Table of Contents
Supply vs Exhaust Ventilation: Complete Airflow Strategy Comparison
Quick Verdict
Supply and exhaust ventilation serve complementary purposes—neither is complete alone.
Bottom Line: Modern buildings require both supply and exhaust ventilation working together. Supply ventilation delivers clean, filtered outdoor air and creates positive pressure preventing uncontrolled infiltration. Exhaust ventilation removes contaminated air at the source and maintains negative pressure in spaces like kitchens, bathrooms, and laboratories. The key is proper balance: typically 5-10% more supply than exhaust to maintain slight positive building pressure while ensuring exhaust from all contaminated zones.
At-a-Glance Comparison Table
| Feature | Supply Ventilation | Exhaust Ventilation | Winner |
|---|---|---|---|
| Building Pressure | Positive (prevents infiltration) | Negative (draws infiltration) | Supply |
| Air Filtration | Filters outdoor air | Cannot filter makeup air | Supply |
| Contaminant Control | Dilution only | Source capture | Exhaust |
| Heat Recovery | Possible with HRV/ERV | Possible with HRV/ERV | Tie |
| Humidity Control | Can dehumidify supply | Cannot control infiltration | Supply |
| System Cost | Higher (conditioning) | Lower (exhaust only) | Exhaust |
| Best For | Clean spaces, offices | Kitchens, labs, bathrooms | — |
How Supply and Exhaust Ventilation Work
Understanding airflow direction and pressure effects is fundamental to system design.
Technical Note: Building pressure is the difference between indoor and outdoor pressure. Positive pressure (supply > exhaust) pushes air out through cracks; negative pressure (exhaust > supply) draws air in through cracks. Neither condition is inherently better—appropriate pressure depends on the application.
Supply Ventilation
Supply ventilation uses fans to push outdoor air into the building:
Airflow path:
- Outdoor air drawn through intake
- Air filtered and conditioned (optional)
- Air distributed to occupied spaces
- Air exits through exhaust openings, envelope leaks, or relief dampers
Pressure effect:
- Supply > Exhaust = Positive pressure
- Air flows outward through envelope cracks
- Prevents uncontrolled infiltration
Benefits:
- Controls air entry point and quality
- Enables filtration and conditioning
- Prevents moisture infiltration (hot-humid climates)
- Provides consistent outdoor air delivery
Exhaust Ventilation
Exhaust ventilation uses fans to remove air from the building:
Airflow path:
- Air removed from contaminated zones
- Makeup air enters through intakes or envelope leaks
- No direct control over makeup air quality
- Negative pressure maintained in exhausted zones
Pressure effect:
- Exhaust > Supply = Negative pressure
- Air flows inward through envelope cracks
- Draws outdoor air through uncontrolled paths
Benefits:
- Captures contaminants at source
- Prevents contaminant spread to adjacent spaces
- Simple, lower-cost systems
- Essential for odor/fume control
Balanced Ventilation
Balanced systems provide equal supply and exhaust:
True balanced (HRV/ERV):
- Separate supply and exhaust fans
- Heat exchange between airstreams
- Minimal pressure differential
- Maximum energy recovery
Quasi-balanced (typical commercial):
- Slight supply-dominant (105-110%)
- Positive overall pressure
- Local exhaust from contaminated zones
- Practical approach for most buildings
Pressure Control: The Strategic Decision
Building pressurization strategy drives system design.
