Combustion Air — The Advanced Methods. Episode eighty-one of the Gas Service Animated series.

The standard method is the front door of section 9.3 — but the code built four more doors for the rooms the front door cannot serve. A single high opening. An equation for tight houses. A fan with a contract. And a blend for the almost-big-enough room. Four methods, four kinds of building.

The one permanent opening method, section 9.3.3.2. One opening, starting within twelve inches of the top of the enclosure, communicating with outdoors directly or by duct. Its size is the larger of two answers: total input divided by three thousand — one hundred twenty thousand gives forty square inches — or the sum of the areas of every vent connector in the space, here about twenty. Forty wins. And the appliance earns clearance: an inch at the sides and back, six inches at the front, tape-verified.

Ducts carry the air only if they follow duct law. One space per duct — never the upper and lower openings sharing a run. An upper horizontal duct must not slope down toward the air source. Galvanized steel or equal. Attic terminations run unscreened; everywhere else, mesh no finer than a quarter inch. And outdoor intakes keep their lowest edge at least twelve inches above finished grade, out of the snow and the mulch.

The known air infiltration rate method — mandatory when a blower door or design data says the house is under zero point four air changes per hour. Two equations, annex-simple: for appliances other than fan-assisted, twenty-one over the A C H, times input over a thousand. For fan-assisted, fifteen over the A C H. At zero point three, the forty-thousand water heater demands twenty-eight hundred cubic feet and the eighty-thousand fan-assisted furnace four thousand — sixty-eight hundred total, more than the standard method asked of a leakier house. Tight construction pays in volume. And no crediting an A C H above zero point six — the code caps the leak you may claim.

The mechanical method, section 9.3.6: when geometry fails, ventilate on purpose. Outdoor air delivered at zero point three five c f m per thousand Btu per hour — forty-two c f m for our hundred twenty thousand — and every appliance interlocked to the air system: no air, no burner. Where exhaust fans run in the same space, their air gets replaced too. The fan signs a contract, and the interlock enforces it.

The combination method, section 9.3.4, for the room that is almost enough. Take the ratio: available indoor volume over required — twenty-three fifty-two over six thousand is zero point three nine. The reduction factor is one minus that: zero point six one. Multiply the full outdoor opening size by the factor: thirty square inches becomes about nineteen each. The room's partial credit shrinks the holes you cut — and the three-inch minimum dimension still stands.

Choosing the door. Room beats fifty-per-thousand — done, unconfined. Generous house air — borrow it with two indoor openings. Real outdoor access — two openings, or one high one at three thousand. Tight construction — the infiltration math, mandatory below zero point four. No geometry at all — mechanical air with the interlock. The method is a diagnosis of the building.

The calculator carries the standard method and the two-openings numbers. The one-opening three-thousand rule with its connector-area check, the infiltration equations, and the mechanical rate you just watched worked by hand — that is deliberate. Some math you should own before a tool owns it for you.

The advanced methods in one breath. One opening: high, the larger of input over three thousand or the connector areas, with clearances. Ducts follow duct law. Tight house: twenty-one and fifteen over the A C H, mandatory under zero point four, capped at zero point six. Mechanical: zero three five c f m per thousand, interlocked. Combination: one minus the ratio scales the openings. Next: the meter becomes your lab instrument.