How Altitude Affects Furnaces, ACs and Heat Pumps

Most HVAC equipment sold in the U.S. is designed and rated at sea level. Put that same furnace, air conditioner or heat pump in Castle Rock or Evergreen and it behaves differently. Burners get less oxygen, fans move thinner air, and the published capacity on the label no longer matches what the system delivers.

None of this means equipment doesn't work at altitude. Millions of Coloradans heat and cool their homes just fine. It means installation has to account for elevation, and a contractor who skips that step can leave you with an undersized system, a furnace that runs rich, or both. This guide explains what changes between 5,000 and 8,000 feet, and what to ask for.

Why thin air matters

Air pressure and density fall as you climb. Air at Denver's elevation is about 15–16% less dense than at sea level. By the U.S. Standard Atmosphere, the gap grows to roughly a fifth around 8,000 feet. Three things follow:

1. Less oxygen per cubic foot for burning gas.
2. Less heat per cubic foot of air. A fan moving 1,000 cfm in Denver moves fewer pounds of air than the same fan in Houston, so it carries less heat to or from your rooms.
3. Lower pressure across gas orifices and coils, which changes how much fuel flows and how refrigerant coils transfer heat.

ACCA's Manual J load calculation includes an altitude correction factor for this. An ACHR News design example uses 0.84 at 5,000 feet for heating airflow calculations and about 0.92 for a dry cooling coil. The same article shows that carrying 36,000 Btu/h of sensible heat takes about 1,200 cfm at sea level but about 1,430 cfm at 5,000 feet. Air volume doesn't change with elevation; the heat it can carry does.

Front Range elevations

"Denver altitude" is a shorthand. The mile marker on the Capitol steps is 5,280 feet, but the metro spans well over 1,500 feet of elevation, and foothills communities sit far higher. Here's how USGS reference elevations compare, with what they mean for a furnace derate and one manufacturer's heat pump capacity factors.

*Calculated using the method in Amana and Goodman furnace manuals and the ACHR News example (4% of input per 1,000 feet of elevation once above 2,000 feet). Your furnace's manual governs; some use different schedules. **Interpolated from Fujitsu's Airstage high-altitude bulletin (heating and cooling); other brands differ. Elevations are the GNIS reference point for each town, from Wikipedia's list of Colorado municipalities by elevation (Evergreen from its Wikipedia entry). Your street can be hundreds of feet higher or lower.

Gas furnaces: combustion derate, orifices and pressure switches

Why a furnace has to be derated

A gas burner needs the right mix of fuel and air. At altitude there is less oxygen in each cubic foot of combustion air. Feed a burner its full sea-level gas rate and it runs rich: it burns incompletely, the heat exchanger runs hotter, and carbon monoxide rises. ACHR News notes that a furnace's input depends on the pressure drop across the gas orifice and on the density and oxygen content of the combustion air, and altitude affects all three.

The fix is to reduce the gas input. The rule in the manuals we reviewed:

• Amana: above 2,000 feet, derate input 4% for each 1,000 feet above sea level. Smaller orifices are needed above 3,500 feet on natural gas the supplier hasn't already derated, and a different pressure switch is required above 4,000 feet.
• Goodman: the same 4% per 1,000 feet, noting that installations above 4,500 feet usually need an orifice change and some models need a pressure switch change.
• ACHR News example: an 80,000 Btu/h-input furnace at 5,000 feet derates to 64,000 Btu/h input. At 80% steady-state efficiency, that's about 51,200 Btu/h of heat delivered.

Schedules vary by manufacturer and model, and ACHR has reported that some premium models need only 2% per 1,000 feet. Always follow the installation manual for the specific furnace.

Orifices, gas heat content and "clocking the meter"

Here's a wrinkle many homeowners never hear about. In some areas the gas utility delivers gas with a lower heating value per cubic foot at altitude, which accomplishes part of the derate before the gas reaches your furnace. Amana's manual notes that "in some areas the gas supplier will derate the gas," and Goodman tells installers to get the local heating value from the utility. That's why the right orifice depends on your actual gas, not just your elevation.

A careful installer confirms the result by clocking the gas meter: timing how long the furnace takes to burn one cubic foot and calculating actual input. Manuals warn not to derate by lowering manifold pressure below the nameplate setting, which can cause poor flame and condensation problems.

Pressure switches and high-altitude kits

Modern furnaces use an inducer fan and a pressure switch to prove the vent is drawing before the burners light. In thin air the inducer creates less pressure difference, so a sea-level switch may not close reliably. That causes nuisance lockouts on cold nights. High-altitude kits typically include a matched switch and orifices.

Colorado's NOx rules add another wrinkle. As covered in our Colorado furnace NOx rules guide, SWEEP noted that some ultra-low-NOx furnaces need a high-altitude conversion kit, estimated at $100–$200 in 2023.

Venting at altitude

Condensing furnaces (90%+ AFUE) vent through PVC pipe. Vent sizing tables in their manuals depend on pipe length, number of elbows and sometimes elevation. ACHR News reported one dealer's example in which a long vent run at altitude needed 3-inch PVC even though the furnace had a 2-inch outlet. Undersized vents can trip the pressure switch or, worse, spill exhaust. When a new condensing furnace replaces an old one that shared a chimney with the water heater, the water heater's venting may need to be resized too.

Air conditioners at altitude

An AC's outdoor coil rejects heat to outdoor air and its indoor coil absorbs heat from indoor air. Thinner air carries less heat on both sides. ACHR's example uses about 0.92 as the cooling adjustment at 5,000 feet, and says manufacturers should publish a derating factor for their equipment. ACHR's design example also illustrates the trade-off: more airflow helps capacity, but only if the ducts can move it at acceptable static pressure.

