The truth about tankless water mist systems in 2024

It’s tempting to rely on tankless water mist systems which claim to never need more than 6 litres per minute – but the decision could cost more than money.

Tankless water mist systems can be greatly impacted by subtle changes. Just as flight was impacted to subtle changes to Smeaton's Co-efficient.
The Wright Brothers successfully secured heavier than air flight by recognising the impact which subtle changes can make to a design – between the lab tests and real world application. Image courtesy of wright-brothers.org

Are tankless water mist systems reliable?

Let’s start with the elephant in the room. Are tankless water mist systems reliable? Would a tankless water mist system be able to protect you, your family, and your home in the event of the fire? Would a tankless water mist system leak?

The honest answer is: it depends. There absolutely is a potential for tankless water mist systems to work as hoped if they are properly designed, tested, and installed. However, there is no silver bullet. In much the same way as central heating varies from home to home, mist and sprinkler systems also need to adjusted from property to property. Some homes may only need a single radiator in their living room, others may need several. Similarly, a tankless water mist system may work for one property but that doesn’t guarantee that it will work for the house down the street.

We’ve looked, in detail, about what is required for a tankless water mist system, as well as a tankless sprinkler system. Our conclusion was that, while it absolutely is possible to install a tankless water mist system, there is a very real risk that a residential mist system could actually need a bigger tank than a sprinkler system for the same property – if it is properly installed.

We have a tank size calculator and guide on how to size residential sprinkler tanks here. To consider the differences between tank sizes, we also go into further detail in our CPD.

The question now turns to how you can check if your system has been properly installed and whether your own property is suitable for a tankless water mist system.

In many cases, tankless systems are attractive to developers because they offer a certifiable solution which saves them the space, the cost, and the co-ordination required by a tank. In some cases, the attractiveness of this potential can override best judgement and systems are sometimes installed which don’t quite work as intended.

Why is 6 litres per minute important?

6 litres per minute is an attractive target for mist installers. It’s high enough for a viable nozzle to operate at, while still being lower than the public mains will supply. 6 litres per minute is around the same amount of water as would fill a home waste paper basket. 6 litres would weigh only 6 kg and can easily be supplied, within a minute, by the majority of domestic water supplies.

Unfortunately, the 6 litres per minute figure reflects the demand from a single nozzle. In practice, systems are required to be able to accommodate more than one nozzle during operation. More importantly, in reality: more than one nozzle is likely to operate in a fire situation.

Where does 6 litres per minute come from?

The 6 litres per minute value seems to have been cemented based on a number of misconceptions:

  1. The misconception that because mist nozzles require less water, the system as a whole must do too;
  2. The misconception that because sprinkler systems work on a limited number of design sprinklers, mist must do too;
  3. The misconception that because a mist system is certified under a fire test, each system installed in a similar manner is certified too.

System demand v nozzle demand

If each nozzle operates at 6 litres per minute, and there are half a dozen nozzles in a room, then the system will require a minimum of 36 litres per minute. This requirement can be even higher if those heads happen to be placed in the most favourable area, which can easily add 20% to the demand. If those heads are also placed at different heights, as tends to be the case with open plan stairways, then a further 20% demand (for each floor) can be required due to static pressure.

It’s also the case that a suppression system which shares an incoming main with other services (such as the bathroom or drinking water) must either install an additional a priority demand valve or demonstrate that they can provide an additional 25 LPM in domestic settings. This increases to 50 LPM in residential settings.

It can be seen then that a mist system, operating at 6 litres per minute, per nozzle, installed as part of an open plan stairway, with 2 heads on each floor of that stairway, can easily require 60 LPM. Given that low flow sprinkler systems can also operate at 60 LPM, it can be seen that relying on mist offers no more of a guarantee of a tankless system than traditional sprinklers do.

