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Using Watermist

Given the variety of watermist systems and their proprietary nature, it is important that the use of watermist does not follow the same general rules of other systems. The system must be matched to the application. As a result, the manufacturer of the system tends to be much more involved in the process and has the responsibility of demonstrating suitability and performance, above and beyond what standards may require for traditional sprinkler systems. This is why the DIOM is the key document to guide specifiers, AHJs and stakeholders. 

3.1 The DIOM – Design, Installation, Operation and Maintenance Manual


The DIOM is the document that contains all the key information from the manufacturer about the watermist system. Given that watermist systems are proprietary solutions, these may vary significantly with application and manufacturer.  These documents should be used by the specifier, approver and any other stakeholder to understand how the system works, and how it should be installed and maintained. It will also detail what are the intended applications of the system and how it has been validated.  What standards have been referenced, what fire test protocols have been used to test the system, what the results were and even what third party laboratories have tested or certified it, if any?

3.2 The Role of Standards and how they should be used

According to the BSI website:


‘In essence, a standard is an agreed way of doing something.


It could be about making a product, managing a process, delivering a service or supplying materials – standards can cover a huge range of activities undertaken by organizations and used by their customers.

If you comply with a British Standard then it’s pretty clear that you take your responsibilities seriously as an organization, and indeed compliance is often taken as evidence of due diligence. It certainly speaks volumes about your attitudes to doing things properly.

However, standards aren’t the same as regulations and following a standard doesn’t guarantee that you’re within the relevant laws. In fact, standards rarely cite the law as legislation could change within the lifetime of the standard.

In a case like this, compliance with the standard will often mean you’re compliant with the relevant legislation, although there are usually ways of being compliant with legislation without using a standard.

A false claim of compliance is likely to put you on the wrong side of the law.’

For a standard to be created, the intellectual property (like a patent) which defines its scope must be in the public domain (expired patent) or the standard must be performance based enough to apply to a wide range of products. The latter is required on the few European standards which manufacturers must comply to place a product in the market. These are called harmonised standards and are the result of European Directives which impose minimum safety of operation of products which include electromagnetic compatibility (EMC), radio emission (RED), electrical safety and pressure vessel containment.

Because the watermist industry is continuously evolving where an ever increasing number of effective watermist applications are discovered, there will not necessarily be a standard to guide manufacturer and buyer on the adequacy of every solution, this may have to be done on a case by case basis by making reference to a similar application.  An example of well established application which only recently has had a standard for it is that of data centres. Prisons cells do not have one, neither do space stations, 2 other applications where watermist is used for fire protection purposes.

Similarly, it is why watermist is not present in other guidance documents such as Approved Document B, which is a mature method based on common practice, but not the only method, to comply to the functional requirements of the Building Regulations.  As watermist becomes the most common method of active fire protection it will progressively be referenced by other guidance documents.

The use of a watermist standard is not a mandatory requirement but when it exists for both the application (say machinery) and product (say high pressure, open nozzle, deluge) in question, then the standard is the most mature agreed guidance available to evaluate that solution.  A product may be outside of the scope of a watermist standard, that does not make it non-compliant, that standard simply does not apply to it.  It is only when a product is within the scope of a standard, refers to it, but does not meet a requirement, that is a non-compliance.

A good standard will condense the accumulated functional performance requirements without being prescriptive. Such a broad standard but performance orientate standard allows for innovative solutions to be checked on whether they perform independently of the implementation (pressure, nozzle position etc).  This is why, for watermist, more important than the relevant standard is the manufacturer’s Design, Installation, Operation and Maintenance guide (DIOM).  This is because nobody knows better than the manufacturer how a proprietary design works.

3.3 Types of Standards

Following the development of watermist as an alternative firefighting medium, the organisations responsible for national and international standards have progressively introduced, published and updated watermist Standards. Commercial, industrial, residential and domestic watermist systems can be specified and installed to a number of standards including those issued by BSI, CEN and NFPA.

