Designing efficient cooling-only HVAC systems requires a structured approach to ensure optimal performance and appropriate equipment sizing. The following 8-step procedure provides a comprehensive framework for designing these systems, where peak cooling loads determine the airflow requirements for system components.
Step 1: Calculate Space Sensible and Latent Loads
The design process begins with a thorough calculation of sensible and latent cooling loads for each space served by the HVAC system:
- Hourly loads are computed for design cooling days across all relevant months
- Heat gains are calculated for each heat source in the space
- These gains are converted to actual loads using the heat balance method
It’s important to note that daylighting controls are intentionally not modeled during design cooling day calculations. This conservative approach prevents understating peak cooling loads, as high daylight conditions would naturally reduce electrical lighting and associated heat gain.
Step 2: Calculate Zone Sensible and Latent Loads
After individual space loads are determined, zone loads are computed by aggregating the loads for all spaces within each zone. This consolidation provides a comprehensive view of cooling requirements across defined building zones.
Step 3: Calculate Required Zone and Space Supply Airflow Rates
Supply airflow rates for zones and spaces are calculated using:
- User-specified sizing methods
- Required supply air temperatures
- User-defined airflow parameters
- Sensible loads calculated in the previous steps
These calculations establish the baseline airflow requirements needed to maintain comfort conditions.
Step 4: Calculate Required Zone Equipment Sizes
With zone airflow rates established, the system can determine:
- Heating capacities for terminal reheat coils (if applicable)
- Requirements for supplemental zone heating units
- Necessary airflow rates for fan-powered mixing box fans
Even in cooling-only systems, some heating capacity may be required for specific zones or applications.
Step 5: Calculate System Airflow Rates
This critical step establishes the required airflow rates for:
- The overall system
- Central supply fan
- Return fan
- Outdoor ventilation air
These calculations ensure adequate air movement throughout the system.
Step 6: Simulate Air System Operation
System operation simulation determines coil loads through a detailed process:
- Simulations are performed for design cooling days in each analysis month
- The specific configuration of system components is considered
- All components and controls specified in the system input are evaluated
- Terminal fans are modeled as single-speed for certain system types to prevent underestimating peak coil loads
- Zone loads are adjusted if diversity factors were specified
- Airflow rates, temperatures, and humidity are computed at each system point
- Coil loads are calculated based on airflow and entering/leaving conditions
This “System Based Design” approach ensures coil sizing results specific to the system type being analyzed.
Step 7: Identify Peak Coil Loads
The coil load profiles generated in step 6 are analyzed to identify maximum cooling coil loads, establishing the basis for equipment selection and sizing.
Step 8: Report Results
The final step compiles comprehensive design data including:
- Space, zone, and system loads
- Space and zone airflow rates
- Zone equipment sizing specifications
- Maximum cooling loads
This systematic approach ensures that cooling-only HVAC systems are properly sized to provide optimal comfort while operating efficiently. The methodology balances technical precision with practical application, resulting in systems that effectively manage cooling loads while minimizing energy consumption.
Properly sized cooling systems not only provide better comfort but also operate more efficiently, reducing energy costs and environmental impact while extending equipment life through reduced cycling and wear.
