Gas water heaters are designed and rated at sea level, and the Denver metro sits about a mile up. A gas heater here has to be built for the altitude or set up for it.
Why altitude matters
A gas burner mixes gas with air, and the air is thinner up here. Water heater makers deal with that in one of two ways: a tank model is tested and certified up to a maximum altitude, or a tankless model is set for the altitude when it's installed. Get it wrong and the burner doesn't burn the way it was designed to. Rheem warns that running a heater above its rated altitude can produce "carbon monoxide gas in excess of acceptable limits."
Tank water heaters: check the label
Most gas tank heaters sold here are certified well above Denver's elevation, but ratings vary by brand and model, and the heater's own label is what counts. Rheem's manual says to check the altitude label for the maximum installation altitude; Bradford White's point to the rating plate.
- A.O. Smith and State ProLine atmospheric models: approved from sea level to 10,100 feet.
- Rheem Performance Plus: every model certified up to 6,000 feet, and some up to 10,200 feet.
- Bradford White: its manuals say the heater was built for sea level to 2,000 feet unless the rating plate says otherwise, so a Bradford White installed in the metro should have a plate showing a higher rating.
Buying a heater online, or from a store outside Colorado? Check the altitude rating on the spec sheet before you buy, and on the label when it arrives.
Tankless water heaters: the altitude setting
A gas tankless heater has to be told how high it is, and the factory setting is for low altitude.
- Rinnai RE series: the default setting is for 0 to 2,000 feet. The installer changes it to setting 02b for 2,001 to 5,400 feet, or 02C for 5,401 to 7,700 feet.
- Navien NPE-A2 and NPE-S2: the altitude is set in the installer menu, and above 5,400 feet the heater needs the high-altitude conversion kit that comes with it.
Navien's conversion guide warns that failing to set up the altitude properly "could cause carbon monoxide poisoning." If you already have a tankless heater, it's worth confirming the altitude setting the next time it's serviced or descaled.
The 5,400-foot line runs through the metro
For tankless heaters, 5,400 feet is where the settings change, and the metro sits on both sides of it. These are the elevations at each city's USGS reference point:
| City | USGS elevation | Above 5,400 ft? |
|---|---|---|
| Commerce City | 5,161 ft | No |
| Denver | 5,276 ft | No |
| Boulder | 5,319 ft | No |
| Arvada | 5,348 ft | No |
| Thornton | 5,348 ft | No |
| Littleton | 5,351 ft | No |
| Northglenn | 5,371 ft | No |
| Westminster | 5,381 ft | No |
| Broomfield | 5,391 ft | No |
| Aurora | 5,404 ft | Yes |
| Wheat Ridge | 5,460 ft | Yes |
| Lakewood | 5,519 ft | Yes |
| Golden | 5,673 ft | Yes |
| Highlands Ranch | 5,696 ft | Yes |
| Centennial | 5,837 ft | Yes |
| Parker | 5,870 ft | Yes |
Each city covers a range of elevations, so your house can sit well above or below its city's reference point. The USGS elevation tool gives the elevation of any point on the map.
Electric water heaters
Electric water heaters heat with elements, not a flame, so there's no altitude rating or setting to check.
Carbon monoxide alarms
A gas water heater is one more reason to keep working carbon monoxide alarms. Colorado law requires an operational CO alarm within 15 feet of the entrance to each bedroom when a home with a fuel-fired appliance, a fireplace, or an attached garage is sold, and when a rental like that gets a new tenant.
Looking at a new heater, or not sure about the one you have? Text a photo of its label to (720) 818-5569. Water heater repair and replacement →
Sources: A.O. Smith ProLine spec sheet · State ProLine spec sheet · Rheem Performance Plus · Rheem installation manual · Bradford White Defender manual · Bradford White Ultra Low NOx manual · Rinnai RE series · Navien high-altitude conversion guide · Colorado Revised Statutes 38-45-102 to 104 · USGS elevations. Checked October 22, 2026.