What Are Dead Legs & How They Pose a Risk to Building Water Systems?

A dead leg is a section of pipework that is either completely unused or has very little flow running through. This includes isolated branch lines, pipe sections with closed valves and pipes with one end-capped.

Dead legs experience periods of no flow which leads to stagnation. Basically, any closed valve not connected to a fixture, appliance or piece of equipment can be considered as a dead leg.

Dead legs are often formed in both industrial and domestic process piping as processes are changed or modified.

​​One of the features of dead legs is that the water that flows through the rest of the pipework might do little to stir the water that ends up trapped in a ‘dead leg’.

Why are dead legs formed?

Before breaking ground, as institutions or corporations plan for expansion or a development firm invests in a new facility, they consider the schematic design for necessary utilities, including the building’s HVAC, electrical, and plumbing systems.

Dead legs may be intentionally installed to facilitate future expansion of a building’s potable water plumbing system to avoid a complete building outage.

Buildings undergo frequent modernization, remodelling and tenant improvements, which mean potable water plumbing changes can result in dead legs. Hospitals, hotels or other buildings can become offenders since they constantly renovate or expand facilities.

Functional dead legs can result from lack of use, such as when different wings of a hospital or floors of a hotel are closed due to low occupancy.

What is the difference between dead legs and dead ends?

In general terms, a dead leg and a dead end are the same: they are sections of pipework that are redundant or very infrequently used.  As a result, they allow water to stagnate within the pipe.

In simple terms, a dead leg is a section of piping that is no longer used, or occasionally used. Dead legs lead to other sections of the water system such as another pipe, an outlet or a valve. Intermittently operated faucets, showers, chillers, heat exchangers, and pumps can also become dead legs depending on how long they are out of service.

In contrast, a dead end (or blind end) does not connect to anything else. The water flow stops when it reaches the end and has nowhere else to go. For example, if a sink has been removed and capped off leaving lengths of pipework containing stagnant water.

Why Are Dead Legs Dangerous?

Redundant pipework is considered to be a persistent and often critical source of risk within water systems.

Perhaps, the biggest risk with the presence of dead legs in water systems is the formation of biofilms.

Biofilms are communities of surface-attached bacteria surrounded by a sticky, gel-like secretion often called slime. Although biofilms start out microscopic in size, they can grow into visible biofouling deposits in just a matter of days.

Without flow, the water in a dead leg does not receive disinfectant or biocide treatment. This allows bacteria to attach to system surfaces to start the biofilm formation process.

Once established, biofilms and associated microorganisms provide a protective environment for Legionella bacteria, entering a building with the source water to multiply to high levels.

Stagnation also reduces the effectiveness of the residual disinfectant often added to inhibit the growth of waterborne pathogens. Once the disinfectant is consumed, and no other water and disinfectant is introduced into the piping due to lack of flow, Legionella and other waterborne pathogens can amplify.

In addition to encouraging the growth of harmful bacteria, dead legs could lead to corrosion, especially if organic matter and stagnant water are present.

All of these factors work together to provide legionella bacteria with the nutrients it needs to grow at an increased rate and lead to a possible outbreak.

How to Identify dead legs?

Adding extra piping often laid out to ease future expansion, could create significant problems down the road for the building owner and occupants.

If the dead leg is put into use it can cause heavy contamination within the water system, which in turn will disrupt the efficacy of the water treatment regime.

As part of your legionella control programme, schematic drawings will map out where any dead legs occur.

A legionella risk assessment will identify these areas of risk and outline any remedial actions to be taken.

How to Manage Legionella Risks from Dead Legs?

While there are treatment options for prevention of, and if necessary, remediation and decontamination after a Legionella event, dead legs in piping systems defy most of these treatments.

Dead legs are often immune to flushing as the exchange of water is inhibited. Similarly, temperature control is no guarantee because of the lack of water flow.

To truly eliminate the growth of legionella bacteria, dead legs have to be designed out of any system before the first pipe is laid. With this design-led approach, it is critical that the traditional T-piece plumbing method (so often used in the UK) is consigned to history.

Loop and serial installations offer a more hygienic approach than traditional T-piece systems as they significantly minimise the risk of standing or stagnant water.

In existing buildings, the best control strategy is to eliminate the dead leg.

In addition, the following points should also be considered:

  1. The responsible person should ensure that legionella risk assessment and schematic drawings are up to date and reviewed regularly.
  2. If the water outlet is used only occasionally and cannot, therefore, be removed, an appropriate flushing regime should be set up. It should be indicated by the legionella risk assessment and proper records of flushing are maintained.
  3. During a period of inactivity, the whole water system can become a large dead leg. It is important to have a robust flushing regime and microbiological sampling regime in place until the building is used.
  4. As stated above, where expansions are planned, it is important to get the design right to minimize the risk of dead legs in water distribution systems.
  5. Comprehensive records must be maintained right from the commissioning stage that show flow rates and temperature profiles for all outlets, water storage tanks and hot water generators.
  6. If necessary, modify circulation pumps, improve pump performance and balance valves for hot water systems.

Review Your Risk Assessment

If you are the designated responsible person, responsible for the upkeep of a hot or cold water system, you should have a legionella risk assessment and risk management procedures in place.

Our qualified engineers are specially trained in identifying areas of risk within a water system, including pipework no longer in use.

If this pipework cannot be removed from the system, our specialist engineers will terminate the pipe so that it is as close to the main as possible and fit drains from the area to reduce risk.

If you are looking for expert guidance on the legionella risk assessment process, look no further. Contact our team today!