Positive Pressure Benefits
| Benefit | Explanation | Application |
|---|---|---|
| Infiltration prevention | Outdoor air enters through controlled intakes, not cracks | All conditioned buildings |
| Moisture control | Prevents humid outdoor air from entering envelope | Hot-humid climates |
| Dust control | Reduces particle entry through envelope | Clean environments, data centers |
| Consistent ventilation | Known supply rate vs uncertain infiltration | Code compliance |
| Energy efficiency | Conditioned air not lost to stack effect | High-rise buildings |
Positive pressure targets:
- Standard commercial: 0.02-0.05" WG (5-12 Pa)
- Healthcare: 0.03-0.05" WG (7.5-12 Pa)
- Clean rooms: 0.05-0.10" WG (12-25 Pa)
Negative Pressure Benefits
| Benefit | Explanation | Application |
|---|---|---|
| Contaminant containment | Prevents spread to adjacent spaces | Labs, isolation rooms |
| Odor control | Captures odors before they spread | Kitchens, bathrooms |
| Process exhaust | Maintains capture at source | Industrial processes |
| Infection control | Prevents airborne pathogen spread | Healthcare isolation |
Negative pressure targets:
- Bathrooms: 0.01-0.03" WG relative to corridor
- Kitchens: 0.03-0.05" WG relative to dining
- Isolation rooms: 0.01-0.03" WG relative to corridor (minimum 3 Pa)
- Laboratories: 0.03-0.10" WG relative to corridor
Pressure Cascade Strategy
Verdict: Pressure Control
Winner: Application-Dependent — Positive pressure is correct for clean zones and most building areas; negative pressure is correct for contaminated zones. The strategy is a design choice based on space use, not a universal recommendation.
Air Quality Control
Supply Ventilation Air Quality
Control over incoming air:
- Filtration: MERV 8-16 typical, HEPA for critical
- Conditioning: Heating, cooling, dehumidification
- Consistent delivery: Known CFM regardless of weather
- Distribution: Directed to occupied zones
Limitations:
- Cannot capture contaminants at source
- Dilution ventilation only for indoor pollutants
- Requires makeup air path for local exhaust
Exhaust Ventilation Air Quality
Control over outgoing air:
- Source capture: Removes contaminants where generated
- Prevents spread: Negative pressure contains pollutants
- Direct removal: Kitchen fumes, bathroom odors, lab vapors
Limitations:
- No control over makeup air quality
- Makeup enters through envelope (unfiltered)
- Infiltration path unknown and variable
Combined Strategy Example
Verdict: Air Quality
Winner: Combined — Supply controls what enters (filtration, conditioning); exhaust controls what leaves (source capture). Both are required for complete air quality control.
Energy Considerations
Supply Ventilation Energy
Energy components:
- Fan power: Moving outdoor air into building
- Conditioning: Heating/cooling outdoor air to supply temperature
- Distribution: Duct pressure losses
Heat recovery opportunity:
- With HRV/ERV: 70-85% of exhaust energy recovered
- Reduces conditioning load significantly
- Requires balanced airflow (supply ≈ exhaust)
Exhaust Ventilation Energy
Energy components:
- Fan power: Moving exhaust air out
- Lost conditioned air: Energy in exhausted air is wasted
- Makeup air conditioning: If makeup is provided
Energy characteristics:
- Lower fan energy than supply (simpler systems)
- No heat recovery unless paired with supply
- Makeup air through envelope = no conditioning control
Energy Comparison
| Scenario | Supply Energy | Exhaust Energy | Notes |
|---|---|---|---|
| Exhaust only | 0 | Medium | Makeup uncontrolled, no recovery |
| Supply only | High | 0 | Can filter/condition, no recovery |
| Balanced (no recovery) | Medium | Medium | Controlled, but no recovery |
| Balanced with HRV | Medium | Medium | Best: 70-85% recovery |
Field Tip: In buildings with significant exhaust (kitchens, labs), dedicated makeup air units with energy recovery from general exhaust can reduce heating/cooling loads by 50-70%. Never leave large exhaust systems without intentional makeup—the resulting infiltration wastes energy and causes comfort problems.
Verdict: Energy
Winner: Balanced with Recovery — Neither supply nor exhaust alone optimizes energy. Balanced systems with heat recovery (HRV/ERV) capture maximum benefit from both strategies.
