Why Tankless Water Heaters Have a 2,000-Foot Elevation Limit

You’re shopping for a new gas appliance, and you see a warning in the specifications, like the one on the Ranein 8.5 GPM heater: “Not recommended above 2,000 feet elevation.”

If you live near sea level, you’d scan right past it. But if you live in Denver (5,280 ft), Albuquerque (5,312 ft), or Salt Lake City (4,226 ft), that warning is one of the most important lines in the entire manual.

It’s not a suggestion. It’s a law of physics.

To understand why, you don’t need a degree in chemistry. You just need to remember the last time you saw a campfire on a mountaintop. It’s often weak, smoky, and struggles to stay lit.

Your gas water heater is just a very carefully controlled fire in a box. And at high altitude, that fire is struggling to breathe.

The Science: Fire Needs Oxygen

Fire has three ingredients: Fuel (natural gas), Heat (the igniter), and Oxygen (from the air).

At sea level, the air is dense and packed with oxygen.
At 5,000 feet, the air is significantly “thinner.” A cubic foot of air contains about 17% less oxygen than at sea level.

Your water heater was engineered at a factory near sea level. It is precisely calibrated to mix a certain amount of gas with a certain amount of dense air to achieve “full gas combustion.”

When you install that same heater in Denver, it’s still injecting the same amount of fuel, but it’s gulping in “thinner” air that doesn’t have enough oxygen.

The Consequences: Inefficiency, Soot, and CO

When gas doesn’t get enough oxygen, it fails to burn completely. This “incomplete combustion” creates two very bad things:

1. Carbon (Soot): The unburnt fuel turns into black, sticky soot. This soot clogs the burner, coats the temperature sensors, and builds up inside the heat exchanger. The unit’s efficiency plummets, and eventually, it will fail completely.
2. Carbon Monoxide (CO): Instead of creating harmless Carbon Dioxide (CO2), the oxygen-starved flame produces Carbon Monoxide (CO), a colorless, odorless, and highly toxic gas.

This is why the problem isn’t just “my heater is less powerful.” It’s “my heater is becoming inefficient, dirty, and dangerous.”

You might think a unit with a “Powerful Blower,” like the Ranein, could solve this. But the blower can’t create oxygen that isn’t there. It’s still just blowing thin, low-oxygen air into the chamber.

The Solution: “Derating” Your Appliance

So, if you live at high altitude, are tankless heaters just not an option? Absolutely not. You just have to use the correct solution.

The industry-standard solution is called “derating.”

Since you can’t add more oxygen, you must reduce the fuel to match the thin air. A certified technician will install a “high-altitude kit” or change the orifice (the gas nozzle) to a smaller size.

The general rule, as set by the International Fuel Gas Code (IFGC), is that a gas appliance’s BTU output must be derated by 4% for every 1,000 feet above 2,000 feet.

Let’s do the math for our 190,000 BTU example unit in Denver (5,280 ft): * First 2,000 ft: No change. * Remaining altitude: 5,280 - 2,000 = 3,280 ft (we’ll call it 3.3 “thousand-foot” increments) * Derating percentage: 3.3 * 4% = 13.2% * Power Loss: 190,000 BTU * 13.2% = 25,080 BTU * Your Heater’s New Max Power: ~164,920 BTU

By “derating” the unit, the technician has restored the proper fuel-to-air ratio. Your heater will now run safely and cleanly, but with a lower maximum power output. This is why you must account for this power loss when sizing your heater (as we discussed in our GPM guide).

Alternatively, many manufacturers (including Ranein’s competitors like Rinnai) sell specific “high-altitude” models that are pre-calibrated at the factory.

That “2,000-foot” warning isn’t just fine print. It’s your first and most important clue to buying the right, safe, and efficient appliance for your high-altitude home.