Designing effective heating-only HVAC systems requires a methodical approach that ensures adequate capacity while avoiding oversizing. The following 8-step procedure provides a comprehensive framework for designing these systems, focusing on peak heating loads to determine appropriate equipment sizing.
Step 1: Calculate Design Heating Loads for Spaces
The design process begins with computing heating loads for all spaces served by the HVAC system:
- Calculations focus on a single design heating hour
- All envelope heat losses are treated as instantaneous (no heat storage delay effects)
- Heat gains from solar, lights, people, and equipment are intentionally ignored
- This conservative approach ensures adequate capacity during worst-case conditions
This methodology provides a reliable baseline for system sizing by focusing exclusively on heat loss factors without the mitigating effects of internal heat gains.
Step 2: Calculate Zone Design Heating Loads
After individual space loads are determined, zone loads are computed by aggregating the heating requirements for all spaces within each zone. This consolidation provides a comprehensive view of heating needs 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
- Space and zone heating loads calculated in the previous steps
These calculations establish the baseline airflow requirements needed to maintain comfort conditions during design heating conditions.
Step 4: 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
These calculations ensure adequate heating capacity is available at the zone level.
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 to deliver the required heating capacity.
Step 6: Simulate Air System Operation
System operation simulation determines coil loads through a detailed process:
- Simulations are performed for the design heating condition
- 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
- Airflow rates and dry-bulb temperatures 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
Results from the air system simulations for the design heating condition are considered to represent the maximum heating coil loads. These peak loads become 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 heating coil loads
This systematic approach ensures that heating-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 heating loads while minimizing energy consumption.
Properly sized heating systems not only provide better thermal comfort but also operate more efficiently, reducing energy costs and environmental impact while extending equipment life through reduced cycling and wear.
