Furnaces and water heaters at altitude

8-3.1 Why does a gas appliance lose output up here?

Air thins with height. One maker's manual, A.O. Smith's BT-65, explains that there is less oxygen per cubic foot as elevation rises, and that overfiring causes sooting and poor combustion. The cure is called derating, which means running the appliance at a lower input than the sea-level figure on its plate.

That manual holds its ratings up to 2,000 feet. Above that, it says "ratings must be reduced at the rate of 4% for each 1000 feet (305 m) above sea level." Its worked example takes a 65,000 Btuh unit to 54,600 at 4,000 feet and 49,400 at 6,000 feet.

Treat those as one model's figures. The BT-65 is a commercial-style water heater, and each appliance's own manual governs. This manual found no furnace maker's table it could cite, so don't apply the example to your furnace or run your own numbers. Get the figure from the installer, out of your unit's manual.

Those are weather stations, not towns, and Cedaredge 3E sits 3 miles east and may be above the town. All five are well over the 2,000-foot line, so assume yours is too and tell the installer your actual elevation.

8-3.2 Why is setup a licensed job?

Getting it wrong can poison people. The same A.O. Smith manual says that above 2,000 feet burner parts must be changed as the National Fuel Gas Code (ANSI Z223.1, section 8.1.2) directs, and that failure can raise carbon monoxide above safe limits. Output falls too, so a unit sized from its sea-level number can come up short on a cold night.

Combustion air is part of it. CSU Extension says all oil and gas systems need combustion air and that the byproducts can be extremely hazardous. As ordinary good sense, don't store things against a furnace or water heater or block its air openings. Soot around a gas appliance is a reason to call a technician and to have §E-2 in mind.

Colorado's rules on who may do what sit in §10-2. In short, Colorado's regulator said in a December 4, 2025 addendum that homeowners may install or replace a domestic water heater on their own home if it isn't for sale, resale, a flip or a rental, and they must still get a permit and inspections. Separately, and as this manual's own advice and not the law: leave the gas connection and the venting to a licensed professional.

A permitted fuel-fired appliance replacement also brings in the alarm rule, as published in Colorado's statutes: an operational CO alarm within fifteen feet of the entrance to each sleeping room, or where a building code specifies. In Mesa County, the sample inspection card from 2023 lists separate lines for water heaters, furnaces, gas piping and venting, and forbids concealing work before inspection.

8-3.3 What should the water heater be set to?

120°F is the number. DOE calls it safe for most of the population, CSU Extension uses the same figure and says to check it at the nearest faucet, and the A.O. Smith manual calls it the preferred starting point, adding that hotter raises scald risk.

Your part is the check. Run the hot tap closest to the heater and hold a kitchen thermometer in the stream. If it's far from 120°F, ask the technician to change it at the next service. On many gas units the dial is part of the gas control, and this manual leaves that alone.

8-3.4 Expansion and the relief valve

Hot water expands, and in a closed system it has nowhere to go. The A.O. Smith manual says a check valve, pressure reducer or meter without bypass makes a closed system, and that the excess pressure can cause tank failure or a weeping relief valve. An expansion tank or similar device may be required. Clifton Water District notes that a dual check yoke may call for a thermal expansion tank, so ask your water provider too.

8-3.5 Filters and the yearly visit

Check the filter monthly and change it on the filter's schedule, within one to three months in the heating season. As ordinary practice, set the furnace's switch to off first, note the arrow on the old filter, and match it with the new one.

Book a yearly check by a trained professional. CDC asks for the same yearly service on heating and water heating equipment. Ask at that visit about sediment in the tank too, because draining hot water carries a scald risk.