General ventilation ducts have no fire resisting properties. They depend on the use of fire dampers to EN1366-2 when crossing fire compartments. The fire damper will close in the event of a fire and secure the compartment.

Ducts for the purpose of removing smoke are the subject of a classification report EN13501-4. This report is provided by a notified body, following a review of the tests conducted to EN1366-1 and EN1366-8 or 9. EN1366-9 ducts are only suitable for single fire compartment applications. They are tested to 600°C. EN1366-8 is a test for multi-compartment ducts and is subject to the full hydrocarbon fire curve, and must be tested with Insulation. Uniquely, the results for the insulation’s performance comes from the EN1366-1 tests.

Both the ductwork and the insulation are subject to the test. They are not treated as individual items, but as a complete system. So why the insulation? When we exhaust smoke/hot gases from the scene of a fire, and cross another fire compartment before reaching atmosphere (outside), we run the risk of a flashover event. The cause of a flashover, the most common type being a hot rich flashover, is thermal radiation feedback. Inside a structure the hot gases radiate heat, and when the area through which it passes has been saturated with the energy (heat), components break down and flames may break out almost at once over all of the surfaces within that heated environment. Smoke ducts to EN13501-4 multi-compartment are insulated to prevent flashover, and thus prevent secondary fires.

Ducts used in kitchen scenarios also have a fire risk, specifically through the presence of grease. Because of oxidisation, the grease has a flash temperature of 180°C, considerably lower than oils used in cooking. Where the ductwork is multi-compartment, there is a risk that a fire in a compartment (other than the kitchen) could reach the kitchen extract. This would cause the grease to ignite and the fire to cascade through all compartments with the kitchen extract passing through them. The ducts are classified to EN13501-3 and tested to EN1366-1. Multi-compartment kitchen extracts require insulation to prevent the flames on the outside, causing the grease on the inside to ignite. In the fire test EN1366-1, a measurement called combustible linings is taken. In this test, an A duct has thermocouples placed inside the duct, inside the furnace, and a measurement is taken of the temperature rise of the ductwork. The duct is tested with insulation and mechanical pressure of -500 PA. The outcome of these measurements is often referred to as the kitchen extract result. Again it is a combination of the metal duct and the insulation that provides the result. Changing the duct construction or insulation used will deliver different unproven results.

Pressurisation ducts are used to keep smoke from penetrating certain critical areas of a building by maintaining higher pressure differentials than normal areas. Smoke inhibits escape and firefighting. A pressurised system is a special form of mechanical ventilation system. Fire dampers cannot be used in such systems. The ductwork is again tested to EN 1366-1 and classified to EN13501-3. Pressurisation ducts are Insulated to prevent heat entering the protected areas.

Passive ducts are used to provide a natural air flow. In many instances they do not contain fire dampers where maintenance and testing could be problematic. Such ducts need to be classified to EN13501-3 and tested to EN1366-1. Passive ducts do not have a mechanical element. In the event of a fire, no air movement is driven in the ducts, and so the ductwork may not require insulation. Where a passive duct traverses an EI wall, the fire designer may require the duct to maintain the I (Insulation) performance of the wall, and may need the duct insulated.