What this means for you:

• A "3-ton" AC delivers less than 3 tons at altitude unless the manufacturer's data says otherwise.
• Ducts sized by sea-level rules of thumb often can't move the extra cfm, so the AC loses capacity and the blower works harder.
• Upsizing by habit is not the answer either. An oversized system short-cycles. A load calculation with the altitude factor is.

If your AC struggles every summer, our AC not cooling guide walks through the checks. For a new system, see AC installation.

Heat pumps at altitude

Heat pumps face the same airflow penalty in both heating and cooling. The evidence:

• NREL lab tests in Golden (about 5,860 feet) measured about 5.5% less capacity for a ducted cold-climate heat pump and 7.4% less for a mini-split than at sea-level airflow, with COP down about 4.7% and 9%.
• ACCA Manual S guidance calls for about an 8–10% derate, a bit more than NREL measured.
• Fujitsu's bulletin for its Airstage heat pumps lists factors of 0.83 at 5,000 feet, 0.80 at 6,000, 0.77 at 7,000 and 0.74 at 8,000, for heating and cooling, and tells designers to size both indoor and outdoor units up accordingly.

The spread is wide, which is the point. Ask your contractor which manufacturer data they used. On top of altitude, heat pump output falls as outdoor temperature drops, so capacity at your county's design temperature (about 3–5°F for most of the metro, colder in Boulder County) matters most. Our heat pump sizing guide covers the Manual J and S steps, and Do heat pumps work in a Denver winter? covers cold-weather performance. For icing and defrost behavior, see Why is my heat pump covered in ice?

Carbon monoxide: the safety side of altitude

An over-fired or poorly adjusted furnace at altitude is a recipe for incomplete combustion, and incomplete combustion makes carbon monoxide. Soot, a yellow or lazy flame, or frequent pressure switch lockouts are warning signs. The safeguards are simple:

• A combustion analysis at installation and during annual service, measuring CO and oxygen in the flue.
• Working CO alarms near every sleeping area, which Colorado law requires in many homes after a sale or permitted work.
• Prompt attention to venting changes, especially after a furnace or water heater replacement.

Our carbon monoxide and furnace safety guide covers the law, alarm placement and what to do if an alarm sounds. MrFixer also offers a carbon monoxide safety check.

Sizing: putting it together

Good sizing at altitude follows the same ACCA sequence as anywhere else, with altitude built in at each step:

1. Manual J with your county's design temperatures and the altitude correction factor.
2. Manual S using the manufacturer's expanded performance data with any altitude derate applied, and for furnaces the derated output, not the nameplate input.
3. Manual D ducts sized for the higher airflow thin air requires.
4. Commissioning: gas input clocked, combustion tested, refrigerant charge and airflow verified.

Two practical consequences. First, an 80,000 Btu/h furnace label overstates the heat it delivers here, so compare quotes on derated output. Second, because furnaces are typically oversized anyway, the derate rarely leaves a home short of heat, but it matters for tight new homes and for heat pumps sized close to the load.

Questions to ask your contractor

• What elevation did you use, and what altitude correction factor is in the load calculation?
• What is this furnace's derated input and output at my elevation?
• Does it need an orifice change or high-altitude pressure switch with our gas supply? Will you clock the meter?
• Will you do a combustion analysis at startup and leave me the readings?
• What capacity does this heat pump or AC deliver at my elevation and design temperature, according to the manufacturer?
• Will you measure static pressure and confirm the ducts can handle the airflow?

MrFixer Heating & Cooling is based in Castle Pines, at about 6,400 feet, and serves the Denver metro and foothills. We install and repair furnaces, heat pumps, mini-splits and dual-fuel systems, with free in-home estimates and financing available. If you have a quote and aren't sure altitude was handled, ask for a free second opinion, see our service areas, or call (720) 466-7983.

Frequently asked questions

• Do furnaces need to be adjusted for Denver's altitude? Yes. Furnace manuals require reducing gas input at altitude. Amana and Goodman call for 4% per 1,000 feet of elevation above 2,000 feet, about 21% at 5,280 feet, achieved with smaller orifices, a high-altitude pressure switch, or gas the utility delivers at lower heat content. The installer should confirm by clocking the meter and testing combustion.
• What is a high-altitude kit for a furnace? A manufacturer kit that adapts a furnace to thin air, typically smaller gas orifices and a pressure switch matched to the lower inducer pressure at altitude. Whether you need one depends on the model, your elevation and your gas supply.
• How much capacity does an AC or heat pump lose at 5,000 feet? It depends on the equipment. NREL measured 5.5–7.4% less heat pump capacity in Golden, ACCA guidance suggests 8–10%, and Fujitsu lists a 0.83 factor (17% less) at 5,000 feet for its Airstage heat pumps. Use the manufacturer's data for the model you're buying.
• Is Castle Rock or Evergreen different from Denver for HVAC? Yes. Castle Rock sits about 6,217 feet and Evergreen about 7,162 feet by USGS reference elevations, compared with 5,280 in Denver. Furnace derates and heat pump capacity corrections grow with elevation, and foothills homes often have colder design temperatures.
• Can altitude cause carbon monoxide problems? A furnace that isn't derated or adjusted for altitude can burn incompletely and produce more carbon monoxide. A combustion analysis at installation and annual service, plus working CO alarms, are the safeguards.
• Does altitude change what size furnace I need? It changes what a given furnace delivers. An 80,000 Btu/h-input furnace derated for altitude produces noticeably less heat than its label suggests, so compare quotes on derated output and base the size on a Manual J load calculation with Colorado design temperatures.