Design sprinklers v operating nozzles

Tankless water mist systems differ from tankless sprinkler systems due to their design requirements. Whereas sprinkler systems can limit the number of heads required to operate down to 2 (in domestic settings, or 4 in residential settings), both tanked and tankless water mist systems must show that they can provide enough water to operate every nozzle within a room. While the individual nozzles may require less water, the system as a whole can sometimes require more as a result of this difference. The relevant passages are below:

This table, from the sprinkler standard, limits the number of heads needed to size the tank. It does not appear in the mist standard.
Mist systems are not necessarily the best option for tankless sprinkler systems.
An extract from the residential mist standard, confirming that all nozzles must be assumed to operate.

The difference between tankless water mist systems and tankless sprinkler systems

We’ve gone into detail on this aspect in our post on tankless sprinkler systems. The key takeaways include:

  • Both sprinkler systems and mist systems can operate directly from the mains without a pump or tank;
  • The low flow, tankless water mist systems require higher pressures than can be provided by the mains. As a result, they always need a pump;
  • In contrast, traditional sprinklers can often operate using the mains flow and pressure so require neither a pump or tank

Aren’t low flow, high pressure, mist systems certified?

Yes… and no. The systems which we’ve seen have only secured Annex C Certification. None of these installers have secured third party certification (such as the firas scheme, which is available to mist installers). In any case, Annex C Certification is the first step in securing system compliance.

What does Annex C Certification (the fire test) look at?

The fire test is defined under 6.1 and Annex C of BS 8458: 2015 and considers:

  • The ability of a system to suppress a fire for 10 minutes (domestic) or 30 minutes (residential) in a defined test environment;
  • The ability of the system to regulate temperatures during the course of the fire;
  • The ability of nozzles, which are outside of the fire area, to hold back from discharging while the other area operates.

The test is used as a baseline which is then referred to when designing a real life system. This includes tankless water mist systems. This is explained at 6.1 of BS 8458: 2015. It is not the case that passing an Annex C test guarantees the effectiveness of a tankless water mist system in a real world application.

Extract from 6.1 of BS 8458: 2015

What doesn’t Annex C (the fire test) look at?

Multiple heads or nozzles in operation

Fire tests do not necessarily confirm that the number of heads or nozzles required to meet the standard are able to operate. It can be seen from plumis’ video that their tankless water mist system passed the Warringtonfire test using only one nozzle. However, the associated report (referred to on the Annex C Certificate itself) makes clear that this does not guarantee that the system would work in a real world fire as a tankless water mist system:

Extract from the 2022 Warringtonfire assessment of the plumis tankless water mist system.

Each home is different. The locations of nozzles will differ in each case. Even identical homes may have differences in the positions of alarms in the ceiling or light switches in the walls which could impact on the number of nozzles in a room, or that nozzles’ ability to tackle the fire. The video below is part of a lab test which is designed to be repeatable. Repeating the test until a design outperforms its competitors in this controlled environment may seem impressive, but its akin to putting someone onto a Premier League football team simply because they’re good at FIFA.

Fire tests exist to demonstrate that a system can meet a baseline level of reliability, but that doesn’t supersede the requirements of the standard itself. The residential mist standard is BS 8458: 2015, which clearly states that water supplies should be capable of supplying all nozzles within a compartment (aka each room):

Mist systems are not necessarily the best option for tankless sprinkler systems.

The fire test requires the test scenario to feature three heads (two expected to operate, with a third external to the compartment, required to not discharge during the test). Regardless, when the system installed in a real world environment has more than two nozzles in the compartment, the system must be able to provide enough water to serve all of those nozzles (up to a room size of 64m2).

This differs from the sprinkler standard, which is based on a limited number of design sprinklers (no more than 2 in a domestic setting). This is for good reason. Fire relies upon three components in order to burn. This is often presented in a fire triangle:

Image courtesy of Fire Action

Traditional sprinklers tackle all three components by:

  • drenching the fuel in water;
  • cooling the fire; and
  • displacing the oxygen with water.

A key selling point with mist is that it doesn’t wet the area that it covers. The downside of this leaves the system only addressing two components of the fire:

  • Cooling the fire; and
  • Displacing oxygen with mist.

