Comparative Climate Impact of Circular versus Rectangular Ventilation Ducts , A Material and Design Perspective

Sammanfattning

The construction and real estate sector is a major contributor to global carbon emissions and climate change. Ventilation systems, although essential for maintaining indoor air quality and occupant health, account for a significant share of building operational emissions. In HVAC systems, duct layouts significantly influence ventilation impacts through both the embodied carbon from material production and operational energy consumption. This study investigates strategies to reduce the environmental impact of ventilation duct systems by evaluating the influence of CO2 emissions due to duct geometry, energy demand, and the ratio of recycled steel content. The study was a theoretical study made by evaluating the manufacturing process and products from Lindab on a section of a ventilation system, both on duct-level and system-level, in a non-residential building in Sweden over a 30-year life cycle. Circular and rectangular duct systems were compared using Revit 2025 and CADvent 7.0 Plug-in for AutoCAD to assess material mass consumption and pressure losses at both component and system levels. In addition, traditional galvanized steel ducts were compared with ducts manufactured from steel made 75% recycled content. A life cycle assessment (LCA) was performed considering stages cradle to gate (A1-A3), Construction process stage (A4 -A5), Operational energy use stage (B6), and End-of-life stage ( C1-C4). Furthermore, three sensitivity analyses were carried out to evaluate the effects of geographical location, building height limitations, and varying airflow on the embodied carbon and efficiency performance of the systems. The results show that duct geometry and material selection significantly influence both embodied carbon and operational energy use. Steel with 75% recycled content substantially reduces CO₂ emissions compared to galvanized steel with 20% scrap content. Circular ducts demonstrated lower material consumption, lower climate impact, and lower energy demand than rectangular ducts. While pressure drop differences between the two geometries were zero at the duct level (one-meter length of straight duct) due to equivalent diameters, system-level differences became more significant because of fittings and other components, which increased operational energy demand and associated carbon emissions. Except in height-limitation scenarios, where rectangular ducts can exhibit superior performance compared to circular ducts at small heights, and under specific conditions such as high CO₂ emission intensity from electricity generation in Germany or other countries that prioritize low operational energy use over material mass considerations, geometric constraints become less critical. Overall, circular duct systems manufactured from steel containing 75% recycled material were identified as the most energy-efficient and environmentally sustainable solution across the other scenarios evaluated. Interestingly, contrary to what might be expected, rectangular duct systems made from 75% recycled steel achieved carbon emissions that were slightly lower than those of galvanized circular duct systems, although the difference was relatively small. In contrast, galvanized rectangular duct systems generated the highest carbon emissions of all the alternatives assessed, making them the least environmentally sustainable option.