Combustion Air — The Standard Method. Episode eighty of the Gas Service Animated series.

A starved flame does not go out politely. It soots, it spills, it makes carbon monoxide — which is why the combustion air math in section 9.3 is a life-safety calculation wearing a geometry costume. The standard method asks one question: does this room hold enough air for these appliances? Fifty cubic feet for every thousand Btu per hour. Let's run it.

Step one: the demand side. Add every gas appliance in the space — nameplate, not guesswork. Furnace, eighty thousand. Water heater, forty thousand. One hundred twenty thousand total. Multiply by fifty and divide by a thousand: six thousand cubic feet of room volume required. That is the number the room has to beat.

Step two: the supply side. Tape the room — length, width, ceiling height. Twenty-four by fourteen by seven: two thousand three hundred fifty-two cubic feet. Short of six thousand — this space is confined. Neighboring rooms only count if they connect through openings without doors, or openings sized by the code. A doorway with a door is a wall that sometimes opens.

Confined space, option one: borrow air from the rest of the building. Two permanent openings — one starting within twelve inches of the top of the enclosure, one within twelve inches of the bottom — each sized at one square inch per thousand Btu per hour. Here, one hundred twenty square inches each, and the floor is one hundred square inches minimum no matter how small the load. Minimum dimension three inches, so a pretty slot that computes but cannot breathe still fails.

Option two: outdoor air, where the divisors change and the openings shrink. Two openings again, top and bottom — but connected directly to outdoors, or through vertical ducts, each opening is one square inch per four thousand Btu per hour: thirty square inches each for our one twenty. Through horizontal ducts, the air fights friction, so the divisor tightens to one per two thousand: sixty square inches each. Direct and vertical share the four thousand. Horizontal pays double.

Step three: what covers the hole derates the hole. Unless the actual free area is known, assume a wood louver passes only twenty-five percent and a metal louver seventy-five. Thirty square inches of net requirement behind a wood louver means one hundred twenty square inches of gross opening. Louvers fixed open — and any screen no finer than quarter-inch mesh. Insect screen on a combustion air opening is a violation with good intentions.

Before you sign it off, hunt the disqualifiers. Doors on the rooms you borrowed from. Motorized louvers without the interlock — proven open before ignition, burner down if they close. Exhaust fans and dryers stealing the makeup air. And tight construction: where infiltration is known below zero point four air changes per hour, the standard method is off the table entirely — the known-infiltration math is mandatory. Next episode's business.

The calculator mirrors the walk: total input, then the room dimensions, and it returns the confined-or-unconfined verdict at fifty per thousand, the opening sizes for each route — one per thousand indoors with the hundred-inch floor, one per four thousand direct or vertical, one per two thousand horizontal — and the louver derates. Same order you just learned. The tape measure is still on you.

The standard method in one breath. Sum the nameplates. Fifty cubic feet per thousand. Tape the room — doors do not count. Short means confined: two openings, high and low — per thousand indoors, per four thousand direct or vertical, per two thousand horizontal. Derate the louvers, mind the three-inch minimum, and hunt the disqualifiers. Next: the advanced methods — one opening, tight houses, and fans that breathe for the room.