To reinforce the system, BS 8458 requires that all of the nozzles in the compartment discharge, regardless of how many there are (up to 64m2). This can have the biggest impact on open plan staircases, which can have a number of heads within the fire area, whilst also being impacted by static pressure (the weight of the water between the highest and lowest nozzles). In some cases, the tank can be bigger on mist systems than they are on sprinkler systems.

Tankless water mist systems, just as any other suppression system, also need to be able to tackles fires which may be located at different point around the room. In some cases (such as those shown below), a single nozzle operating would not be able to tackle each part of the fire:

Battery fire (laptop)
Battery fire (hoverboard)
Kitchen fire
Alcohol fires
Fires caused by pets
Cats v candles
Examples of fires which may not be controlled by a single nozzle or head

Leakage

The usual contenders for tankless water mist systems operate at a higher pressure than standard equipment is designed to operate at. The pumps, valves, pipework/hoses, and other components all need to be capable of withstanding pressures which can be more than ten times higher than traditional sprinkler systems. Mist is a developing technology and so some installers rely upon components which are primarily designed for alternative applications while specialist equipment is developed, tested, and certified. The results aren’t always the best.

Extract from the plumis operating manual

Sprinkler systems are tried and tested, having first been created as early as the 18th century. The equipment used is well developed and scrutinised. Further, we only use equipment which is either firas, LPCB, or FM approved, to guarantee that your property will not suffer damage from leaks.

In contrast, mist systems are using less developed components which are known to leak. How much they leak isn’t clear, but it is clear that it’s enough to notice. While a 5 minute wait at standard working pressure may seem reasonable, it is a world away from the water regulations’ requirement of half an hour at 1.5 x working pressure, followed by 90 minutes of monitoring.

Even more concerning, parts are known to leak during operation. While this may seem manageable, there appears to be a risk that a sufficient level of water would not reach the nozzle to operate. It may even be the case that, while the nozzles tackle a fire in one room, the leaks upset electrics in another and create a secondary fire. In any case: a system which is supplied with 6 litres per minute and leaks, will not be providing the nozzles it serves with 6 litres per minute.

Concerns about the reliability of ancillary components led to the creation of BS 8366: 2019, which introduced a minimum standard for the components themselves – but not all mist systems have achieved this accreditation.

Other services using the same supply

When an infill is shared with other services, BS 8458 requires that water supplies (including tankless water mist systems) be able to provide the following:

  • An additional 10% of the flow required for the mist system itself; and
  • An additional 25 litres per minute for domestic installations, or 50 LPM for residential installations (unless a priority demand valve is installed)

The fire test doesn’t factor these requirements into its process. This is left to the final, real world, design itself.

Worst case scenario

In his 2003 book, “A mathematician plays the stock market”, John Allen Paulos wrote:

“the only thing certain in life is uncertainty”.

Let’s consider this then from the perspective of an engineer who has been asked to assess the reliability of a fire suppression system. Would you be willing to guarantee that your test will reflect the worst case scenario? Would you even be able to predict the worst case scenario? What if your test could predict and reflect the worst case scenario? Would that guarantee that it could also predict performance in the second worst case scenario?

Absolutely not. There are simply too many variables. The number of floors within a fire compartment could differ between properties. The particulars of the fire may change. The footprint may differ. The spread may differ. The fuel may differ. What if there’s a blackout at the time of the fire? While a good lab test will consider a range of scenarios, no certification company could ever be expected to confirm reliability in the worst case scenario.

The same is, of course, true for tankless water mist systems. While fire tests have been carried out and passed by reputable companies for tankless water mist systems: these tests simply set a baseline for performance for the system components themselves under their strict laboratory setting (which needs to be repeatable). The fire test also only considers the responsiveness to fire.

The certification companies themselves make clear that the test results relate only to the conditions of the test and that small differences to the composition may significantly affect performance and invalidate the results. The following is from the 2022 Warringtonfire assessment of the plumis tankless water mist system. Plumis is not deliberately being chosen out. The same limitations apply to any fire test for any system, including ICO, and traditional sprinkler systems.

Extract from the 2022 Warringtonfire assessment of the plumis tankless water mist system.

