Workshop: vent sizing. Episode seventy-four of the Gas Service Animated series.

Chapter thirteen on the board today, with the tables this channel transcribed and verified twice. The grammar from the common-venting episode: NAT Max for draft-hood appliances, FAN Min and FAN Max for fan-assisted — and NA means exactly what it says.

Problem one. A one-hundred-thousand-Btu fan-assisted furnace, alone on a Type B vent through the roof: total height twenty feet, lateral five feet, Type B connector. Size the vent.

Table thirteen-one-a, the twenty-foot height block, five-foot lateral row. Three-inch: FAN window twenty-one to seventy-one — too small. Four-inch: twenty-nine to one forty-three — one hundred sits inside. Four-inch vent. And notice the discipline: the answer is a window, not a ceiling — one hundred had to clear the minimum too, because an oversized vent can fail a fan appliance on the low end.

Problem two. A forty-thousand draft-hood water heater, own Type B vent, height fifteen feet, lateral two. But the installer used single-wall connector into the vent. Which table, and what size?

Single-wall connector moves you to table thirteen-one-b — same vent, different connector, lower numbers. Fifteen-foot height, two-foot lateral: three-inch NAT Max reads forty-seven. Forty fits — three-inch vent. On the Type B connector table it would read forty-eight; the single-wall column gives away capacity, and on a bigger load that difference changes the answer. The connector is part of the vent.

Problem three, the rules question. That same water heater vents into a masonry chimney whose flue measures ninety square inches inside. Its draft-hood outlet is three inches — area about seven square inches. Legal?

The seven-times rule from thirteen-one: the chimney's area can't exceed seven times the appliance outlet area. Seven times seven is forty-nine — and this flue is ninety. Too big: the flue never warms, draft never establishes, condensation eats the masonry. The fix is a listed liner sized to the tables — the orphan story's cure, on the board.

The vent calculator carries both connector tables — use it to check the pencil, and to feel how the FAN window moves as height and lateral change. Then close the book knowing why the answer is the answer.

Vent sizing in one breath. Pick the table by connector. Height block, lateral row, then the window: NAT under the max; FAN inside min and max. Respect NA, and respect seven-times on masonry. Three problems, three habits. Next workshop: the combustion numbers.