3.3.1 System Fire Performance Standards


These are the most common standards in watermist given that design and installation methods will vary significantly with the manufacturer and yet, its effectiveness in addressing the fire must be tested.  The standard offers a communised, agreed method, to determine whether a system performs well or not: there are clear pass criteria to determine whether these systems perform. The fire loads used for the fire test may be little resemblance to the reality of the application simply because the testing protocols seek repeatability more than realism as a way to evaluate systems consistently.  The standard may also set some prescriptive minimum requirements to ensure system reliability such as water supply, AMAO, power supply and period of operation.  The standard(s) used to validate the system’s fire performance, if applicable, should be detailed in the manufacturer’s DIOM.  An example of such a standard is prEN 14972-5. Fixed firefighting systems. Watermist systems. Test protocol for car garages for automatic nozzle systems;

3.3.2 Component Reliability Standards

There is a significant amount of accumulated knowledge of the ways in which water-based, gaseous, fire fighting components can fail over time depending on the material they are made and the environment they are exposed to.  Some of the tests or best practices embedded in these standards may include ageing tests, over-pressure tests, and material choices.  The standard(s) used to validate the reliability of the key components should be detailed in the manufacturer’s DIOM. Example of such a standard is BS 8663.

3.3.3 Design, Installation and Maintenance Standards

These tend to be limited in the watermist ecosystem as installation varies significantly with manufacturer and would, therefore, be found in the DIOM itself.  Examples of such details would be: position and orientation of nozzles, type of fittings used, water availability requirements.  Curiously, these tend to be the most common standards for the sprinkler ecosystem because it is so commoditised: components are interchangeable and there is no proprietary system.  An example of such a standard is prEN 14972-1. Fixed firefighting systems. Watermist systems. Design and installation and NFPA 750: 2015. Standard on Water Mist Fire Protection Systems. 

3.4      Current Watermist British and European Standards

Watermist standards provide recommendations for the design, installation, commissioning and maintenance of fixed watermist systems for low and high pressure watermist covering both residential and domestic, and commercial and industrial, applications.  


3.4.1 BS 8458 Residential and domestic watermist

These apply only to wet pipe systems using automatic nozzles to occupancies not exceeding 20m in height and to domestic occupancies. BS 8458 primarily covers watermist systems used for life safety purposes, but can also apply to property protection for the following: 

•    Residential - apartments, residential homes, HMOs, boarding houses, care facilities and dormitories etc. 
•    Domestic - individual houses, flats and maisonettes etc. 
Important note: If the fire/fuel loading in any given occupancy exceeds that which would normally be found in a residential or domestic living room, kitchen or bedroom, or if the fire hazard is greater than that of a conventional residential or domestic occupancy, then this Draft for Development will not be suitable for use. Key indicators of adverse fire hazard include significant volumes of videotapes, books, paper and institutional catering facilities, and the configuration of the building (e.g. high ceilings, large volumes), ventilation and its contents. In those circumstances, DD 8489 should be used. 


3.4.2     BS 8489 Commercial and industrial watermist

BS 8489-1 (see 2.1(a) above) may be applied to hazards involving the following: 


  • Local applications - involving flammable liquid fires, as detailed in DD 8489-4

  • Combustion turbines and machinery spaces - with volumes up to and including 80m3, as detailed in DD 8489-5

  • Industrial oil cookers - as detailed in DD 8489-6

  • Low hazard occupancies - as detailed in DD 8489-7

3.4.3 The European draft Standard prEN 14972

prEN 14972 is a draft standard that is scheduled for publication (Part 1) in 2020.  There are another 17 parts that define the fire test protocols to validate the different applications such as office, data centre, residential etc.

The structure of the standard follows the logic of the watermist ecosystem and differs from other suppression systems because of the breadth of applications: 

  • The exact specification of the system and the intended application must be defined in the DIOM of that system. That specification must have been tested on the fire test specific to that application to demonstrate it has been validated.  

