Howard Passey discusses fire risks inherent in kitchen extract ductwork, and RISCAuthority’s work to establish a certification scheme
FIRE SPREADING into and through kitchen extract ductwork continues to be a significant concern to property owners, occupiers and insurers. Such systems are considered to be a high fire risk element, particularly when serving commercial catering environments, and the scope and scale of damage is often disproportionate to the size of the establishment.
The fire risk is posed primarily where poor standards of maintenance and cleaning allow for the build up of grease and fats in the extract ductwork, which itself is often extensive and may be routed through a premises, exacerbating the potential for significant damage.
Many will be familiar with high profile fires where spread into and through kitchen extract ductwork has led to significant spread and losses, including those at Heathrow Airport, River Café and the Hard Rock Café. However, following the London Fire Brigade ‘Latest Incidents’ feed shows that such fires are not isolated incidents, nor necessarily infrequent – often with significant loss and inconvenience to public and communities. Statistics for 2016/17 published by the government indicate that in non domestic premises in England there were 918 fires where cooking oil or fat was the first item or material ignited, and 796 fires where cooking oil or fat was responsible for the development of the fire.
Sadly, kitchen extract systems are not always given the level and frequency of attention they demand, because extract ducting from the kitchen canopy often runs hidden above ceilings, or in ducts and risers or other void spaces in a building. Until a fire occurs, there may be no other obvious signs of danger.
Efficiency and ignition
Of course, extract systems are designed to remove the grease from the airflow above a cooking range; however, although the filters installed in the hood are designed to and will trap grease particles, they can never be 100% efficient, and a significant volume of particles will pass beyond the filters and into the extract ductwork.
This allows for a potentially flammable coating to form on the inside of the canopy, extract plenum, ductwork fan blades and turning vanes. It is recognised that different types and styles of cooking will produce different types and extents of grease deposit, and that they are all likely to have different flashpoints and ignition temperatures; what is clear, however, is that most can be readily ignited and will burn.
Often fires during cooking are confined to the pan, hob or surface with no development beyond. However, given that cooking in commercial environments is predominantly sited beneath an extract canopy and associated ductwork, flames caused through a flare up of ignited cooking oil vapours may impinge on the underside of the canopy and primary filters, igniting or otherwise quickly heating the hood, filters and grease deposits. A fire may also start inside the ductwork simply because the temperature of hot gases entering are high enough to ignite the grease.
On the face of it, the risks associated all appear a result of poor cleanliness. However, ensuring that ductwork is cleaned effectively is not just a post installation concern, but a risk that can be improved by good design and installation. For example, designing a ductwork system that is as short as possible, using filters that reduce the potential for build up of grease and fats immediately above the cooking range; installing radial rather than right angled bends at changes in direction; and ensuring that all areas of the system are accessible for cleaning, will offer significant benefits once the system is operational.
Guidance and filters
Legislative drivers aside, guidance exists for designers in the form of the Building and Engineering Services Association (BESA) publication DW172, Specification for Kitchen Ventilation Systems. BESA also publishes TR19, Internal cleanliness of Ventilation Systems. Insurers also published in 2006 their own guidance in RC44, Recommendations for fire risk assessment of catering extract ventilation in response to the number and significance of fires of this nature. The intention is to update RC44 once the revisions to TR19, currently underway, are complete.
In terms of filters, there are a variety of different styles that can be used. Mesh type filters are rated as a high relative risk due to the mesh layers holding the grease extracted from the airstream as it passes, hence significant deposits can build up.
Baffle type filters comprise a number of interlocking vanes, which form a two pass grease removal device. Grease laden air passes through the filter by a series of forced changes in direction and speed, and some of the grease particles are deposited on the vertical vanes. Deposited grease is then drained off into a collection drawer, which has to be regularly cleaned.
Assuming regular cleaning and maintenance is undertaken, relative safety ratings for filter types are described in RC44 – assuming that regular cleaning and maintenance is undertaken – showing the higher relative risk presented by mesh filters particularly.
In order to ensure that the ductwork can be effectively cleaned, greater flexibility in cleaning methods is available if the ductwork is easily accessible, although there are systems that can clean ductwork remotely. To facilitate this, ductwork sections should be fitted on their side (not the base, to prevent grease leaking out) with access doors at a maximum of 3m centres (or less where only arm access is possible), and either side of any internal equipment such as fans and attenuators and at changes of direction.
