Seasonal Total Equivalent Warming Impact (STEWI) is a comprehensive environmental metric that quantifies the total global warming contribution of an HVAC system over its operational lifetime, expressed in kilograms of CO2 equivalent (kg CO2e). It accounts for both direct emissions caused by refrigerant leakage and indirect emissions resulting from energy consumption, while factoring in seasonal variations in system performance and energy use. By combining these elements into a single figure, STEWI enables meaningful comparisons between different systems, refrigerants, and design strategies based on their real-world climate impact.
Technical Details and Calculation
The STEWI calculation incorporates several key variables: refrigerant charge (kg), annual refrigerant leakage rate (typically estimated at 2% to 10% depending on system type and regulatory assumptions), refrigerant Global Warming Potential (GWP), annual energy consumption (kWh), regional electricity emission factor (kg CO2/kWh), and expected system lifetime (commonly 15 to 25 years for commercial equipment). The general formula sums the direct warming impact (refrigerant charge × leakage rate × GWP × system lifetime, plus end-of-life refrigerant loss) with the indirect warming impact (annual energy consumption × electricity emission factor × system lifetime).
Because STEWI incorporates seasonal energy use patterns rather than relying solely on steady-state performance data, it reflects actual operating conditions more accurately. Energy consumption data may be sourced from building energy simulations, seasonal performance ratings such as SEER or SCOP, or real-world monitoring systems. Electricity emission factors vary by region and grid composition, so STEWI results for identical equipment can differ significantly depending on geographic location.
Applications in HVAC Design and Refrigerant Selection
STEWI serves as a decision-support tool in several practical contexts:
- Comparing the overall environmental impact of systems using different refrigerants, such as R-410A (GWP of 2,088) versus R-32 (GWP of 675) or R-290 (GWP of 3)
- Evaluating trade-offs between a low-GWP refrigerant with lower energy efficiency and a higher-GWP refrigerant with superior seasonal performance
- Informing equipment procurement decisions for projects targeting sustainability certifications or carbon reduction goals
- Guiding maintenance strategies by illustrating the warming impact of various leakage rate scenarios
Related Standards and Regulatory Context
STEWI aligns with broader environmental assessment frameworks referenced in standards such as ISO 16757 and EN 378. It is closely related to Total Equivalent Warming Impact (TEWI), which uses a similar methodology but may not always emphasize seasonal variation to the same degree. Life Cycle Climate Performance (LCCP) extends the analysis further by including emissions from refrigerant manufacturing, transportation, and system disposal. Key regulatory drivers that make STEWI increasingly relevant include the Kigali Amendment to the Montreal Protocol, the EU F-Gas Regulation (Regulation 517/2014 and its 2024 revision), and EPA regulations under the AIM Act in the United States.
Practical Significance
STEWI highlights a critical insight for HVAC professionals: selecting a refrigerant based solely on its GWP value can be misleading. A system using a lower-GWP refrigerant that consumes significantly more energy over its lifetime may produce a higher total warming impact than a well-optimized system using a moderate-GWP refrigerant. By accounting for both direct and indirect emissions within a seasonal performance framework, STEWI encourages holistic system optimization rather than single-variable decision-making.