Altitude Effects on Air Properties

Overview

Altitude significantly affects air properties critical to HVAC system design and performance. As elevation increases, atmospheric pressure decreases exponentially, reducing air density and altering thermodynamic behavior. These changes impact equipment capacity, fan power requirements, combustion processes, and heat transfer characteristics.

Standard atmospheric conditions are defined at sea level (elevation = 0 ft or 0 m) with a barometric pressure of 101.325 kPa (14.696 psia, 29.92 in. Hg) and temperature of 15°C (59°F). HVAC equipment rated at these conditions requires correction when applied at higher elevations.

Atmospheric Physics

Barometric Pressure Variation

Atmospheric pressure decreases with altitude due to the reduced weight of the air column above. The relationship is governed by the barometric formula derived from hydrostatic equilibrium and the ideal gas law.

Simplified Barometric Equation (U.S. Units):

P = P₀ × (1 - 6.8754 × 10⁻⁶ × Z)^5.2559

Where:

• P = atmospheric pressure at elevation Z (psia)
• P₀ = sea level pressure = 14.696 psia
• Z = elevation above sea level (ft)

Simplified Barometric Equation (SI Units):

P = P₀ × (1 - 2.25577 × 10⁻⁵ × h)^5.2559

Where:

• P = atmospheric pressure at elevation h (kPa)
• P₀ = sea level pressure = 101.325 kPa
• h = elevation above sea level (m)

Standard Atmosphere Model

The International Standard Atmosphere (ISA) defines atmospheric properties as a function of altitude. The troposphere (0 to 11,000 m or 0 to 36,089 ft) exhibits a linear temperature decrease of 6.5°C per 1000 m (3.57°F per 1000 ft) known as the environmental lapse rate.

Temperature Variation:

T = T₀ - L × h

Where:

• T = temperature at altitude h (K or °R)
• T₀ = sea level temperature = 288.15 K (518.67°R)
• L = temperature lapse rate = 0.0065 K/m (0.00357°R/ft)
• h = elevation (m or ft)

Pressure-Altitude Relationship (Troposphere):

P = P₀ × (T/T₀)^(g/(R×L))

Where:

• g = gravitational acceleration = 9.80665 m/s²
• R = specific gas constant for air = 287.05 J/(kg·K)
• g/(R×L) = 5.2559 (dimensionless exponent)

Air Density Correction

Air density decreases with altitude following the ideal gas law:

ρ = P/(R×T)

Where:

• ρ = air density (kg/m³ or lb/ft³)
• P = absolute pressure (Pa or psia)
• R = specific gas constant = 287.05 J/(kg·K) or 53.35 ft·lbf/(lb·°R)
• T = absolute temperature (K or °R)

Density Ratio:

σ = ρ/ρ₀ = (P/P₀) × (T₀/T)

Where:

• σ = density ratio (dimensionless)
• ρ₀ = sea level density = 1.225 kg/m³ (0.0765 lb/ft³)

For constant temperature assumption (conservative for HVAC calculations):

σ ≈ P/P₀

Altitude Correction Tables

Barometric Pressure vs. Elevation

Temperature Correction (Standard Atmosphere)

Performance Corrections

Fan Performance at Altitude

Fan airflow (volumetric flow rate) remains constant regardless of altitude when operating at the same pressure differential. However, mass flow rate, power consumption, and pressure capability are affected.

Mass Flow Rate:

ṁ = σ × ṁ₀

Where:

• ṁ = mass flow rate at altitude (kg/s or lb/min)
• ṁ₀ = mass flow rate at sea level
• σ = density ratio

Fan Static Pressure:

ΔP_s = σ × ΔP_s0

Where:

• ΔP_s = static pressure at altitude (Pa or in. w.g.)
• ΔP_s0 = static pressure at sea level

Brake Horsepower:

BHP = σ × BHP₀

Where:

• BHP = brake horsepower at altitude
• BHP₀ = brake horsepower at sea level

Motor Selection Implications:

At 5,000 ft elevation (σ = 0.832), a fan requiring 10 HP at sea level requires only 8.32 HP. However, motor cooling is compromised due to reduced air density. Standard motors may require derating or special cooling provisions above 3,300 ft (1,000 m) per NEMA MG 1.

Cooling Equipment Capacity

Air-cooled condensers and evaporative cooling equipment experience reduced capacity at altitude due to decreased air density and modified heat transfer coefficients.

Condenser Capacity Correction:

Q = σ^n × Q₀

Where:

• Q = capacity at altitude (Btu/hr or kW)
• Q₀ = capacity at sea level
• n = exponent typically 0.85 to 0.95 (manufacturer-specific)

Evaporative Cooler Correction:

Evaporative cooling effectiveness decreases due to:

• Reduced air mass flow through media
• Lower convective heat transfer coefficients
• Altered psychrometric properties

Capacity reduction approximates:

Q = σ × Q₀ × (1 + 0.1 × (1 - σ))

Typical capacity loss: 3-4% per 1,000 ft above 1,000 ft elevation.

