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Elevated-velocity (EV) flushing uses increased shear forces to destabilise and mobilise material from internal pipe surfaces - enabling bulk mass transfer to drain. At commissioning, this includes swarf, sediment, corrosion products and other debris; in operation, it extends to the removal of accumulated biofilm.
Similar to debridement in chronic wound care, it is a mechanical intervention - removing accumulated biofilm rather than attempting to treat it in situ.
It is not a one-off treatment, but part of a controlled, repeatable regime for managing a chronic and unavoidable condition within water systems.
See EV flushing in action:
Flushing kits are available for Horne taps, TMVs and shower systems.
Elevated-velocity flushing can be applied during commissioning and throughout the operational life of a water system. By increasing flow velocity and removing restrictions, higher shear forces are achieved - mobilising accumulated material for removal to drain.
This is not simply a cleansing flush, but a controlled mechanical intervention to support system performance and water quality.
A simple flushing kit enables this process. Typically, only one or two are required per site, supporting commissioning compliance, protecting precision components and contributing to ongoing system management.
The following sections explore how elevated-velocity flushing can be applied in practice, and the role of Flushing Kits in supporting this approach.
Elevated-velocity flushing is a controlled method of increasing flow velocity within a water system to maximise shear forces at internal pipe surfaces.
It should not be confused with a routine duty flush at the outlet, which simply runs water at normal regulated flow rates to reduce stagnation. Elevated-velocity flushing instead bypasses outlet restrictions and thermostatic controls to achieve significantly higher velocities, enabling the mobilisation and removal of accumulated material from within the system.
The benefits of this method include:
Elevated-velocity flushing is a crucial step in the commissioning of new plumbing installations and is referenced within BS EN 806 and BS 8558. It should be carried out prior to the commissioning and operation of installed thermostatic mixing valves, shower valves and other precision components, ensuring debris is removed and systems operate as intended from the outset.
Mechanical Cleaning of Pipe Surfaces
The primary function of elevated-velocity flushing is the mechanical cleaning of internal pipe surfaces.
During construction and connection, swarf, sediments, particulate matter and other debris can enter the system, posing a risk to downstream fittings and precision components. In operation, corrosion and precipitation products may also accumulate, providing a substrate for biofilm attachment and development over time - impacting water quality and system performance.
Elevated-velocity flushing introduces a temporary hydraulic regime that departs significantly from normal flow conditions. The resulting increase in flow velocity and shear forces at the pipe wall promotes the abrasion and mobilisation of adhered material and accumulated deposits.
Once dislodged, these materials are entrained within the flowing water and removed to drain, returning the internal condition of the pipework to a more manageable state.
BS EN 806-4 (Water System Commissioning) and its complementary guidance BS 8558 require a cleansing high-speed flush, using wholesome water, of new or modified domestic water system (DWS) pipework prior to commissioning and operating its specialist components.
This applies to new installations, systems that have undergone refurbishment or alteration, and those returning to service after periods of low or no use (for example, during the Covid-19 shutdown). The purpose is to remove contamination introduced/accumulated during installation or after stagnation, prior to normal operation.
Key requirements and considerations include:
Horne thermostatic mixing valves, taps and shower systems are designed to support compliance with these requirements. Dedicated access points to hot and cold water supplies are provided upstream of flow-restricting components (check valves, mixing valves, flow regulators and outlet fittings) and downstream of integral isolating valves
In combination with the full-bore isolating ball valve and flushing kit adaptor, this configuration enables unrestricted discharge and the achievement of maximum system flow velocities.
This design approach, combined with the flushing kit accessory, supports effective removal of sediments during commissioning and, once operational, the mobilisation and removal of accumulated biofilm - contributing to improved system cleanliness, performance and longevity.
References:
Following commissioning, ongoing domestic water system (DWS) quality can be supported through regular elevated-velocity (EV) flushing. This assists in managing waterborne and retrograde pathogens that reside within biofilm, which inevitably forms on surfaces in contact with non-sterile water supplies.
Understanding Biofilm Formation
Biofilm is a structured community of microorganisms embedded within a protective and adhesive matrix. It readily forms on internal pipe surfaces in water systems.
Biofilm development typically progresses through five stages:
The Bio-Load in Water Systems
Hans-Curt Fleming, a leading biofilm researcher, estimates that approximately 95% of a water system’s microbial load resides on pipe surfaces, with only 5% suspended in the water as planktonic organisms.
This highlights the importance of targeting surface-associated biofilm, rather than relying solely on treatment of the bulk water.
Elevated-velocity flushing promotes large-scale, controlled stage 5 dispersion of biofilm. By increasing shear forces, accumulated material is destabilised and mobilised from pipe surfaces, allowing it to be removed to drain.
This process does not eliminate biofilm entirely, but reduces its accumulated mass and returns the system to an earlier, more manageable state (maximum to stage 2, irreversible attachment).
Opportunistic Premise Plumbing Pathogens (OPPP)
OPPP are a group of microorganisms that thrive within biofilm in plumbing systems and can pose health risks in institutional and commercial environments. Common examples include:
The increasing use of low-flow fittings to conserve water can introduce unintended challenges - particularly in older water systems designed for higher demand and flow rates.
In institutions such as hospitals, where plumbing systems are often extensive and ageing, reduced flow rates can create low-energy hydraulic conditions. These favour the accumulation of deposits and the development of biofilm, increasing the need for intervention to maintain water quality.
Reduced flow rates can also affect drainage systems originally designed for higher volumes. Lower velocities make it more difficult for accumulated material to be transported effectively, increasing the risk of deposition, blockages and disturbance during use.
This can contribute to aerosolisation and the potential for retrograde contamination at the water system periphery - particularly in sensitive environments such as healthcare settings.
Elevated-velocity flushing can help to periodically mobilise and remove accumulated material within both water and associated drainage systems. In doing so, it supports a more effective hydraulic regime - promoting bulk mass transfer and contributing to improved whole-system hygiene and performance.
Chemical treatments are generally most effective against free-floating (planktonic) bacteria within a water system. However, as highlighted by researcher Fleming, this represents only a small proportion of the total microbial load - typically around 5% - with the majority residing within biofilm on pipe surfaces.
As a result, chemical approaches alone often fall short in controlling established biofilm. The inherent structure and resilience of biofilm limit the effectiveness of antimicrobial agents, highlighting the importance of complementary approaches.
Protective Biofilm Structure
Biofilms consist of microorganisms embedded within an extracellular polymeric substance (EPS), which acts as a protective barrier.
Promotion of Antimicrobial Resistance (AMR)
Exposure to sub-lethal concentrations of antimicrobial agents can contribute to the development of resistance mechanisms within biofilm communities.
Unintended Consequences
Chemical interventions can also have broader ecological and system-level effects:
These limitations highlight the importance of mechanical intervention. Elevated-velocity flushing removes accumulated biofilm as a non-selective bulk mass transfer to drain - mobilising all constituent microorganisms and associated material.
By physically removing this mixed population, rather than targeting specific species, the process reduces the selective pressures and inter-species interactions that can drive resistance, persistence and pathogenic dominance - supporting a more controlled and manageable system condition.
Elevated-velocity flushing provides a mechanical, non-selective approach to removing accumulated biofilm and associated material:
A simple Flushing Kit enables this process - supporting both commissioning and ongoing system management.