Designing effective HVAC systems requires a methodical approach that balances technical precision with practical application. The 9-step procedure outlined below provides a structured framework for designing systems that deliver both cooling and heating functionality, ensuring optimal performance and efficiency.
Step 1: Calculate Space Sensible and Latent Cooling Loads
The design process begins with computing sensible and latent cooling loads for each space served by the HVAC system. This involves:
- Calculating hourly loads for design cooling days across all relevant months
- Computing heat gains from each source in the space
- Converting these gains to loads using the heat balance method
Below is a clean and valid HTML table that provides a structured overview of calculating hourly loads for design cooling days, computing heat gains, and converting these gains to loads using the heat balance method. This table is designed for HVAC-ENG.com visitors.
Importantly, daylighting controls are not modeled during design cooling day calculations. This conservative approach prevents understating peak space cooling loads, as high daylight conditions would reduce electrical lighting and associated heat gain.
Step 2: Calculate Space Sensible and Latent Heating Loads
For heating loads, calculations focus on a single design heating hour with several key differences from cooling calculations:
- All envelope heat losses are treated as instantaneous (no heat storage delay effects)
- Heat gains from solar, lights, people, and equipment are ignored
- This approach provides a conservative heat loss estimate for proper system sizing
Step 3: Calculate Zone Sensible and Latent Loads
Zone loads are determined by aggregating loads for all spaces within each zone, accounting for both cooling and heating conditions. This consolidation provides a comprehensive view of thermal requirements across the building.
Step 4: Calculate Required Zone and Space Supply Airflow Rates
Supply airflow rates are calculated based on the more demanding requirement between cooling and heating conditions:
- Typically, cooling conditions dictate design supply airflow due to smaller temperature differentials
- In certain climates, heating loads may require larger airflow rates
- Calculations utilize user-specified sizing methods, supply air temperatures, and airflow parameters
Step 5: Calculate Required Zone Equipment Sizes
With zone airflow rates and design heating loads established, the system can determine:
- Heating capacities for terminal reheat coils
- Requirements for supplemental zone heating units
- Necessary airflow rates for fan-powered mixing box fans
Step 6: Calculate System Airflow Rates
This step establishes required airflow rates for:
- The overall system
- Central supply fan
- Return fan
- Outdoor ventilation air
Step 7: Simulate Air System Operation
System operation simulation determines coil loads by:
- Performing simulations for design cooling days in each analysis month and for design heating conditions
- Considering the specific configuration and controls of system components
- Modeling terminal fans as single-speed for single zone CAV systems, fan coils, VRF indoor units, and water source heat pumps
- Omitting changeover controls for 2-pipe hydronic systems to capture required peak capacities
- Computing airflow rates, temperatures, and humidity at each system point
- Calculating coil loads based on airflow and entering/leaving conditions
This “System Based Design” approach ensures coil sizing results specific to the system type being analyzed.
Step 8: Identify Peak Coil Loads
The coil load profiles generated in step 7 are analyzed to identify maximum cooling and heating coil loads, establishing the basis for equipment selection3.
Step 9: Report Results
The final step compiles comprehensive design data including:
- Space, zone, and system loads
- Space, zone, and fan airflow rates
- Zone equipment sizing specifications
- Maximum cooling and heating coil loads
This systematic approach ensures that HVAC systems are properly sized to provide optimal comfort while operating efficiently. Right-sizing begins with accurate load calculations but extends through the entire design procedure to deliver properly conditioned air throughout the building.
A well-designed HVAC system impacts not only indoor air conditions but also energy efficiency—a critical consideration as global energy demands continue to rise and fossil fuel resources face depletion in the coming decades.
