Altitude and the Four Percent Rule. Episode eighty-seven of the Gas Service Animated series.

Take a perfectly tuned appliance a mile up and it quietly becomes a different appliance. Same orifice, same manifold pressure, same everything — except every cubic foot of gas and air now holds fewer molecules. The orifice never got the memo. This is the altitude episode: the rule, the math the trade argues about, and the orifice table that settles it.

The physics in one move: a fixed orifice meters volume, not molecules. At altitude, the same volumetric flow of gas meets air that carries less oxygen per cubic foot — on both sides of the flame at once. Run the sea-level input into that thin air and combustion runs rich toward incomplete — soot, carbon monoxide, the whole familiar failure family. So the input comes down, or the orifice changes. That is the entire why.

The rule is section 11.1.2. Nameplate ratings apply in full up to two thousand feet — no derate. Above two thousand, three permitted paths: reduce four percent for each one thousand feet above sea level before selecting the appliance; or as the authority having jurisdiction permits; or per the listed manufacturer's instructions. Run the code text at Denver's mile: five thousand two hundred eighty feet — about twenty-one percent off a one hundred thousand nameplate. About seventy-eight thousand nine hundred Btu per hour.

Now the version half the trade learned: four percent per thousand feet above two thousand — which at the mile gives only thirteen percent — about eighty-six thousand nine hundred. That is traditional practice, and it is the lenient math — the code text and the annex orifice table both count from sea level. Our field calculator now shows both numbers, code basis first, so the difference is never a surprise on a rooftop. Know which one your jurisdiction expects before you argue with an inspector.

In practice, path three outranks the arithmetic: the manufacturer's instructions. Many modern appliances are certified for altitude — commonly to forty-five hundred feet, some to ten thousand — with no derate at all, or with their own kit schedule by elevation band. The rating plate and the installation manual carry that answer. The four percent math is what you run when the manufacturer is silent — not instead of reading.

The hardware route lives in annex E — informational, and elegant. Select the orifice for the manufacturer's sea-level rated input, then read the altitude equivalent straight from the annex table: it already includes the four percent per thousand. A number thirty orifice at sea level becomes a thirty-one in the three-to-five-thousand band, and a thirty-five at ten thousand — smaller drills for thinner air. Orifice work is qualified-technician work with the manufacturer's kit — never a field-drilled guess.

Whatever path sets the input, the proof is the same discipline. Install the kit orifices. Set manifold pressure to the rating plate's spec for that elevation. Then verify: clock the meter against the derated target — using the local heating value, because at altitude the Btu in each metered cubic foot has shifted too — or confirm manifold pressure and orifice size. And close with combustion analysis: air-free CO in range is the signature that the thin-air math actually landed.

The calculator's last tab: nameplate and elevation in, the code-basis derate out — four percent per thousand above sea level — with the traditional above-two-thousand variant right beneath it, and the section to cite is eleven point one point two. At or below two thousand feet it tells you the truth every tech loves: no derate required. Done.

Altitude in one breath. Two thousand feet is the line. Above it: four percent per thousand — counted from sea level in the code text — or the A H J, or the manufacturer, who usually has the real answer. Orifices step down by the annex table, kits only. Verify by clocking against the derated target with the local heating value, and prove it with air-free CO. That closes the Methods season — ten episodes, seven calculators, and no black boxes left on the truck. Every number you punch in, you can now put on a whiteboard.