HVAC Load Estimation: Infiltration Heat Gain and Loss

Infiltration represents uncontrolled air exchange between outdoor and indoor environments, significantly impacting both heating and cooling loads in all building types. Accurate infiltration estimation is essential for proper HVAC system sizing, energy consumption prediction, and maintaining desired indoor environmental conditions.

Essential Infiltration Standards

Professional HVAC engineers utilize comprehensive infiltration databases to ensure accurate load calculations and effective building envelope design strategies.

Core Infiltration References

Standard	Section	Pages	Coverage Focus
2017 ASHRAE Fundamentals	Section 18.3	483, 484	Infiltration load calculation methods and factors
2017 ASHRAE Fundamentals	Sections 16.11, 16.13, Tables 10, 11	441, 445	Air leakage measurement and building envelope performance
2005 CIBSE Guide B	Section 1.3, Table 1.5	14	European infiltration standards and calculation methods
2006 CIBSE Guide A Environmental Design	Section 4.7, Tables 4.13-4.17	160, 161	Building air tightness and infiltration modeling
Carrier Part 1 Load Estimating	Chapter 06, Tables 43, 44	94-96	Practical infiltration applications for load calculations

Fundamental Infiltration Concepts

Driving Forces for Air Movement

Infiltration occurs due to pressure differentials created by multiple natural and mechanical forces:

Wind pressure effects:

• Windward surfaces: Positive pressure driving air into buildings
• Leeward surfaces: Negative pressure drawing air from buildings
• Wind velocity correlation: Pressure increases with square of wind speed
• Building geometry: Height, orientation, and shape influence pressure distribution

Stack effect (thermal buoyancy):

• Temperature differential: Indoor-outdoor temperature differences create buoyancy forces
• Building height: Taller buildings experience greater stack effect
• Neutral pressure plane: Level where indoor and outdoor pressures are equal
• Seasonal variations: Heating season upward flow, cooling season potential reversal

Mechanical system impacts:

• Supply-return imbalances: Fan systems creating building pressurization or depressurization
• Exhaust systems: Local exhaust creating negative pressure zones
• HVAC operation: System cycling affecting building pressure relationships

Air Leakage Pathways

Building envelope penetrations provide paths for uncontrolled air movement:

Primary leakage areas:

• Windows and doors: Frame-to-wall interfaces and operating components
• Wall penetrations: Electrical, plumbing, and mechanical system pass-throughs
• Building joints: Wall-to-roof, wall-to-foundation, and construction material interfaces
• Envelope discontinuities: Architectural features and building system integrations

ASHRAE Infiltration Calculation Methods

Section 18.3 Load Calculation Applications

ASHRAE methodology provides multiple approaches for infiltration load estimation:

Air change method:

• ACH assumption: Typical values by building type and construction quality
• Load calculation: Q = V × ACH × ΔT × 1.08 (sensible) + V × ACH × Δω × 0.68 (latent)
• Simplicity advantage: Easy application with minimal building-specific data required
• Accuracy limitations: May not reflect actual building performance

Crack method:

• Leakage area calculation: Sum of individual envelope component leakage areas
• Pressure differential application: Correlation between pressure and airflow rates
• Component-specific factors: Different leakage characteristics for windows, walls, doors
• Enhanced accuracy: Better representation of actual building envelope performance

Tables 10 and 11 Air Leakage Data

Comprehensive leakage area data for various building components:

Window leakage rates:

• Residential windows: 0.1-0.6 cfm/ft of crack at 25 Pa pressure differential
• Commercial windows: 0.06-0.37 cfm/ft² of window area at 75 Pa
• Window quality impact: High-performance windows significantly reduce leakage
• Installation quality: Poor installation can increase leakage by 200-300%

Wall and envelope leakage:

• Frame construction: 0.3-3.0 cfm/ft² of wall area at 75 Pa
• Masonry construction: 0.1-1.5 cfm/ft² of wall area at 75 Pa
• Insulation impact: Continuous air barriers significantly reduce leakage rates
• Construction quality: Workmanship critically affects air tightness performance

CIBSE European Standards

European Air Tightness Approach

CIBSE standards reflect European emphasis on building envelope performance:

Table 1.5 applications:

• Air permeability classifications: Building categories based on measured air tightness
• Design infiltration rates: Conservative assumptions for various construction types
• Testing requirements: Mandatory blower door testing for many building types
• Performance targets: Specific air tightness goals for energy efficiency compliance

European Calculation Methodology

Advanced infiltration modeling incorporates:

Tables 4.13-4.17 comprehensive data:

