Air Balancing
Air balancing ensures that each zone receives the correct airflow for cooling, heating, and ventilation. This section covers airflow distribution and register selection.
Airflow Requirements
Section titled “Airflow Requirements”Zone Airflow
Section titled “Zone Airflow”Each zone requires airflow based on:
- Peak cooling airflow - To meet peak sensible load
- Minimum ventilation - ASHRAE 62.1 requirements
- Heating airflow - For systems with air-side heating
System Airflow
Section titled “System Airflow”Total system airflow is the sum of zone airflows, adjusted for:
- Diversity (zones don’t all peak simultaneously)
- Safety factors
- Duct leakage allowance
Register Selection
Section titled “Register Selection”Supply Registers
Section titled “Supply Registers”Select supply air outlets for each zone:
- Ceiling diffusers - Square, round, or linear
- Sidewall registers - For perimeter heating
- Floor registers - For underfloor systems
Register Properties
Section titled “Register Properties”- Throw - Distance air travels from the register
- Spread - Width of the air pattern
- Noise level - NC rating at design airflow
- Pressure drop - Static pressure loss
Sizing Registers
Section titled “Sizing Registers”Size registers based on:
- Zone airflow requirement
- Maximum face velocity (typically 500-700 FPM)
- Throw requirement for the space
- Acceptable noise level
Return and Exhaust
Section titled “Return and Exhaust”Return Air
Section titled “Return Air”Configure return air for each zone:
- Return grille sizing
- Return plenum vs. ducted return
- Transfer air between spaces
Exhaust Air
Section titled “Exhaust Air”For spaces with exhaust requirements:
- Exhaust airflow (from space type)
- Exhaust fan selection
- Make-up air requirements
Air Balance
Section titled “Air Balance”Verify air balance for each zone:
- Supply = Return + Exhaust + Transfer
- Positive pressure for critical spaces
- Negative pressure for restrooms, kitchens
Critical Path Analysis
Section titled “Critical Path Analysis”Pressure Drop Calculation
Section titled “Pressure Drop Calculation”HVAKR calculates the pressure drop through the duct system:
- Duct friction losses
- Fitting losses
- Terminal unit losses
- Register losses
Critical Path
Section titled “Critical Path”The path with the highest pressure drop:
- Determines fan static pressure
- Used for duct sizing
- Identifies balancing challenges
Balancing Dampers
Section titled “Balancing Dampers”For paths shorter than critical:
- Dampers add pressure drop
- Balance to match critical path
- Shown on duct layout
Ventilation Distribution
Section titled “Ventilation Distribution”Zone Ventilation
Section titled “Zone Ventilation”ASHRAE 62.1 ventilation for each zone:
- Breathing zone outdoor air (Vbz)
- Zone air distribution effectiveness (Ez)
- Zone outdoor air fraction (Voz)
System Ventilation
Section titled “System Ventilation”Total system outdoor air:
- Sum of zone requirements
- Adjusted for system efficiency
- Multiple spaces equation if applicable
Ventilation Verification
Section titled “Ventilation Verification”Check that each zone receives adequate outdoor air:
- Primary air at minimum airflow
- System outdoor air fraction
- Critical zone requirements
Best Practices
Section titled “Best Practices”- Size registers for throw - Match throw to room dimensions
- Check noise levels - Verify NC ratings are acceptable
- Balance return and exhaust - Account for all air leaving the zone
- Verify ventilation - Ensure ASHRAE 62.1 compliance
- Document the critical path - For commissioning reference