The altitude story

High-altitude water heater sizing: what changes above 2,000 feet

Thinner air carries less oxygen, so gas appliances burn differently at elevation. The codes and manufacturers treat it as a real engineering constraint, not a footnote. Here's the whole story, with sources you can check.

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Flagstaff Water Heaters LLC · ROC 367669 · Arizona Registrar of Contractors · Licensed, bonded and insured

The rule itself

What the National Fuel Gas Code actually says

Section 11.1.2 of NFPA 54 (the National Fuel Gas Code) draws one line: appliance input ratings apply as printed at elevations up to 2,000 feet. Above that, input must be reduced: per the manufacturer's instructions, the code's rate, or as the local authority approves. We don't publish our own percentages, because the right adjustment is the one in your unit's manual. That is the table we size from.

Where the code line falls Diagram: from sea level to 2,000 feet, ratings apply as printed. Above 2,000 feet input is adjusted downward per manufacturer and code. Winslow and Flagstaff both sit above the 2,000-foot line. Usable input Elevation (feet above sea level) 2,000 ft as printed adjusted per manufacturer + code Winslow 4,866 Flagstaff 6,909
Every town we serve is above the National Fuel Gas Code's 2,000-foot line, Flagstaff by nearly 5,000 feet. Elevations are approximate public figures. Illustration only: the slope is exaggerated for clarity. The reduction for a given heater is the one printed in its manual.
Where models divide

Manufacturers handle altitude three different ways

This is why model selection in Flagstaff is a spec question. Three real examples from the major brands' own documentation:

Manufacturer altitude ranges vs Flagstaff Range chart from zero to nine thousand feet. A.O. Smith outdoor tankless is certified from zero to six thousand feet, which ends below Flagstaff. Rheem natural-gas high-altitude orifice kits commonly band five thousand one to eight thousand feet, which covers Flagstaff. Rinnai's dip-switch altitude band of five thousand two hundred one to seven thousand seven hundred feet also covers Flagstaff. Flagstaff at six thousand nine hundred nine feet is marked. A.O. Smith outdoor tankless: certified to 6,000 ft Rheem NG high-altitude kit: 5,001–8,000 ft band Rinnai setting for ~7,000 ft homes: 5,201–7,700 Flagstaff 6,909 02,000 4,0006,000 8,000ft
The ember line is Flagstaff. Notice the first bar ends below it: a certified ceiling is a hard stop, not a suggestion. Ranges come from the manufacturers' own documentation, and the Rinnai row shows the setting for Flagstaff-elevation homes specifically (other elevations use other bands). Always confirm against the manual for the exact model.
The same data as text
DocumentationRangeCovers Flagstaff (6,909 ft)?
A.O. Smith outdoor tankless certification0 – 6,000 ftNo; ends below Flagstaff
Rheem NG high-altitude orifice kits (common band)5,001 – 8,000 ftYes
Rinnai DIP-switch band (setting for Flagstaff-elevation homes)5,201 – 7,700 ftYes
The field errors

Two common sizing mistakes

Sea-level sizing at 7,000 feet

A unit picked off the box label runs undersized here: recovery lags, showers queue, and the unit works harder than it was picked to. The fix is sizing from the altitude table on day one.

Double-derating

The opposite error: adjusting the appliance and devaluing the fuel, stacking two corrections where the documentation calls for one. The manual specifies which adjustment applies. We follow it exactly, and the inspection checks the work.

What we actually do on every install

  • Pull the altitude table from the manual for the exact model going in.
  • Shortlist only models certified for your elevation.
  • Apply the documented adjustment: orifice kit, switch setting, or rating table.
  • Record what was set, and hand the unit to the inspector to verify.
Call (928) 592-2763 Tankless at altitude
Call (928) 592-2763