Total Equivalent Warming Impact (TEWI) is a metric that quantifies the overall contribution of a refrigeration or air conditioning system to global warming over its entire operational life. It accounts for both the direct effect of refrigerant emissions released through leakage and end-of-life losses, and the indirect effect of carbon dioxide produced by the energy consumed during system operation. TEWI is expressed in kilograms of CO2 equivalent (kg CO₂e), providing a single value that enables meaningful comparison between different system configurations, refrigerant choices, and efficiency levels.
Technical Details and Calculation
TEWI is calculated by summing direct and indirect warming contributions using the following general formula:
TEWI = (GWP × L × n) + (GWP × m × [1 − α]) + (E × β × n)
- GWP = Global Warming Potential of the refrigerant (based on a 100-year time horizon)
- L = Annual refrigerant leakage rate (kg/year)
- n = System operating lifetime (years)
- m = Total refrigerant charge (kg)
- α = Refrigerant recovery/recycling factor at end of life (0 to 1)
- E = Annual energy consumption (kWh/year)
- β = CO₂ emission factor for electricity generation (kg CO₂e/kWh), which varies by region and energy source mix
Refrigerant GWP values range widely: R-410A carries a GWP of 2,088, R-32 has a GWP of 675, and natural refrigerants such as CO₂ (R-744) have a GWP of 1. Typical annual leakage rates for commercial systems fall between 2% and 15% of total charge, depending on system type and maintenance quality. The indirect emission factor (β) can range from approximately 0.05 kg CO₂e/kWh in regions with predominantly renewable grids to over 0.9 kg CO₂e/kWh in coal-dependent regions.
Practical Significance in HVAC Design
TEWI provides engineers and facility managers with a holistic view of environmental impact, preventing decisions based on a single variable. For example, selecting a low-GWP refrigerant may not reduce overall warming impact if the system operates with poor energy efficiency. In many climates and grid conditions, the indirect energy-related component accounts for 60% to 90% of total TEWI, underscoring the importance of high-efficiency system design alongside responsible refrigerant selection.
TEWI analysis is commonly applied when:
- Comparing alternative refrigerants for new equipment specifications
- Evaluating chiller or rooftop unit replacements
- Assessing the environmental benefit of improved insulation, controls, or heat recovery
- Supporting sustainability reporting and corporate carbon reduction goals
Related Standards and Terms
The TEWI methodology is referenced in standards and technical documents published by ASHRAE, the International Institute of Refrigeration (IIR), and ISO 817. It is closely related to, but distinct from, Life Cycle Climate Performance (LCCP), which further includes emissions from manufacturing, transportation, and disposal of system components.
- Global Warming Potential (GWP)
- Life Cycle Climate Performance (LCCP)
- Life Cycle Assessment (LCA)
- Seasonal Energy Efficiency Ratio (SEER)
- Refrigerant Management and Leak Detection
- EPA Section 608 (refrigerant handling regulations)