  • The EN contains many application fire test protocols with the expectation to add many more as these become established methods for testing watermist systems.  These are based on the many testing protocols created by the certification laboratories worldwide, including FM, VdS and UL. 

  • Once this standard is published, certification bodies can provide certification of systems based on the EN fire test protocols.  A standard with such a broad number of applications and contributing stakeholders will help mature the fire testing protocols worldwide so that manufacturers do not need to certify to so many national standards which differ little in practice.

  • With the publication of the EN the existing British Standards (BS 8458 and BS 8489) would be withdrawn so that the EN becomes BS EN 14972. It is also important to note that the 2 existing UK standards only cover a small proportion of the watermist ecosystem as per the table below:


The prEN 14972 will not be a harmonised standard so it will not require a CE mark against that standard for compliance with the Construction Products Regulations (CPR).

3.5 The role of Approvers/Certifiers/Listing Bodies/Testing laboratories

A National standard only allows manufacturers to declare self-compliance and to fire test against the standard.  A test report may simply show a test result but it may not be conclusive (that in fact, that product fails the tests). A self-declaration of compliance may also be false (which is illegal). Certification goes well beyond that. Certification is a third-party test and underwriting of the performance of the system.  Again, certification is not mandatory but certain stakeholders will not accept systems that are not certified, such as commercial insurers covering business continuity simply because of the much higher credibility that a certified system provides.

As a consequential trend to the development of national and international standards, certification laboratories have also been publishing approvals documents for watermist approvals. These are authored by the labs that will carry out the certification, such as BRE has the LPS 1283, UL has UL 2167 and FM has FM 5560. American National Standard for Water Mist Systems.  It is important to note that these are not the same as national and international standards despite being referenced by them. Instead, they are a means to demonstrate that the requirements in those standards (and frequently more) have been met by the system. The following have a progressive level of credibility a) self-declaration of compliance b) testing by a third party c) verification by a third party d) certification by a third party.

Certification laboratories which have experience with watermist internationally are: FM, UL, VdS, BRE.

3.6 Types of Certification
3.6.1 Full System Certification

This involves the certification of the fire performance for the specific application but it tends to include the reliability testing of key components that affect fire performance such as nozzles. This is the case for UL 2167 and LPS

As FM5560 states:


‘The requirements of this standard reflect tests and practices used to examine characteristics of water mist systems. Water mist systems having characteristics not anticipated by this standard might be acceptable if performance equal, or superior, to that required by this standard is in the sole opinion of testing organization, demonstrated, or if the intent of the standard is met. Alternatively, water mist systems that meet all of the requirements identified in this standard might not be acceptable if in the sole opinion of testing organization, other conditions which adversely affect performance exist or if the intent of this standard is not met.

Certification is based upon satisfactory evaluation of the product and the manufacturer in the following major areas:

1.4.1 Examination and tests on production samples shall be performed to evaluate:

  • The suitability of the product;

  • The performance of the product as specified by the manufacturer and required by the testing organization; and, as far as practical,

  • The durability and reliability of the product.

1.4.2 A thorough review of the proposed water mist system “Design, Installation, Operation and Maintenance” manual.’

3.6.2     Certification of equipment 

As it is typical with other fire protection equipment, certification of the components is a common process. It is important to note that it does not validate whether the application is fit for purpose, instead, it only validates that the components will not fail to perform its function over the long term. A certified component provides evidence the component has been third party scrutinised and therefore should be reliable but it is not a guarantee in its own and should not be confused with the system performance validation.  There should also be a link (through a reference in the system’s DIOM) on what components and what certifications have been concluded for which applications.

Therefore, specifiers of watermist systems should ensure that the components that make up a watermist system are listed as approved by a recognised approval body. It is essential that specifiers are supplied with copies of the relevant listing or certificates, which should normally be included in the tender submission or specification document.  

