Inspection and testing of your ventilation system(s) should be carried out generally and where practicable in accordance with TR19 Guide to Good Practice Cleanliness of Ventilation Systems. Heavy Use – 12-16 hours per day – 3 Monthly Moderate Use – 6-12 hours per day – 6 Monthly Light Use – 2-6 hours per day – 12 Monthly The need for specialist cleaning of extract systems will depend on the level of usage of the cooking equipment, types and quantity of cooking and other risk factors such as vulnerability of the system to ignite. Typical cleaning intervals are shown in TR19 Table 11: Frequency of Cleaning: TR19 Table 9 sets out guideline maximum levels of grease deposit and actions recommended. The actual recommended action timing will depend on the rate of accumulation, the risk vulnerability of the system and site and any particular warranties imposed by the building insurers. Wet Film Thickness Test Measurement – Recommended Action 200Î¼m as a mean across the system – Complete cleaning required Any single measurement above 500Î¼m – Urgent local cleaning required