Reading the Vent Tables — 13.1(a). Episode eighty-four of the Gas Service Animated series.

Table 13.1(a) is a machine: feed it two geometry numbers and it hands back a diameter that will carry the flue gases without spilling and without condensing. But machines have operating manuals, and this one's manual is where venting jobs go right or wrong. Today: the single-appliance table, cell by cell.

First, confirm you are even in the right table. The header says it all: number of appliances, single. Type, Category one. Connection, directly to the vent — and per the annex, a Type B double-wall connector belongs here too. Single-wall metal connector? Different table, 13.1(b), next episode. And the whole 13.1 family serves only chimneys and vents not exposed to the outdoors below the roof line.

The two inputs. Total height H: from the draft hood outlet or flue collar, vertically to the vent termination — the annex figures draw it exactly. Lateral L: the horizontal distance the connector travels to reach the vertical vent. The L equals zero rows are earned, not assumed — straight vertical, no elbows at all. Every lateral row already includes two ninety-degree elbows in its numbers.

Draft hood appliance — read the NAT Max column only. Forty-thousand water heater, fifteen feet of height, five of lateral: run down the three-inch column to the fifteen-five row — forty-five. Forty fits under forty-five, so three-inch carries it on the table. The annex worked example is the same dance at bigger numbers: a one-twenty draft hood appliance at ten and five lands on a five-inch vent — one twenty-two on the single-wall table the example actually uses; the Type B cell here reads one twenty-four.

Fan-assisted changes the question: the input must land inside a window — at or above FAN Min, at or below FAN Max. The eighty-thousand furnace at fifteen and five: three-inch tops out at sixty-five — fail. Four-inch runs thirty to one-thirty — eighty lands inside. Why a minimum at all? An oversized vent moves flue gas slow and cool; cool gas condenses, and condensation is corrosion on a schedule. The window protects the vent from your generosity.

Some cells just say N A — and the footnote is blunt: not allowed. Rule 13.1.1: where the table shows N A, that configuration shall not be used. Thirty feet of height with thirty of lateral kills the entire three-inch column. You do not interpolate around an N A, you do not round past it — you change the geometry, the diameter, or the venting approach.

Between rows, the code is generous and precise. Interpolation is expressly permitted; extrapolation beyond the table is prohibited, both directions. Twelve feet of height, four-inch, lateral five: the ten-foot row reads seventy-seven, the fifteen-foot row eighty-seven. Twelve is two-fifths of the way — seventy-seven plus four is eighty-one. Clears an eighty-thousand appliance by a thousand Btu. Or skip the algebra with the conservative pairing: lower row for the max, higher row for the min.

The boundary rules that finish the job. Extra elbows beyond the built-in two cost five percent of max each at forty-five degrees or less, ten percent up to ninety. Connectors never upsize more than two sizes over the appliance outlet. Where the vertical vent is larger than the connector, the vent diameter sets FAN Min and the connector diameter sets the max — and the vent's flow area never exceeds seven times the appliance outlet area. Below six feet of height, or past the table's edge — engineering methods, not optimism.

Table 13.1(a) in one breath. Confirm the header — single, Category one, Type B world, interior. Measure H and L honestly. Draft hood: down the NAT column, first cell at or above the load. Fan-assisted: inside the window, min and max. N A means no. Interpolate freely, extrapolate never, count your extra elbows, respect the seven-times cap. Next: single-wall metal, common vents, and the masonry boundary.