Components which typically undergo certification are: 


  • control valves, water pumps and inert gas pressure cylinders; 

  • distribution pipework, couplings and fittings; 

  • watermist nozzles; 

  • fire detection  and alarm control panels, fire detection sensors, alarm sounders, indicators and actuators;  

  • non-electrical equipment such as pneumatic, hydraulic or mechanic control equipment; 

  • manual actuation equipment. 

3.6.2     Certification of installers

Because watermist is most frequently a proprietary system (as opposed to a generic one like a sprinkler) there is little demand and therefore little interest in installer certification for watermist systems.  Instead, it is common that manufacturers to establish a distributor network of licensed and approved contractors who enter into contractual agreements with the manufacturers to install systems according to the DIOM.

This effectively creates a direct line of accountability between the customer, installer and manufacturer.  This means the manufacturer is more closely involved with the applications its products are being utilised in and thus take more responsibility in ensuring that installations are being carried out correctly given it is their brand and reputation that is on the line. Failure of a specific product does not hide in the reliance on generic standards or generic installer certification.

As a result, there are only a few certification bodies listing watermist installers: FIRAS and IFC.  Even though the approval of an installer under the FIRAS or IFC scheme, is an endorsement of competency in the installation of watermist systems generically, for a manufacturer it enough for the demonstration of competency for the installation of a specific watermist system.  Instead, more important, is evidence that the installer has been trained and audited by the manufacturer and can demonstrate competence with that specific product.  This is also common in other types of proprietary fire protection system, like Ansul for wet chemical fire protection.  The consequence is that there is more onus on the manufacturer to quality control installations which results in a higher overall quality of operation.

It is also common for manufacturers to undertake installation projects themselves to ensure the quality of installation is closely monitored.  It is therefore key that, whether an installer has third party certification or not, the installer has evidence that they have been deemed competent by the manufacturer to install the watermist system in question.

3.7 Choosing a Watermist system

Before any fire protection system is specified, the following must precede its specification:

  1. The problem has been identified and objectively defined

  2. The constraints of possible solutions have been observed

  3. Solutions which are fit-for-purpose have been identified

This applies to any fire protection system whether they are water-based or not, mature or innovative.  Unfortunately, prescriptive guidance creates the unintended consequence of skipping these steps for more mature systems with the expectation that due diligence has been carried out by history, forgetting whether existing performance is good or whether it can be improved.  Watermist forces this correct case-by-case due diligence work to be carried out.  Watermist frequently solves a water supply or storage constraint because it is a water-efficient technology, which is why it is widely adopted in the maritime industry. However, only when the above assessment and comparative cost x benefit assessment is made is that this can be validated and watermist’s advantage made evidently.

The most competent professional to lead the above is a fire engineer, that is their role in society. It is therefore critical that a fire engineer with demonstrated competence is consulted when their involvement is necessary.  

Watermist has been used mostly for asset and business continuity purposes but it is gaining traction in domestic and residential applications too for improving tenability and means of escape for occupants.  
Consultation with the relevant stakeholders should first take place to seek approval where any protection system is being considered, regardless of whether the system is an elective install or to meet Building Regulations or for business continuity:


  • The building control authority. 

  • The fire service authority. 

  • The water supply authority (Note: It is a requirement to gain consent under Water Regulations as defined in the Water Regulations Guide). 

  • The insurer(s) of the premises and premises’ contents.

  • Asset managers

  • Resident Association

The DIOM should be able to answer all questions from any of these stakeholders, whether provided by a contractor or the manufacturer.  Once this approval has been given, it is essential that the specifier seeks a written technical specification from the watermist contractors, authorised by the manufacturer, on how the system will be designed, installed, commissioned, tested and maintained in accordance with the DIOM. 

Performance = Effectiveness x Availability

As with any fire protection system, the system has to be able to perform in the event of a fire, for that it needs to be demonstrated to be effective at the intended application.  It will only be effective if the system is suitable for the application and if it has been designed and installed correctly so that it works as intended.   It must also be available (or uptime, ready to work when required).

