How Water Flushing Regime Controls Legionella in Building Water Systems?

The safety of building water systems must be taken very seriously. It is the responsibility of the statutory duty holder to keep the water systems safe by preventing the risk of legionella outbreak which could lead to Pontiac Fever & Legionnaires’ disease.

The most common way to prevent the growth of legionella bacteria in water systems is by controlling the water temperature. Hot water temperatures must be above 50°C within 1 minute of opening and outlet and cold water temperatures below 20°C after 2 minutes of opening the outlet.

However, building managers face challenges in maintaining the required temperatures due to a plethora of reasons. Shortfalls in the design and installation could make it practically impossible to maintain the temperature of the water system. Combined with other factors such as the availability of nutrients, stagnant water could prove to be a breeding ground for legionella bacteria.

In such cases, flushing the water systems ensures that water is flowing and is not allowed to stagnate. A regular flushing regime minimises the potential for exposure of the occupants by the immediate removal of contaminated water through either the entire facility or a localised area of the water distribution system.

In this blog, we discuss why every organization should consider including a flushing regime in their water safety plan.

What is flushing?

Flushing is an exercise of replacing stagnant water with the introduction of fresh cold water containing residual disinfectant. Flushed water carries away waterborne Legionella and dislodges biofilm lining the inside of the pipes and fittings, which harbours Legionella.

Water flushing moves water systematically through sections of a water distribution system, creating a scouring action to clean the line.

Periodically flushing outlets can be an effective means of achieving a regular changeover of the water contained in the little used outlet and its associated supply pipework.

Flushing pipes is a precaution that should be taken after any shutdown of more than a few days. Depending on the complexity of a building’s water system, facility managers should develop a comprehensive water management program for this process. It should not replace a facility-specific water management plan.

Importance of flushing in legionella control

The extent of water use is one of the most important factors affecting water quality. Where stagnation occurs or water use is low, cold water temperatures can increase significantly and there is the potential for Legionella growth.

For example, in a hotel during the offseason, there could be rooms that are often empty. In this situation, a plan should be put in place to ensure these little-used taps, showers, other water outlets, and sections of the water system are thoroughly flushed through regularly.

Showers are very important in this regard because of their capacity to generate aerosols and their potential to be under-utilised.

It is also worth noting that flushing might be required in circumstances where the usual risk of legionella is low. For instance, in a residential property that is rented out to tenants, there may be very little risk of the bacteria being able to spread.

However, if that property is left without a tenant present for several weeks or months, the water in the system will not be flushed through because of a lack of use. In this case, the landlord should make sure that flushing takes place weekly to ensure the system is well-maintained and safe for the next tenant to use.

When Legionella is detected within your water distribution system, flushing is one of several options available to reduce the immediate exposure to patients/residents and should be performed in conjunction with procedures for checking disinfection residual and temperature.

Where to undertake flushing?

Flushing should be based on the location and source (tap/shower) of the Legionella detection, which may affect an entire facility or only a limited number of the outlet(s) in a localised area.

Thoroughly flushing all onsite water plumbing systems in every building where water has been sparsely used or stagnant is important, as flushing replaces stagnant water with fresh and high-quality water.

Both the hot and cold plumbing systems within the buildings should be flushed by opening valves, faucets, and outlets at all points of use. Points of use within a building may include:

  • Kitchen and breakroom faucets used for drinking water or food preparation
  • Drinking fountains and water bottle dispensers
  • Bathroom sinks and showers
  • Toilets & urinals
  • Ice machines and refrigerators with ice makers
  • Water features that generate aerosols (fountains, spas, etc.)
  • Faucets and fixtures accessible to the public, including children, elderly and immunocompromised individuals

Some onsite water plumbing systems contain numerous areas where water is stored; these systems should be drained and flushed with fresh cold water. These areas include, but are not limited to:

  • Cooling towers
  • Humidifiers
  • Hot water storage (some buildings have more than one type of heating system and hot water storage)
  • Hot water recirculating loops

How to flush hot & cold water systems

Flushing procedures should be based on a risk assessment of the water systems in the building/institution concerned. A flushing protocol is only effective where the water system is adequate and the water supply is not contaminated. This particularly applies where there are water storage tanks.

Flushing in stages is best. Begin at the main service line and include all the plumbing, storage tanks, fixtures, and equipment such as water fountains and ice machines. The first flush pulls out stale water, while follow-up flushes draw freshly-treated water through the building. The longer service is interrupted the more flushing is needed.