The Wright Brothers: Do slight variations really make that big of a difference?

Slight changes can make the most crucial difference to a design. Especially when it comes to fluids. This isn’t unique to tankless water mist systems. If it wasn’t for the Wright Brothers re-evaluating Smeaton’s co-efficient, we would never have cracked heavier than air flight in 1903. The change was small, but it was the breakthrough needed to take us from an 12 second flight on Kitty Hawk in 1903, to military planes operating within 10 years during the Italo-Turkish War in 1911. More progress was made in those ten years than in the hundred years which came before. By the end of the First World War, Britain had established the RAF and military planes were a standard resource.

How big was the difference? Smeaton’s co-efficient had been in use since the 1700s and used to assess the density of air when designing windmills. Originally, it was found to have a value of 0.005. A number which engineers had failed to recognise the significance of due to its size.

The Wright Brothers’ genius laid in their willingness to reassess all factors relied upon for flight. Using a home made wind tunnel, the bicycle repair brothers refined Smeaton’s co-efficient, from 0.005 to 0.0033. The numbers are tiny but this small fraction represented more than a 30% change in the density of air. While the established authorities in the arena, including Samuel Langford, a Director of the Smithsonian Institution, had the resources to achieve heaver than air flight, and were anticipated to do so: they failed to recognise the impact of small changes.

Engineers have learned from these mistakes. When it comes to tankless water mist systems, just as with other lab tested systems, the certification company is clear:

Extract from the 2022 Warringtonfire assessment of the plumis tankless water mist system.

What is needed to certify an installation for building regulations?

Although plumis state on their website that their system is compliant with building regulations. We’re not convinced. At least, not when its installed in the way in which in which we’ve seen that it has been marketed. It’s our professional opinion that there is no sprinkler or mist system on the market which can guarantee compliance without a tank without carrying out a proper design assessment for the particular property.

Plumis are not alone in making the claim that their system can guarantee compliance as a tankless water mist system, but their promotional material appears to be littered with claims which aren’t backed up by the engineering. Plumis, alongside any other company who believes that their system is able to operate, in all domestic settings, without a tank, are invited to reach out to us if there is something which has been overlooked. The most fundamentally required aspect of any competent engineer is their ability to accept when they are wrong and follow the science. We’re open to this, but the science seems to be telling us otherwise.

When it comes to the building control regulations themselves: England, Scotland, and Wales all require a system which either is, or is equivalent to, BS 9251 (the standard for traditional residential sprinkler systems). We offer both traditional sprinklers and mist systems which meet this criteria, including tankless water mist systems (where appropriate).

We do not believe that a system which relies upon a single nozzle operating in the most favourable area, and provides no tank, is equivalent to BS 9251. Many reasons are discussed above and can be summarised as:

  • A single head operating neither matches the criteria of BS 8458 (which may be considered equivalent to BS 9251 in some circumstances) nor the minimum number of heads operating for a traditional sprinkler system;
  • The components used haven’t been been subjected to the same levels of testing and are known to leak;
  • Relying solely upon empirical data (lab tests) without verifying the performance via calculation for each installation leaves too many gaps in the design process.

The future of tankless water mist systems

There are a lot of positives in plumis’ design and the system absolutely has potential. High pressure mist undoubtably has a future and a market.

The purpose of this article is not to stomp out that progress, but to ensure that it continues. Claiming that these systems can guarantee performance without a tank is misconceived at best and downright dangerous at worst. If there isn’t enough water for the system to operate, someone will eventually get hurt.

Our position is that engaging in a dialogue, around where the technology is truly at, is the only way to ensure progress. We believe that the technology has taken us as far as it can. It’s now for the water boards to improve their supplies, as we’ve written about before.

We wish only the best to these manufacturers, but equally believe in the importance of managing expectations in order to move forward.

Have we missed something?

If any manufacturer or developer believes that they have managed to crack tankless water mist systems, we are more than interested in discussing your solution and amending our view on this.

In the meantime, developers, manufacturers, and installers are welcome to explore more of our articles in the Sprinkler Academy.

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