Quick Answer: Gas tankless water heaters lose roughly 4% of combustion capacity per 1,000 feet of elevation because thinner mountain air holds less oxygen for the burner — but the altitude where manufacturers require you to actually do something about it isn’t a single number. Rinnai’s factory default covers 0-2,000 feet (610m) before a DIP switch or altitude kit is needed, while Rheem’s units run unadjusted up to 3,280 feet (1,000m). Electric tankless units sidestep the combustion issue entirely, since there’s no flame to derate, though colder groundwater at elevation still raises the temperature-rise demand the same way it does in any cold climate.
Every brand roundup on this site treats GPM as climate-dependent, never an absolute number — and altitude is the piece of that equation buyers in Denver, Boise, Flagstaff, and dozens of other Mountain West cities run into that flatland installers rarely think about. A unit that fires perfectly at sea level can be running underpowered, or in the worst case producing excess carbon monoxide from incomplete combustion, at 6,000-7,000 feet if it was never adjusted for the elevation.
Why altitude derates a gas burner in the first place
Combustion needs a precise mix of fuel and oxygen. As elevation climbs, air pressure drops and so does the concentration of oxygen available to the burner — the same volume of air simply contains less of what the flame needs. Manufacturers and installers use a standard industry rule of thumb: about a 4% loss in burner capacity per 1,000 feet of elevation gain. At 5,000+ feet — a starting elevation for cities like Denver — that stacks up to roughly a 20% loss in effective BTU output if the unit is running with an unadjusted factory orifice and gas valve setting meant for near-sea-level air.
| Elevation | Approx. combustion capacity loss (unadjusted) |
|---|---|
| 2,000 ft (610m) | ~8% |
| 5,000 ft (1,524m) | ~20% |
| 7,000 ft (2,134m) | ~28% |
| 10,000 ft (3,048m) | ~40% |
Figures scaled from the widely-used 4%-per-1,000-ft combustion-derate rule applied to gas-fired appliances generally. Exact loss varies by unit and burner design — always confirm against your specific model’s altitude table rather than this general rule alone.
Why “high altitude” isn’t the same threshold across brands
This is the detail that trips up buyers who assume altitude adjustment is a single universal cutoff. It isn’t — it’s set per manufacturer, and sometimes per model line within the same manufacturer.
| Brand | Unadjusted range | Above that |
|---|---|---|
| Rinnai (most models) | 0-2,000 ft (610m) | DIP switch or altitude table adjustment required |
| Rinnai RHFE-201FA (outdoor) | 0-10,000 ft (3,048m) | No adjustment needed on this specific model |
| Rheem (most tankless models) | 0-3,280 ft (1,000m) | DIP switch adjustment required |
A home at 2,500 feet — a real elevation for cities like Reno or Colorado Springs’ lower neighborhoods — clears Rheem’s unadjusted threshold with zero changes needed, but the exact same house needs Rinnai’s altitude adjustment procedure for a Rinnai unit, per Rinnai’s own published “high altitude is above 2,000 ft” definition. See our Rinnai vs. Rheem comparison for the rest of how these two brands differ before assuming altitude behaves the same way across both.
It also isn’t uniform within one brand’s own lineup: Rinnai’s outdoor RHFE-201FA is rated to run unadjusted all the way to 10,000 feet, while other Rinnai indoor models default to the same 0-2,000-foot DIP switch range referenced above. Reusing an altitude rating from one model number for a different model, even from the same manufacturer, is a documented way installs come up short — the same “confirm against your specific model” pattern that applies to breaker counts on our electrical requirements guide and BTU input on our gas line sizing guide.
Electric tankless: no combustion derate, but groundwater still matters
Electric tankless units heat water with resistance elements, not a gas flame, so there’s no combustion air problem and no altitude-based capacity loss to plan around — a genuine advantage for a mountain-elevation install where running gas line is also often more expensive.
That doesn’t make elevation irrelevant to an electric install, though. Groundwater temperature at altitude tends to run colder, the same variable that already drives this site’s core sizing rule: GPM output is climate-dependent, never an absolute number. A high-altitude home with 40°F groundwater demands more temperature rise from the same electric unit than a lowland home with 60°F groundwater, even though neither elevation nor cold groundwater touches the unit’s rated kW capacity the way combustion derating touches a gas unit’s rated BTU. See our electric vs. gas tankless comparison for how the two technologies handle cold-climate temperature rise differently.
Rinnai High Altitude Kit / Natural Gas Orifice Set
- Addresses the exact gap this guide covers — Rinnai's factory DIP switch default only covers 0-2,000 ft, and most models need this kit or the equivalent DIP switch reset above that line.
- Confirm your exact model number's altitude table before ordering — the RHFE-201FA outdoor model is rated to 10,000 ft unadjusted while other Rinnai models are not, and using the wrong model's kit won't match your unit's orifice sizing.
- Installation should be performed or verified by a licensed technician; an incorrectly sized orifice at altitude is a documented cause of incomplete combustion and elevated carbon monoxide output.
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Combustion Analyzer / CO Meter (Gas Appliance Diagnostic)
- Directly verifies the thing altitude derating actually threatens — incomplete combustion and elevated CO output — rather than just assuming an installed altitude kit solved the problem.
- Especially relevant above 5,000 ft, where an unadjusted burner can lose roughly 20% of its rated combustion capacity per the derate table above even while still igniting and appearing to run normally.
- Standard tool for any licensed gas technician doing high-altitude commissioning; worth having on hand for the final verification step even if the adjustment itself is professionally installed.
How to choose
- Know your exact elevation, then check your specific model’s altitude table. Don’t assume a flat 2,000-foot rule applies across brands — Rheem’s own unadjusted range runs to 3,280 ft, a full 1,280 ft higher than Rinnai’s default cutoff.
- Above 2,000-3,280 ft depending on brand, budget for an altitude kit or DIP switch adjustment as part of the install, not an optional extra — see our installation guide for how this fits into overall install cost.
- Going electric sidesteps the combustion-derate question entirely, but still size GPM for your actual (likely colder) groundwater temperature — see our best electric tankless water heater picks and our sizing guide.
- Never reuse one model’s altitude rating for a different model, even within the same brand — Rinnai’s own lineup spans a 0-2,000-ft default DIP switch range on some models to a 10,000-ft unadjusted rating on others.
- Verify combustion quality after any altitude adjustment, especially above 5,000 ft where the unadjusted capacity loss reaches roughly 20% — see our troubleshooting guide if you suspect an ignition or performance issue post-install.
The bottom line
Altitude doesn’t have one universal cutoff for tankless water heaters — it has a brand-specific one, and sometimes a model-specific one within the same brand. Rinnai’s factory default stops at 2,000 feet; Rheem’s stops at 3,280 feet; a handful of specific models like Rinnai’s outdoor RHFE-201FA are rated to run unadjusted all the way to 10,000 feet. The physics behind all of it is consistent — roughly a 4% combustion-capacity loss per 1,000 feet of elevation for any unadjusted gas burner — but the point at which a manufacturer requires you to actually do something about it depends on the exact model in front of you, not a rule of thumb from a different unit. Electric tankless heaters skip the combustion question entirely, though colder high-altitude groundwater still raises the temperature-rise demand the same way it does in any cold climate. Pull your specific model’s altitude table before you install, not after a burner starts running underpowered at 7,000 feet.