The access doors should be as large as possible, given the size and style of ductwork used. TR19 provides a table of recommended sizes. Probably the most important factor however is the frequency of cleaning of the extract system.
It is not uncommon to find protocols in place for an annual clean of the system which, depending on the frequency, type and duration of cooking is likely to be entirely inadequate. The table above shows the cleaning frequencies (in months) described in TR19.
Of course, the frequency of cleaning of the ductwork is only one factor in maintaining the cleanliness of the system. RC44 provides additional guidance in support recommending that:
- the insides of all filter housing and grease collection trays should be cleaned weekly
- where removable filters are fitted, they should only be removed when the system has been shut down, to prevent unfiltered air entering the ducts – these filters may be
- put in a dishwasher or hand washed to remove grease
- cartridge filters having integral grease collection reservoirs should be cleaned at least twice a week
- an extract plenum behind filters is part of the design, and grease collected in this area should be removed by regular cleaning at least twice a week
It also highlights the ineffectiveness of creating so called ‘fire breaks’ by cleaning small areas around access panels. Cleaning methodologies also vary, from the most common hand wipe/scrape, through to steam cleaning and high pressure water washing. Chemicals may of course be used. It is important to ensure that the method of cleaning is appropriate for the type and layout of the extract system and type of grease deposits that have built up.
Often the ventilation duct will run through several floors, touching the structure of the building in a number of places, which creates a potential risk of ignition of combustible materials in close proximity. Recommendations in DW172 make direct reference to this risk, stating: ‘If a fire originates in or is transferred to the ductwork system, then it may spread beyond its original location ... the ductwork route shall therefore take account of this risk, have a minimum separation of 500mm between uninsulated ductwork and any combustible material.’
Whilst this guidance pertains more specifically to the design and installation of a system, good standards of housekeeping around the ductwork, in areas like risers or roof voids, must also be observed to minimise the potential for ignition and fire spread.
It should also be recognised that despite the presence of fire suppression systems in the canopy and ductwork, this does not negate the need or reduce the requirement for thorough extract cleaning at regular intervals. Indeed, the design and siting of suppression system pipework and heads can impact significantly on the ability to clean effectively if poorly sited.
Work is currently underway at BAFE to develop a scheme for designers, installers and maintainers of kitchen fire suppression systems, to help address these problems.
Despite the presence of design and cleaning guidance, it was noted that the prevalence of fire in extract systems continued to cause concern, and hence in order to raise standards, a more robust approach was required. In response, RISCAuthority approached the Building Research Establishment (BRE) with a view to establishing a certification scheme through the Loss Prevention Certification Board for kitchen extract ductwork cleaning.
The result of this liaison was the publication of LPS 2084, Requirements for the LPCB approval and listing of companies carrying out inspection, cleaning and maintenance of ductwork systems. The standard includes specific requirements for the inspection and cleaning of kitchen extract ductwork, the primary requirements being that initial inspection shall include:
- a survey of level of grease present throughout ductwork – including photographic evidence
- status of filters
- details of existing cleaning regime
- typical usage levels
- identification of suppression systems and their maintenance regimes
- identification of restricted access areas
- a proposal to address restricted access issues to improve future cleaning
and also that the post cleaning report should include the following:
- a survey of the cleaned ductwork system, to include photographic evidence
- status of cleaned filters
- detailed schematic of the ductwork system, showing areas of restricted access
- suggested remedies for areas of restricted access
- details of agreed ongoing cleaning/maintenance regimes
It is understood that there has been a high level of demand for certification from companies working in this market, and at the time of writing, three were already listed on the redbooklive website.
It is hoped also that the introduction of the scheme will raise the profile of the risks associated with kitchen extract ductwork with fire risk assessors.
Although assessors will be familiar with the more obvious and visible fire risks, and the need to ensure that appropriate records of inspection and maintenance are available, there is a need for better awareness of the hidden risks in ventilation and extract systems, and for clear and robust reporting on cleaning activities by contractors.
As with other safety related services, technologies in the field of extract ductwork cleaning continue to evolve. In preparation for the planned update to RC44, as described above, RISCAuthority is also exploring systems using biological cleaning agents, such as Ecofix from Quintex.
The cleaning agent is introduced into the kitchen extraction and ventilation system as a mist via a system of pipework and nozzles. The agents attach themselves to any grease, oils and fats, breaking down their carbon structures into water and carbonic gas, and leaving a small biomass residue
Howard Passey is principal consultant at the Fire Protection Association. For more information, view page 5