Flush cold and hot water at all water points of use (faucets, showers, toilets, drinking fountains, and water-using devices such as dishwashers and refrigerators/ice makers) to replace the water that has been standing in the pipes. The flushing time can vary by the plumbing configuration and type of outlet being cleared

  • It is important to flush the service line that runs from the water main to the building before flushing the rest of the building’s plumbing system.
  • Special consideration should also be given to pipe loops and onsite water storage to ensure these are adequately flushed and maintained.
  • Water-using devices may require additional cleaning steps in addition to flushing (e.g., discarding ice). Consult the device manufacturer’s maintenance instructions.
  • Flush the hot water until the water reaches its maximum temperature. Where possible, hot water at the tap should reach at or above 50°C. Anti-scalding controls and devices may limit the maximum temperature.

Consider checking water quality parameters, such as temperature, pH, and disinfectant levels, in the water entering the building and at points of use after flushing to verify that fresh water is being flushed through the entire plumbing system. Achieving stable temperature, pH, or disinfectant levels can be a good indicator that the system has been adequately flushed.

How often should flushing be done?

According to HPSC’ Guidelines for Control of Legionellosis in Ireland, flushing should be carried out at least weekly to reduce the risk of Legionella and other bacterial growth.

But in high-risk populations such as healthcare facilities and care homes, a risk assessment may indicate the need for more frequent flushing – i.e. twice weekly.

The duration of flushing should be based on a risk assessment but at a minimum, the procedure below should be followed:


Run showers for six minutes weekly as follows:

  • Run cold for three minutes
  • Run hot for three minutes once water is hot


Run individual hot and cold taps weekly as follows:

  • Run cold for three minutes
  • Run hot for three minutes once the water is hot.

Mixer taps

  • Run with the lever in the coldest position for three minutes weekly
  • Run with the lever in the hottest position for three minutes weekly
  • Ensure that hot water comes out hot when in the hot position and cold when in the cold position

The flushing procedures for hot and cold water services are shown in the table.


Where a single TMV serves several multiple showerheads, it is important to ensure that these showers are flushed frequently. Where an outlet is not used for more than a week it must be flushed until the temperature of the water at the outlet has reached the pre-determined temperature set by the thermostatic mixing valve.

Follow the Best Practices

Flushing may seem easy, but proper care must be taken to ensure the process is carried out correctly.

Cold water should be used to flush the cold water system and hot water to flush the hot water system.

The period of flushing must be sufficient to remove all stagnant water leading to the outlet. The number of outlets that can be flushed simultaneously will depend on the capacity of the water heater and the flow capability of the system

Water emitted from the outlet during flushing should be purged to drain safely. Flushing should be carried out in a safe manner that minimises aerosol production e.g. removing showerheads before flushing.

Properties with water treatment systems like reverse osmosis, filtration and/or water softeners may need those systems serviced in accordance with manufacturer recommendations. Bacteria can grow in stagnant water, so replacing water filters is generally recommended. Always consult and follow manufacturer guidelines for disinfecting a building’s water treatment systems.

Hard-copy or electric records should be recorded, including a list of the individual outlets flushed; the date, time and duration of flushing; biocide concentration; any other observations; and the initials of the person carrying out the work.

Having a system in place to help you identify outlets that are used infrequently is also very important. This may mean involving others within your organisation to find out how often outlets are used, as well as designers and installers.

Reopening Work Premises Safely

Stagnant water poses several health risks, including the growth of waterborne pathogens such as Legionella species, as well as increased potential for corrosion that results in the release of lead from pipes and plumbing fixtures.

With buildings shut down or infrequently used during the COVID-19 pandemic, water is not circulating through plumbing systems and has become stagnant. As a result, disinfectants in the water dissipate, potentially allowing microorganisms, bacteria and pathogens to grow within plumbing systems.

Such systems may have been out of use for a significant time and in most cases cannot simply be used straight away. The system may require recommissioning as if new (that is thorough flushing, cleaning and disinfection and/or controlled flushing of outlets such as taps, showers and toilets) prior to returning to use and reopening of the building.

Risk assessment review and water testing should also be considered as part of the recommissioning plan.

How We Can Help?

Responsibility for health and safety falls to the employer or to those who maintain control over the building – usually the duty holder and the responsible person. Failure to fulfil this responsibility can have dire consequences for public health, facility owners and their employees, not to mention the duty holder themselves.

Addressing stagnant water concerns may seem daunting to building owners who have never had to consider these impacts before.

Water Treatment Ireland Ltd delivers a range of legionella control solutions to mitigate the risk of a legionella outbreak. Our team of experts support duty holders to keep their facilities safe and comply with relevant health and safety legislation.

If you would like to speak with one of our specialists about managing legionella risks in your facility, feel free to reach out to us.