Combustion Equipment Derating

Fuel-burning appliances require altitude derating due to reduced oxygen availability. The oxygen mass fraction remains constant at 23.2% by mass, but the oxygen density decreases proportionally with air density.

Altitude Derating Factors (Natural Gas):

Burner Orifice Resizing:

Gas flow through orifices follows:

Q = C × A × √(ΔP/ρ_gas)

For constant input rate at altitude, orifice area must increase:

A_altitude = A_sea level × √(P₀/P)

Alternatively, gas pressure regulators can be adjusted to compensate for altitude.

Code and Standard References

ASHRAE Standards

ASHRAE Standard 55 - Thermal Environmental Conditions:

• Defines standard atmospheric pressure for comfort calculations
• References 101.325 kPa (14.696 psia) as standard
• No explicit altitude adjustments for comfort criteria

ASHRAE Handbook - Fundamentals, Chapter 1:

• Standard atmosphere properties tabulated to 30,000 ft
• Psychrometric chart corrections for altitude
• Air property equations and correlation factors

ASHRAE Standard 37 - Methods of Testing for Rating Electrically Driven Unitary Air-Conditioning and Heat Pump Equipment:

• Standard rating conditions at sea level
• Altitude correction procedures for capacity and efficiency

Building Codes

International Mechanical Code (IMC):

• Section 304.5: Altitude compensation for combustion air
• Section 915: Venting adjustments for reduced barometric pressure
• Table 304.5.1: Combustion air openings sized for altitude

International Fuel Gas Code (IFGC):

• Section 303.6: Altitude derating of fuel-burning appliances
• Appendix A: Altitude-corrected vent tables

NFPA 54 (National Fuel Gas Code):

• Section 3.6: Gas equipment altitude adjustments
• Requires derating above 2,000 ft elevation

Equipment Standards

ANSI Z21 Series (Gas Appliance Standards):

• Equipment tested and listed for specific altitude ranges
• Common ranges: 0-2,000 ft, 2,000-4,500 ft, 4,500+ ft
• Field conversion kits required for different altitude ranges

NEMA MG 1 (Motors and Generators):

• Section 14.4.2: Service factor adjustments for altitude
• Standard motors rated to 3,300 ft (1,000 m)
• Derating of 1% per 330 ft (100 m) above 3,300 ft
• Temperature rise increases due to reduced cooling

Design Considerations

System Sizing

Heating Equipment:

• Select boilers and furnaces with altitude-appropriate ratings
• Account for 4% capacity loss per 1,000 ft above 2,000 ft (gas)
• Verify combustion air calculations per IMC requirements
• Consider condensing equipment less affected by altitude

Cooling Equipment:

• Apply manufacturer-specific correction factors
• Increase equipment size to compensate for reduced capacity
• Verify fan motor selection accounts for both power reduction and cooling limitation
• Consider evaporative cooling effectiveness reduction

Ventilation Systems:

• Design for volumetric flow rate requirements (CFM or L/s unchanged)
• Recognize reduced mass flow impacts contaminant dilution
• Adjust energy recovery equipment for density changes
• Size exhaust fans for reduced static pressure capability

Ductwork and Piping

Friction Loss:

Friction loss in ducts remains primarily velocity-dependent. For constant volumetric flow:

ΔP_f = f × (L/D) × (ρ × V²/2)

At altitude:

• Velocity (V) unchanged for same CFM
• Density (ρ) reduced
• Friction pressure drop (ΔP_f) reduced proportionally

Velocity Pressure:

VP = ρ × V²/2 = σ × VP₀

Velocity pressure readings decrease with altitude for identical velocities, affecting:

• Pitot tube measurements
• Terminal device selection
• Duct fitting loss coefficients (total pressure basis)

Control Strategies

Pressure-Based Controls:

• Duct static pressure setpoints should be maintained in absolute terms
• Differential pressure controls unaffected (measure pressure difference)
• Barometric pressure compensation required for absolute pressure sensors

Air Quality Controls:

• CO₂ sensors measure concentration (ppm) unaffected by altitude
• Contaminant dilution requires same volumetric ventilation rates
• Mass-based calculations (lb/hr, kg/s) must account for density reduction

Temperature Controls:

• Psychrometric processes follow reduced pressure relationships
• Saturation pressure of water vapor follows Antoine equation
• Humidity ratio (lb_water/lb_dry air) calculations require local barometric pressure

Psychrometric Effects

Moisture-Carrying Capacity

At reduced pressure, air’s moisture-carrying capacity increases for a given dry-bulb temperature.

