How do air transfer grilles affect smoke leakage rates in doors?

Smoke leakage considerations 

When specifying air transfer grilles for fire and smoke doors, it’s vital that all components are considered that affect smoke containment. It is possible to create airways through fire resistant constructions and still meet the requirements of Building Regulations and the recommendations of BS 9999. Doorsets should be tested to BS EN 1634-3 as a complete system in order to accurately calculate the leakage rate and ensure it remains below the permissible 3m3/m/hr or 15m3/hr across a single door.

There are a number of factors that can affect the smoke leakage rate of a door with an air transfer grille, including:

• The type of air transfer grille 
• The size and location of the air transfer grille 
• The maintenance of the air transfer grille
• Door seal configuration 

Types of air transfer grilles

There are two distinct types of air transfer grilles: fire resistant air transfer grilles and fire & smoke resistant air transfer grilles. Fire resistant air transfer grilles are designed to withstand the heat of a fire and prevent the passage of flames and hot smoke. Fire & smoke resistant air transfer grilles are designed to prevent the passage of flames and cold smoke.

Size and location of the air transfer grille 

The size and location of the air transfer grille can also affect the smoke leakage rate of the door. A larger grille will allow more smoke to pass through, while a smaller grille will allow less smoke to pass through. The location of the grille is also important. If the grille is located at the bottom of the door, it is more likely to allow smoke to pass through than if it is located at the top of the door.

Maintenance of the grille

The maintenance of the grille is also important. Lorient intumescent air transfer grilles, including those positioned behind cover grilles, will require periodic cleaning with a damp cloth. The use of intumescent materials means there are no moving parts. The product is trouble free in operation and easy to maintain – periodic testing is unnecessary.

Door seal configuration

As demonstrated in the graph, there is a relationship with the type of perimeter seal and the operable configuration. In order to maximise the size of the air transfer grille, careful consideration of the perimeter seals is required. As the example shows, once you take into account the threshold or hardware interruption, then the maximum size of the air transfer grille in the door will become critical. In this instance, the 200mm x 200mm is comfortably below the threshold.

Fig 1. Smoke leakage determined under the conditions of BS 476-31.1 / BS EN 1634-3 at a pressure differential of 25Pa. Total smoke leakage of door leaf and 200mm x 200mm LVN20S air transfer grille related to door perimeter. 

Air transfer grille size variation 

Figure 2. demonstrates the smoke leakage rates of the door leaf and varying sizes of LVN20S fire & smoke resistant air transfer grilles in relation to the type of perimeter seals. Choosing a higher performing perimeter seal will provide a greater degree of headroom when it comes to specifying the air transfer grille size for the doorset. 

Specifiers should obtain from the manufacturer the leakage rate if the doorset cannot be tested as a complete solution for primary evidence purposes.

Fig 2. Smoke leakage determined under the conditions of BS 476-31.1 / BS EN 1634-3 at a pressure differential of 25Pa (positive). Total smoke leakage of door leaf and LVN20S air transfer grille related to door perimeter.

Fig 2.