connector part of the table — NAT Max for a draft hood appliance, the FAN window for a fan-assisted one, each at its own rise R. Lookup three: the combined input, against the common vent part — in the column that matches the pairing: FAN plus FAN, FAN plus NAT, or NAT plus NAT.

Worked, fast. Forty-thousand draft hood water heater, eighty-thousand fan-assisted furnace, sharing a vent. Connector one: the water heater at its rise, NAT Max column — passes. Connector two: the furnace inside its FAN window — passes. Common vent: one hundred twenty combined, read in the FAN plus NAT column at the total height — the height measured from the highest draft hood outlet or flue collar to the common vent outlet. And one materials rule with teeth: mix a single-wall connector into the system and the single-wall table sizes the common vent — the meaner table governs.

The common-vent boundary rules. Connector horizontal length: eighteen inches per inch of diameter — a four-inch connector gets six feet — and past that, ten percent off the capacity per multiple. The common vent's area never smaller than the largest connector. And the minimum and maximum input checks in 13.2.19 bracket both appliances. These are the checks that catch the almost-right design.

Now the masonry boundary — the seven-times rule, and the code states it twice. A connector and chimney flue serving a draft hood appliance: effective area at least the draft hood outlet, and not greater than seven times that outlet. A three-inch draft hood outlet is about seven square inches; seven times that is forty-nine. The grand old interior tile flue at ninety square inches? More than seven times the outlet — too much cold masonry for that little appliance to heat. It will condense, it will spall, it will spill. The fix has a name.

The liner. Drop a listed corrugated metallic liner inside the flue and size it from the Type B tables at eighty percent of the max — the FAN Min unchanged, one long-radius ninety already baked into the penalty. A lined flue counts as interior, so the ordinary tables apply again. Inspect and clean the chimney first — always — and where a flue once burned solid or liquid fuel, it gets a cleanout and a fresh look before gas ever moves in. Exterior bare tile is a special, colder story: strict conditions, minimum input floors by winter design temperature — and below minus eleven, the code recommends no exterior masonry configuration at all.

The second vent episode in one breath. Single wall: same geometry, meaner numbers — forty-seven against forty-eight, and a floor at thirty-eight. Connectors rise, small enters high. Common venting is three lookups — two connectors, one combined — and mixed metal means the meaner table. Masonry: not less than the outlet, not more than seven times it — or line it at eighty percent and move on. Next: the four numbers your analyzer lives by.