Saturation Humidity Ratio:

W_s = 0.622 × (P_ws / (P - P_ws))

Where:

• W_s = saturation humidity ratio (lb_water/lb_dry air)
• P_ws = saturation pressure of water at dry-bulb temperature (psia)
• P = barometric pressure at altitude (psia)

At 5,000 ft (P = 12.228 psia) and 75°F (P_ws = 0.4298 psia):

W_s = 0.622 × (0.4298 / (12.228 - 0.4298)) = 0.0227 lb_w/lb_da

At sea level (P = 14.696 psia) and 75°F:

W_s = 0.622 × (0.4298 / (14.696 - 0.4298)) = 0.0187 lb_w/lb_da

Increase in moisture capacity: 21.4%

Enthalpy Calculations

Moist air enthalpy:

h = c_p × T + W × h_fg

Where:

• h = enthalpy (Btu/lb_da or kJ/kg_da)
• c_p = specific heat of dry air = 0.240 Btu/(lb·°F)
• T = dry-bulb temperature (°F)
• W = humidity ratio (lb_water/lb_dry air)
• h_fg = latent heat of vaporization ≈ 1061 Btu/lb at 85°F

Altitude affects W but not the fundamental enthalpy equation.

Psychrometric Chart Corrections

Standard psychrometric charts are generated for sea level pressure (14.696 psia). For high-altitude applications:

1. Use altitude-specific charts (available for standard elevations)
2. Apply corrections to standard chart
3. Utilize psychrometric software with barometric pressure input

ASHRAE provides charts for:

• Sea level (14.696 psia)
• 2,500 ft (13.91 psia)
• 5,000 ft (12.23 psia)
• 7,500 ft (10.91 psia)

Combustion Analysis

Stoichiometric Requirements

Complete combustion requires specific air-to-fuel ratios by mass. At altitude, volumetric flow must increase to maintain mass flow.

Natural Gas (CH₄) Stoichiometry:

CH₄ + 2O₂ → CO₂ + 2H₂O

Stoichiometric air requirement: 17.2 lb_air/lb_fuel

Volumetric Flow Correction:

Q_air,altitude = Q_air,sea level × (1/σ) = Q_air,sea level × (P₀/P)

At 5,000 ft (σ = 0.832), combustion air volume increases by 20% for the same mass flow and heat input rate.

Excess Air Requirements

Excess air percentages typically increase at altitude to ensure complete combustion:

Flue Gas Venting

Draft pressure in natural draft venting systems decreases with altitude:

ΔP_draft = ρ_outdoor × g × h × (1 - T_outdoor/T_flue)

Reduced outdoor air density (ρ_outdoor) decreases buoyancy force, requiring:

• Taller chimneys or vent stacks
• Power venting systems for reliable operation
• Oversized venting per altitude-corrected tables

Practical Applications

Equipment Selection Checklist

High-Altitude Installations (>3,000 ft):

1. Verify equipment altitude rating and applicable range
2. Apply manufacturer capacity correction factors
3. Select burners with altitude-appropriate orifices or kits
4. Confirm motor service factors and cooling provisions
5. Size ductwork for volumetric flow requirements
6. Adjust temperature and pressure setpoints
7. Specify altitude-appropriate psychrometric charts
8. Verify combustion air provisions per local codes
9. Calculate system performance at design elevation
10. Document altitude corrections on design drawings

Field Testing Adjustments

Air Balancing:

• CFM measurements remain standard practice
• Velocity pressure corrections: VP_corrected = VP_measured × (P₀/P)
• Pitot tube traverse calculations unaffected (velocity computed correctly)
• Capture hood measurements compensate internally

Combustion Testing:

• CO₂ readings interpreted identically (concentration-based)
• Draft measurements reflect actual reduced values
• Oxygen readings require altitude-aware analyzers
• Temperature differential measurements unchanged

Pressure Measurements:

• Static pressure differentials directly comparable to design
• Absolute pressure references require altitude baseline
• Manometer readings in inches water column (in. w.g.) are direct

Summary

Altitude fundamentally alters HVAC system design and performance through reduced atmospheric pressure and air density. Key impacts include:

1. Pressure: Exponential decrease following barometric equation (4% per 1,000 ft)
2. Density: Proportional decrease affecting mass flow and thermal capacity
3. Combustion: Oxygen availability reduces, requiring equipment derating (4% per 1,000 ft above 2,000 ft)
4. Fans: Power requirements decrease but motor cooling compromised
5. Heat Transfer: Air-side capacity diminishes with reduced density
6. Psychrometrics: Moisture capacity increases at elevation
7. Venting: Natural draft reduced, requiring taller stacks or mechanical assist

Proper altitude compensation ensures code compliance, occupant comfort, and system efficiency in high-elevation installations. Design engineers must apply appropriate correction factors from equipment manufacturers and verify selections against applicable standards including ASHRAE Handbook data, IMC requirements, and NEMA motor ratings.

References

• ASHRAE Handbook - Fundamentals, Chapter 1: Psychrometrics
• ASHRAE Standard 55: Thermal Environmental Conditions for Human Occupancy
• International Mechanical Code (IMC), Chapter 3: General Regulations
• NEMA Standards Publication MG 1: Motors and Generators
• International Standard Atmosphere (ISO 2533)
• NFPA 54: National Fuel Gas Code
• ASHRAE Standard 37: Methods of Testing for Rating Unitary Equipment