Application-Specific Recommendations
When to Use Supply-Dominant Ventilation
Use supply-dominant (positive pressure) for:
- Office buildings and commercial spaces
- Healthcare clean zones (patient rooms, ORs)
- Clean rooms and controlled environments
- Data centers and computer rooms
- Retail spaces
- Schools and educational facilities
- Assembly spaces (theaters, conference rooms)
- Buildings in hot-humid climates
Supply-dominant design:
- Supply = 105-110% of total exhaust
- Filtered, conditioned outdoor air
- Positive building pressure (0.02-0.05" WG)
- Local exhaust from bathrooms, janitor closets
When to Use Exhaust-Dominant Ventilation
Use exhaust-dominant (negative pressure) for:
- Commercial kitchens
- Bathrooms and restrooms
- Laboratory fume hoods
- Hospital isolation rooms
- Industrial process areas
- Paint booths and spray areas
- Welding shops
- Parking garages (CO control)
- Any space generating contaminants
Exhaust-dominant design:
- Exhaust sized for contaminant capture
- Dedicated makeup air (supply) for balance
- Negative pressure relative to adjacent zones
- May be positive relative to outdoors if makeup exceeds exhaust
Balanced Ventilation Applications
Use balanced ventilation for:
- Tight residential construction
- Passive house / net-zero buildings
- Any building wanting maximum heat recovery
- Mixed-use buildings with multiple pressure requirements
- Buildings in extreme climates (maximize recovery)
System Design Considerations
Supply System Components
| Component | Function | Design Consideration |
|---|---|---|
| Outdoor air intake | Air entry point | Location away from exhaust, pollutants |
| Filters | Particle removal | MERV 8-16, pressure drop |
| Heating/cooling coil | Tempering | Size for design conditions |
| Supply fan | Air delivery | Sized for duct pressure + filter |
| Ductwork | Distribution | Equal friction or velocity reduction |
| Diffusers | Room distribution | Throw, noise, draft risk |
Exhaust System Components
| Component | Function | Design Consideration |
|---|---|---|
| Capture hoods/grilles | Contaminant collection | Capture velocity, hood design |
| Ductwork | Air transport | Material for contaminant, velocity |
| Exhaust fan | Air removal | Sized for system pressure |
| Discharge location | Air release | Away from intakes, property line |
| Makeup air source | Balance | Dedicated unit or transfer air |
Pressure Control Strategies
Building pressure control:
- Monitor: Pressure sensor at representative location
- Control: Modulate supply VFD or relief damper
- Target: 0.02-0.05" WG positive (adjustable)
- Alarms: High/low pressure alerts
Zone pressure control:
- Monitor: Pressure sensor in controlled zone
- Control: Modulate supply or exhaust damper
- Target: Per zone requirements
- Interlock: Fail-safe with exhaust critical zones
Common Mistakes to Avoid
| Mistake | Impact | Prevention |
|---|---|---|
| Exhaust without makeup | Excessive negative pressure, drafts, door problems | Size supply/makeup to match exhaust |
| Over-pressurizing building | Doors difficult to open, energy waste | Target 0.02-0.05" WG only |
| Kitchen exhaust without makeup | Back-drafting, smoke complaints | Dedicated makeup air for kitchen |
| Lab exhaust without supply | Negative cascade, spread of contaminants | Zone-by-zone pressure control |
| Ignoring stack effect | Pressure varies with height | Consider stack effect in high-rise |
| No pressure monitoring | Unknown building condition | Install building pressure sensor |
Related Tools
Use these calculators for ventilation design:
- Fresh Air Flow Calculator - Determine outdoor air requirements
- Kitchen Hood Calculator - Size kitchen exhaust systems
- Parking Ventilation Calculator - Design parking exhaust
Key Takeaways
- Supply ventilation: Delivers controlled outdoor air, creates positive pressure, enables filtration
- Exhaust ventilation: Removes contaminated air, creates negative pressure, captures pollutants
- Both are needed: Supply for clean zones, exhaust for contaminated zones, balanced for overall building
- Positive pressure standard: Most buildings target 5-10% supply-dominant for positive pressure
- Heat recovery: Balanced systems enable 70-85% energy recovery from exhaust air
Further Reading
- Understanding Fresh Air Flow - Ventilation requirements
- Understanding Kitchen Hood - Kitchen exhaust design
- Natural vs Mechanical Ventilation - Ventilation strategies
References & Standards
- ASHRAE Standard 62.1: Ventilation for Acceptable Indoor Air Quality
- ASHRAE Handbook—HVAC Applications: Chapter 34, Kitchen Ventilation
- ASHRAE Handbook—Fundamentals: Chapter 16, Ventilation and Infiltration
- IMC (International Mechanical Code): Chapter 5, Exhaust Systems
Disclaimer: This comparison provides general technical guidance. Building ventilation design requires detailed analysis of occupancy, activities, and local codes. Always consult with qualified engineers and verify compliance with local requirements before finalizing ventilation strategies.