Once a watermist system has been deemed the most appropriate, three conditions need to be satisfied for a watermist system to perform as intended:

a)    the system chosen must be suitable for the fire load (the application) it is being proposed.
b)    the system must be designed, installed and commissioned correctly
c)    the system must be maintained so that it is ready to operate (available)

3.8 Suitability

Are there reference fire tests or real-life tests to validate that the solution will work as intended for the chosen application? What is the priority intent of the application?  Does the third-party testing/ verification/ certification make reference to the type of application and the key parameters to get it to work?  In the case of declaration of compliance to a standard, can that be verified by third parties?  The items below provide an example of a non-exclusive list of considerations:


3.8.1 Means of escape

Watermist can improve tenability within protected spaces by rapidly cooling the room/space temperature sufficiently enough to afford safe passage away from the seat of the fire. This feature can significantly increase the chances of survival of personnel working within these protected spaces or for vulnerable occupants who are not expected to evacuate quickly from the room of origin. 
In addition, watermist can play a vital role in modern glass buildings. There are several inherent problems in the use of glazing for compartmentation or on escape routes. By maintaining the integrity and insulation of glass panels, watermist systems can allow people to pass by a fire and not be exposed to untenable heat levels.


3.8.2  Smoke precipitation

Smoke is made up of mostly solid substances dispersed in a gas. To date, watermist under normal conditions cannot fully wash smoke to a level comparable with a dedicated extraction system. Nevertheless, watermist has been shown to be capable of capturing some water-soluble gases, minimal soluble and non-soluble gases such as carbon monoxide cannot be captured but can be dispersed (with local concentration reduction). 


3.8.3 Visibility reduction 

Watermist may have two effects on visibility that must be considered relevant to the evacuation of normally occupied areas. These are:


  • reduction of light and visibility (possibly down to a few metres); 

  • light diffusion in various directions and a loss of visual contrast that could lead to people within the area becoming disoriented.


With these facts in mind, the system designer should specify measures to safeguard staff working in normally occupied areas. These measures should include a reasonable time delay for staff to evacuate the area prior to the system being activated. This time delay can be calculated by determining travel distances required by staff to reach designated safe exit routes from the protected area. 


3.8.4 Electrical conductivity 

Conductivity is critical when higher voltages are involved, even in extremely fine droplets of a watermist system, because even demineralised water can become conductive as the water droplets become contaminated during system discharge by combustion particles and smoke. Therefore, it is vital that complete shutdown of exposed electrical equipment is achieved in the event of a fire. Observing the distance and positioning of nozzles in relation to electrical equipment outlined in the appropriate standards and the manufacturer’s design and installation manual can also reduce levels of conductivity.

3.8.5 Water discharge times  

 Water discharge times will vary depending upon whether the watermist system is designed for residential and domestic or commercial and industrial applications:

10min water discharge time for domestic applications; 

30min water discharge time for residential applications; Both residential and domestic systems should meet the pass criteria for the total time of discharge with all nozzles operating in the room concerned.)

for industrial extinguishing systems which are required for local applications involving flammable liquid fires, combustion turbines up to and including 80m3, industrial oil cookers, and low hazard occupancies, the duration should be at least twice the time taken to extinguish the fire and to prevent reignition.

for commercial suppression systems using automatic nozzles, the duration should be commensurate with the nature of the hazard as defined in the relevant standard that applies. 


3.8.6 Ventilation considerations

In instances, the very fine water droplets of watermist behave more like a gas than a liquid and can be affected by air movement and/or the size, character and internal layout of a building.  These occur where the mist has little momentum from the nozzle and relies fully on the Archimedes Principle for travel to the base of the fire and thus can be affected by other fluid flows. In all cases where this situation might occur, as established by risk assessment, especially in industrial applications where forced ventilation may be significant, any ventilation system must be shut down prior to system discharge.