• Climate-specific factors: Regional wind patterns and temperature differentials
• Building geometry corrections: Height, exposure, and orientation adjustments
• Construction detail impacts: Thermal bridging and air barrier continuity effects
• Seasonal variations: Different infiltration rates for heating and cooling seasons

Quality assurance integration:

• Measured vs. predicted: Correlation between design assumptions and testing results
• Performance verification: Post-construction air tightness validation requirements
• Continuous improvement: Building performance feedback into design practices

Carrier Load Calculation Applications

Practical Implementation Methods

Tables 43 and 44 provide application-oriented infiltration data:

Building type considerations:

• Residential buildings: 0.35-1.0 ACH typical infiltration rates
• Commercial buildings: 0.1-0.5 ACH depending on construction and maintenance
• Industrial buildings: 0.5-2.0 ACH reflecting operational and construction factors
• High-performance buildings: <0.1 ACH achievable with proper design and construction

Load Calculation Integration

Systematic infiltration assessment:

1. Building characterization: Construction type, age, and envelope condition evaluation
2. Climate data application: Local wind patterns and temperature differential analysis
3. Pressure relationship assessment: HVAC system impacts on building pressurization
4. Seasonal variation consideration: Different infiltration rates throughout the year

Infiltration Load Impact Analysis

Sensible Heat Load Effects

Temperature-driven infiltration loads:

Heating season impacts:

• Cold air infiltration: Outdoor air requiring heating to indoor temperature
• Calculation method: Qs = 1.08 × CFM × (Ti – To)
• Peak load considerations: Design day temperature differentials
• System sizing implications: Infiltration can represent 20-40% of total heating load

Cooling season impacts:

• Hot air infiltration: Outdoor air requiring cooling to indoor temperature
• Humidity load interaction: Combined sensible and latent cooling requirements
• System capacity effects: Peak infiltration during high wind conditions

Latent Heat Load Effects

Moisture-driven infiltration loads:

Humidity ratio differentials:

• Load calculation: Ql = 0.68 × CFM × (ωo – ωi)
• Climate dependency: Significant in humid climates during cooling season
• Dehumidification requirements: Additional system capacity for moisture removal
• Indoor air quality impacts: Humidity control and comfort considerations

Modern Infiltration Control Strategies

Building Envelope Improvements

Air tightness enhancement methods:

Construction details:

• Continuous air barriers: Unbroken envelope sealing systems
• Advanced window systems: High-performance fenestration with superior sealing
• Penetration sealing: Comprehensive sealing of all envelope openings
• Quality assurance: Construction oversight and testing verification

Mechanical System Integration

Controlled ventilation strategies:

Balanced ventilation approach:

• Energy recovery ventilation: Heat and moisture exchange between exhaust and supply air streams
• Demand-controlled ventilation: Variable outdoor air based on occupancy and indoor air quality
• Building pressurization control: Slight positive pressure to reduce infiltration
• Zoned ventilation: Different ventilation strategies for different building areas

Advanced Infiltration Modeling

Computer Simulation Methods

Sophisticated analysis tools:

Multizone modeling:

• Building pressure calculations: Detailed pressure relationships throughout buildings
• Dynamic analysis: Hourly infiltration variations based on weather and operation
• System interaction modeling: HVAC system impacts on building air movement
• Energy consumption prediction: Annual energy use including infiltration effects

Performance-Based Design

Measured performance integration:

Blower door testing:

• Design validation: Confirming predicted vs. actual building envelope performance
• Commissioning tool: Identifying and correcting air leakage problems
• Continuous monitoring: Long-term building envelope performance tracking
• Retrofit optimization: Targeted improvements based on measured performance

Quality Assurance and Verification

Design Validation Methods

Infiltration load accuracy requires systematic verification:

Validation procedures:

• Building envelope assessment: Detailed review of construction documents and specifications
• Climate data verification: Confirming appropriate weather data for calculations
• System interaction analysis: Understanding HVAC system impacts on infiltration
• Performance testing: Post-construction verification of actual vs. predicted infiltration rates

Ongoing Performance Management

Long-term infiltration control:

Maintenance considerations:

• Envelope degradation: Age-related increases in air leakage rates
• Seal replacement: Periodic renewal of weatherstripping and caulking
• System rebalancing: Maintaining proper building pressurization
• Performance monitoring: Tracking energy consumption patterns indicating infiltration changes

Accurate infiltration estimation remains fundamental to successful HVAC system design, directly impacting equipment sizing, energy consumption, and indoor environmental quality while providing opportunities for significant energy savings through improved building envelope performance.