Altitude BTU Derate: How to Size Furnaces and ACs for High Elevations

What “Altitude BTU Derate” Really Means

When you live or manage property at higher elevation think Denver around 5,000 feet the air is thinner. Thinner air has less oxygen and lower density. Furnaces need oxygen for clean combustion, and air conditioners rely on air density to move heat. That’s why equipment “loses” capacity at altitude. Pros call that loss a derate. For gas furnaces, a common rule of thumb is ~4% less input per 1,000 ft. So a furnace sized for sea level may be too “big on paper” but too weak in practice up high unless adjusted. Cooling equipment also drops capacity because fans move less air and heat transfer is slower. This guide walks you through how to size, select, and tune systems for altitude without guesswork. If you want a quick sizing sanity check, see The Furnace Outlet’s Sizing Guide.

Why Thin Air Changes Furnace Performance

Gas furnaces are calibrated for the oxygen and pressure found near sea level. Go higher, and the burner gets less oxygen per cubic foot of air. The result can be:

• Weaker flames and incomplete combustion

• Soot formation and nuisance safety shutoffs

• Lower usable BTU output than the nameplate suggests

Manufacturers address this with high-altitude kits or burner orifice changes that reduce fuel flow to match the oxygen available. Technicians also verify gas pressure and combustion with instruments don’t skip that step. The common practice is to derate furnace input about 4% per 1,000 ft to keep combustion safe and efficient. If you’re shopping, look for models and accessories that explicitly state altitude compatibility in the specs, or browse furnaces that note high-elevation options.

AC at Altitude: Cooling Capacity and Airflow

Air conditioners and heat pumps move heat by circulating refrigerant and pushing air across coils. At altitude, the air is less dense. Two things happen:

1. Coils exchange less heat, so cooling capacity falls.

2. Fans move fewer CFM for the same speed, and compressors work harder.

That combination can make a “perfect-on-paper” 3-ton system feel undersized in a mountain home. To help offset this:

• Favor variable-speed blowers and inverter-driven outdoor units for better control at varying loads.

• Ensure ducts are right-sized and static pressure is reasonable; small static problems hurt more up high.

• Consider modern R-32 systems with efficient coils and compressors, like residential condensers.

Denver vs. Dallas: A Plain-English Comparison

Here’s how a typical high-altitude market (Denver) differs from a low-altitude one (Dallas):

• Air pressure & oxygen: Denver is lower; Dallas is normal.

• Furnace combustion: Denver needs ~20% input derate at ~5,000 ft; Dallas needs none.

• Cooling capacity: Denver drops, Dallas delivers full nameplate capacity.

• Fan performance: Denver airflow is reduced; motors work harder.

• Energy use: Denver homes often spend 30–50% more to reach the same comfort.

• Humidity: Denver air is dry; whole-home humidifiers are commonly recommended.

Bottom line: a “3-ton, 100k BTU” spec sheet means different real-world performance at altitude. Expect to right-size and tune more carefully in Denver than in Dallas. When in doubt, ask for equipment that’s rated or adjustable for elevation and confirm the setup with a combustion/airflow test.

How to Calculate a Furnace Derate (With Example)

Use this simple approach for gas furnaces:

Step 1: Find elevation. Example: 5,000 ft.
Step 2: Apply rule of thumb. 4% × 5 (thousands) = 20% derate of input.
Step 3: Adjust input. A 100,000 BTU/h input furnace becomes 80,000 BTU/h input at setup.
Step 4: Estimate output. Multiply by AFUE. If AFUE = 96%:
0.96 × 80,000 = 76,800 BTU/h effective output.

Important notes:

• Manufacturer instructions always win over rules of thumb.

• Derate applies to input; output depends on AFUE and real-world setup.

• Coordinate with your installer to swap orifices, verify gas pressure, and perform combustion analysis.

• Shopping? Start with furnaces that list high-altitude kits.

Sizing AC/Heat Pumps Up High (Without Overbuying)

Because cooling capacity falls with altitude, you may be tempted to “just buy bigger.” A better plan is to size right and prioritize control:

• Use a Manual J load or lean on the Sizing Guide for a data-driven starting point.

• Prefer variable-speed or inverter systems to match shifting loads and keep humidity in check.

• Ask for manufacturer altitude performance data (capacity vs. elevation).

• Verify duct design and blower settings; airflow is the make-or-break variable up high.

Explore efficient, flexible options: R-32 condenser that maintain performance over a wider range of conditions.

Installation Tweaks That Pay Off at Elevation

At altitude, setup quality matters even more than equipment brand. Ask your installer to:

• Change burner orifices or apply altitude kits per the manual.

• Set gas pressure correctly and verify with combustion analysis.

• Measure static pressure and airflow (CFM); adjust blower speed or tap settings.

• Confirm duct sizing and fix choke points; small returns crush airflow up high.

• Use proper line set sizes and lengths for refrigerant performance.

Consider hardware that helps the cause:

• Variable-speed motors (ECM) to hold airflow steady

• Larger heat exchangers/coils for better transfer

If you need a turnkey solution, look at packaged units that specify altitude ranges.

Comfort Isn’t Just Temperature: Managing Dry Air

Mountain air is famously dry. Dryness makes you feel cooler in winter and less comfortable overall. It also affects wood floors, instruments, and nasal passages. What helps:

• A whole-home humidifier integrated with your furnace/air handler

• Smart thermostat control to balance humidity and temperature

• Slowing the blower slightly (when safe) to improve dehumidification in summer

Proper humidity (about 30–50% RH) reduces static shocks, eases breathing, and lets you set the thermostat a bit lower in winter for similar comfort. Ask your contractor about compatible humidification in the Accessories section and dial in settings after installation. If you’re comparing ducted vs. ductless, note that ductless mini-splits with inverter compressors can manage sensible loads efficiently, but you may still want a dedicated humidity strategy.

Energy Use: What to Expect on the Utility Bill

Because equipment works against lower air density and homes see larger temperature swings, high-altitude systems often consume 30–50% more energy for the same comfort level compared to low altitude. Plan accordingly:

• Tighten the envelope first: air seal, add insulation, upgrade windows where practical.

• Use programmable or smart thermostats and realistic schedules.

• Maintain equipment: clean filters, coils, and check blower wheels.

• Right-size the system rather than oversize; oversized units short-cycle and waste energy.

If you need help sorting through options, The Furnace Outlet’s Design Center can assist with a plan that fits your elevation and home.