For residential and domestic applications, such as private homes and care homes, this same concern is raised when considering high rise properties where natural draughts might be significant.  These should be part of the assessment of the system chosen with data obtained from the intended application.  Larger locations such as shopping centres and theatres where there might be large atria, fire testing is frequently carried out to demonstrate its performance in such spaces. 

3.8.7 Protection enclosure integrity

Although the use of watermist for area protection (total flooding systems) is less susceptible to room leakage than gaseous fire suppression systems, it is the responsibility of the installer to specify the acceptable equivalent leakage volume for any protected area on the basis of tests.    

3.8.8 Nozzle positioning

Nozzle positioning is a key factor in the success of watermist system design, so appropriate notice must be taken of the presence of potential obstacles, because low momentum lightweight watermist droplets may swirl around obstacles and may not provide the three-dimensional penetration which is a feature of gaseous fire suppression systems. It is essential that nozzle positioning should strictly follow the requirements outlined within the manufacturer’s design and installation manual (DIOM). 

3.8.9 Nozzle heights and spacing

Installation of nozzles at heights and spacings that has been validated by the nozzle manufacturer’s tests is a crucial requirement. Compliance ensures the correct amount of water is discharged with enough kinetic energy to reach the seat of the fire and to ensure the efficient generation of water droplets to extinguish or suppress the fire, as described in the manufacturer’s design and installation manual.  
In general, maximum heights for nozzle installation are between 5m and 6m. However, greater nozzle heights could be considered under special circumstances if validated by the nozzle manufacturer’s third party-certificated tests. 
It is equally vital that compliance with minimum distances is also strictly adhered to, especially for object protection applications.  The discharge from nozzles that are too close together may impede the formation of the required water droplet characteristics at the seat of the fire or even cause unwanted turbulence at the seat of the fire.

3.9 Design, Install and Commissioning

Are there means to verify installers competence in designing, installing and commissioning? Is there evidence of their training and auditing by the manufacturer or third party? Are there design tools to aid and check installer work, to avoid misinterpretation (or deviation) from rules?  Can the system fully replicate a fire scenario as part of the commissioning process?

The watermist contractor must provide the client/specifier with a copy of their ‘Watermist DIOM’ that contains all the design and installation rules for the referenced system. Particular attention should be given to the condition of the distribution pipework. It should be confirmed that prior to installation, the pipework has been cleaned and is completely free of any sharp edges, swarf or debris that could impair the functional efficiency of the system. Systems which can be discharged by every nozzle as part of the commissioning process should do so to demonstrate its ability to work.  In the case of automatic nozzles, evidence that the water supply for the AMAO can be provided (either through tank or water demand surplus to peak demand).

Commissioning and snagging of the works must be undertaken by a commissioning engineer who reports directly to the watermist contracts manager.  
The commissioning engineer must also take responsibility for testing all aspects of:


  • electrical detection and actuation in accordance with the DIOM and standards referenced in the DIOM

  • any mechanical actuation checks of watermist equipment; 

  • the fully charged state of all water storage vessels, main water control valves and required gas cylinders; 

  • a complete functional check of all re-settable valves and actuators unless testing of these components resulted in system discharge through the watermist nozzles; 

  • a full system discharge test that may be required where appropriate. 

3.10 Uptime and Maintenance

Is there a way to validate that an installed system will operate as intended? Does commissioning include the discharge of nozzle(s)?  Will occupants be aware of the system? What are the mandatory and desired maintenance regimes? What are the potential failure modes? How are these addressed? If something goes wrong, what are the potential consequences?  How is this notified to the responsible person?

A user’s operating and maintenance manual should include detailed maintenance instructions covering all the individual weekly, monthly, quarterly and annual test procedures for all the mechanical and electrical components that make up the overall watermist system.
The full list of these components and their test requirements can be found in the manufacturer’s DIOM which will take into consideration any applicable standards or certification carried out on that system.  

Want to learn more:


  1. An Introduction

  2. Watermist Systems Classifications

  3. Using Watermist

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