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entilation systems are critical for human comfort, health, and the service life of building systems. Dirty ductwork restricts


airflow, forcing fans and motors to work harder, increasing energy consumption and reducing heat recovery effectiveness. Accumulated debris can also compromise indoor air quality, leading to respiratory issues, allergies, and reduced productivity. TR19 Air, the accepted industry specification


for internal cleanliness, provides guidance for achieving legal requirements under the Health & Safety at Work Act 1974 and the Occupiers Liability Act 1984. Its purpose is to keep systems clean, remove harmful microbial contamination, reduce fire risk, maintain performance, and safeguard occupant health.


How dirty ducts impact heat recovery efficiency


Heat recovery systems rely on the free flow of air through ductwork to transfer heat efficiently. When ducts are contaminated with dust, moisture or other debris, airflow is restricted, reducing heat exchange performance. Dirty heat recovery coils, thermal wheels, and cross-over coils operate at low thermal efficiency, leading to higher energy bills and increased wear and tear. Moisture in ductwork encourages mould growth,


releasing spores that can cause respiratory problems. Poor drainage and blocked coils further reduce system efficiency and increase maintenance costs.


TR19 Air: best practice specification


TR19 Air sets out measurable cleanliness standards for new and existing ductwork, aligned with BS EN 15780. It defines cleanliness quality classes (Low, Medium, High) and acceptable dust accumulation levels. For example, offices and healthcare areas require stricter standards than warehouses or storage rooms. Regular inspection and cleaning as appropriate


are essential. TR19 Air recommends minimum inspection intervals based on system quality class - typically every 6 to 12 months for most commercial systems. Cleaning should be programmed after a system is built but before final commissioning to ensure a clean start.


Risk assessment and verification


A suitable risk assessment is required to identify contamination levels and determine cleaning needs. Visual inspection is the primary method, supported by quantitative tests like the European Vacuum Test (EVT) and Deposit Thickness Test (DTT). Acceptable contamination levels are defined for supply, recirculation, and extract ductwork. Cleaning is required when these levels are exceeded, or when airflow is reduced by 15% or more. Post-clean verification is crucial. After cleaning,


surfaces should be visibly clean and meet stringent post-clean levels (<0.3 g/m² using EVT). A post-


works verification report (PWVR) must document the extent of cleaning, methods used, testing results, and recommendations for future maintenance.


Cleaning methods and access


Adequate provision of access panels is essential for effective cleaning. Mechanical brushing, compressed air, hand vacuuming, and wet cleaning methods are used depending on contamination type and duct material. Cleaning should follow the direction of airflow to prevent cross-contamination. Safety, risk assessment, and COSHH compliance are paramount.


Why dirty ventilation systems undermine heat recovery performance – and how to fix It


Heat recovery systems are increasingly specified to close the energy performance gap in modern buildings. Despite their promise, many fail to deliver their full efficiency in operation due to dirty ventilation systems, however. Gary Nicholls, managing director of Swiftclean, a ventilation hygiene specialist and co-author to BESA TR19 Air guidance, explains why ventilation duct hygiene must be viewed as more than just a maintenance issue


Advice for contractors and maintenance teams


¡ Define cleanliness quality class: Agree on the cleanliness class for each system and document it. ¡ Plan access: Ensure sufficient access panels are installed for inspection and cleaning. ¡ Schedule regular inspections: Follow TR19 Air’s recommended intervals and adjust based on risk assessment. ¡ Verify cleanliness: Use visual and quantitative methods to confirm compliance. ¡ Document everything: Maintain detailed records, including post-clean reports, photographs, and schematic drawings. ¡ Select qualified contractors: Choose specialists with BESA AHT-Air Hygiene Technician qualifications and proven track records. Dirty ventilation systems are a hidden threat to


heat recovery performance, energy efficiency, and regulatory compliance. By following the TR19 Air specification and prioritising ventilation hygiene, contractors and maintenance teams can safeguard building performance, reduce energy costs, and ensure legal and insurance requirements are met. Ventilation duct hygiene should be considered an essential element of both building efficiency and safety.


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