To resolve backdraft caused by excessively long exhaust hood ducts, prioritize duct length reduction, proper insulation, and pressure balance adjustments. Shorten duct runs to minimize resistance, install high-quality backdraft dampers at transitions, and ensure the system complies with ASHRAE 90.1 standards for airflow efficiency. Regularly inspect for condensation and optimize fan performance to maintain negative pressure in exhaust zones.

Excessively long ducts increase friction and pressure drop, reducing airflow effectiveness and creating negative indoor pressure that draws exhaust gases back into occupied spaces. A 2021 study by ASHRAE found that ducts exceeding 50 feet in length can experience up to 40% pressure loss, increasing backdraft risks. Condensation in lengthy ducts, often due to poor insulation, further degrades airflow and promotes mold growth. For example, a 2018 NFPA report noted that 60% of kitchen exhaust system failures in commercial buildings stemmed from duct length exceeding design limits. Shortening ducts to 30-40 feet, as recommended by ASHRAE, reduces pressure drop and improves compliance with backdraft prevention codes. Additionally, installing variable speed fans and backdraft dampers can mitigate pressure imbalances. A 2020 HVAC Journal analysis showed that systems with dampers saw a 35% reduction in backdraft incidents. Proper maintenance, including annual duct inspections and insulation upgrades, ensures long-term reliability. These measures collectively address the root causes of backdraft while enhancing energy efficiency and indoor air quality.