Let’s begin by reminding ourselves about fire itself, which will put into context how fire resistant doors fulfil their function.
The 3 necessities for a fire
- Heat – the temperature must be sufficiently high to cause ignition; a spark would suffice
- Fuel – the material which the fire will burn
Once the heat source has caused ignition, and as long as there is sufficient oxygen and fuel, the fire will continue to burn. As it does, a large amount of heat will be produced which will flow from regions of high temperature to regions of low temperature. This process in turn causes the fire to spread.
Now let’s look at how heat is transferred.
The 3 main methods by which heat is transferred
- Conduction – the transmission of heat through materials. When there is sufficient heat present, it may be enough to ignite fuel through other objects. Combustible materials are most susceptible to heat transmissions
- Convection – the transmission of heat within a liquid or gas which is due to their difference in density. Heated liquid or gas expands and becomes lighter, thereby becoming displaced by their heavier counterpart. When this happens, oxygen is drawn in, further inciting the chemical chain reactions. In an enclosed setting, such as in a confined office space, the movement of the fire will most likely be forcing the gases lower in height as the heated gases spread along ceilings and walls. Superheating then occurs in the fire, thereby causing it to rise further, but this time, carrying with it products of incomplete combustion, such as embers
- Radiation – the transmission of heat by waves travelling until heat is absorbed by other objects. An example of this would be a bar heater or open fireplace radiating onto a drying rack or curtain
This is the simplest way to spread fire. A lit match, for example, can easily burn paper. The more objects the fire gets in contact with, therefore, the bigger the probability that the fire will be able to spread faster.
Fire resistant doors can delay the spread of fire
If a fire does start, fire resistant doors can play a vital role – some models can resist fire for up to 240 minutes. So having examined the conditions for a fire to start and then spread, how do they achieve this?
Well, as you’d expect, through brilliant design … and lots of experience! It starts of course with their positioning – where is the ideal location for them to perform their primary functions of allowing people to evacuate quickly and safely while delaying the spread of the fire and smoke.
We often talk about a doorset or a complete door assembly when we talk about fire resistant doors i.e. not just the door and its frame but all of the associated hardware (e.g. locks, latches, hinges, closers and so on).
Intumescent fire door seals should be fitted to the stiles and head of a fire-resisting doorset. These seals are fitted into grooves cut into the door or the frame, or alternatively, can be surface mounted. As soon as the temperature in the vicinity of the strips exceeds 200°C, usually about 10-15 minutes after the start of a fire, the seal swells and seals the gaps between door and frame.
As smoke spread is an even greater threat to life and property than flames, particularly in the early stages of a fire, fire doors might also have to be fitted with a ‘cold smoke’ seal to prevent the ingress of smoke around the door edges (such fire doors would be specified as FDs fire doors). Exceptions apply where the leakage of smoke is essential for detecting a fire early.
Glazing may range from a small vision panel in a door to a glazed screen for maximum light transmission and safety. Ordinary glass cracks when exposed to heat and is liable to fall out fairly early in a fire. Fire resisting glass can withstand exposure to the heat condition in a fire test for at least 60 minutes before it reaches a temperature high enough to soften it. This is mainly because, with clear FR glazing, nearly 50 per cent of the incident heat is transmitted through the glass by radiation.
All dedicated fire doors, other than those to locked cupboards and service ducts, should be fitted with fire door closers. To be effective these must be capable of closing the door from any angle of opening and should be strong enough to overcome the resistance of any latch or sealing system. They should conform to BS EN 1154:1997 Building Hardware – Controlled Door Closing Devices. Door closing devices fitted to fire-resisting doors are required to perform one of two functions, dependent on whether or not a latch is fitted to the door. These functions are considered “essential” in terms of the ability of the doorset to achieve its intended fire resistance rating:
- Latched door: To close the door in a controlled manner into a position where the latch engages. In this case, once the latch is engaged, such closers will have no further essential role to play.
- Unlatched door: To close the door in a controlled manner into its frame or, in the case of double swing doors, to its dead centre closed position, and maintain this condition for a period during fire exposure until the heat activated sealing system takes over the role of maintaining the door in the closed position.
Please refer to our technical guides for further details.
Talk to the experts
As we’ve often said, it’s a complex topic with lots to think about, not least the regulations. You need to get it right first time – people’s lives are at risk!
Our team of experts is always on hand to guide you through all the regulations and give you sound and pragmatic advice based on years of experience and successful installations.
We look forward to helping you.