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Stagnant water advice

If you have a building that's been empty for longer than a week or your water use has gone down significantly, you may have stagnant water in your system. Here's what to do.


What is stagnant water?

Water that has been held in your water tank or pipes for any longer than 24 hours will deteriorate and become stagnant. This is due to loss of chlorine residual, warming, and interaction with the plumbing system materials.

When this happens, it creates the right environment for bacteriological growth. Most commonly, this can lead to legionella colonisation but can also cause taste and odours, discolouration and increased levels of dissolved metals.

If this happens and you try to use the water system as normal, you could potentially expose yourself and others to legionella or other microbes, harmful levels of contaminants or foul-smelling and tasting water. We recommend establishing a control or water safety plan tailored to your water system.

Stagnant water FAQs

Read our FAQs for further information about stagnant water and other things to consider when reopening a premises.

If you are an employer, you will have a duty of care to your employees and anyone visiting your premises. The management of your water system also falls under the COSSH Regulations and Health Safety at Work Act.

If water usage has changed then the Risk Assessment is likely to be invalid for the current situation. The good news is that your Legionella Risk Assessor will already have a good understanding of your building and should be able prescribe a temporary scheme of control to recover the situation.

BSEN 806-5 Specification for installations inside buildings conveying water for human consumption – operation and maintenance. This standard provides specific requirements for the operation and maintenance of installations inside buildings. Section 6 of this document provides details on operation, section 12 provides details on maintenance, Annex A provides details on frequency for inspection and maintenance and Annex B provides details on inspection and maintenance procedures.

BS 8558 Guide to the design, installation, testing and maintenance of services supplying water for domestic use within buildings and their curtilages.

Guidance on BSEN 806. This standard provides additional guidance on best practice and section 6 provides details on water quality checks for stored water.

Legionella Code of Practice is freely available along with information on water safety plans for buildings.

You should be cautious and seek professional advice on re-commissioning your water system.

Find a water hygiene expert

This all depends on the size and complexity of your building's water system and whether any parts of the system have been in use. It's always best to get expert advice specifically for your water system but to give an idea of what's involved:

  • A small water system which perhaps just has a male and female toilet and kitchen with a simple combination boiler providing hot water may require just a thorough and safe flush of all parts of the water system and cleaning and descaling of outlets.
  • A larger building may have cold water tanks, more complex hot water plants and water-recirculating systems. These more complex buildings will require a risk assessment and a recovery scheme of control prescribed by an expert.

Certain things on water systems pose risks and require routine checks and maintenance. For example, water tanks, hot water storage vessels, hot water circulating systems, showers, thermostatic mixer taps and valves, expansion vessels and the design of the water system and pipework itself. The HSE guidance helps explain what you need to do.

We suggest flushing your hot and cold outlets one more time and if the water quality seems ok then it's probably fine. If you haven't got one, it's a good idea to arrange for a Legionella Risk Assessment

It's also worth making contact with your Assessor to explain the situation as they could point out something you haven't thought of.

You should flush your systems by opening each outlet for at least two minutes. For outlets connected directly to the mains, this should result in the water temperature dropping as water from the underground water main passes to the tap. 

When flushing outlets, spray should be minimised to reduce Legionella risk and tank-fed pipework should be flushed after the tank has been drained down.

You'll know that fresh water is running from the outlet when:

  • It's generally accepted that the water appears clear and normal.
  • Cold systems - When the cold temperature drops to within a degree or two of the incoming supply to the premises or tank.
  • Hot systems - When hot water outlets remain above 50℃ for a minute or so. (55℃ in healthcare premises).

Firstly, you should assume any stagnant water flushed may contain Legionella bacteria. So, you'll need to avoid exposing yourself or others to this flushed water. 

You should reduce water spray, for example, by placing towels over outlets or discharging outside taps or shower heads into a refuse sack.

Depending on the design of your water system, there could be a number of appropriate flushing methods. Think about how water normally flows through your building.

  • Where do the hot and cold water supplies come from?
  • Are there cold water tanks, hot water vessels, hot water circulation systems, point-of-use water heaters and outside taps?

Once you know how hot and cold water is distributed through your building and what any tanks serve, it's a good idea to flush outlets nearest those sources. Run the outlets for a few minutes, drawing fresh hot and cold water. Then close those outlets and systematically move through the building, drawing fresh water as you go, working towards the furthest points away from those hot and cold sources. It's probably better to work on one system at a time but this can get complicated. If you're not sure then seek expert advice (don't forget to operate appliances too).

Sources of supply:

Cold water sources

  • Water Main - This could be directly from the public main and without any onsite cold water tanks, in which case you should start drawing water from outlets closest to the incoming supply or main internal stop valve.
  • Tanks - Any tank should be cleaned and disinfected before you attempt to draw water from it. This is to avoid drawing any additional stagnant water through your water systems. Once the tank is clean, determine the flushing plan to draw water from outlets nearest that tank, working towards outlets furthest away.

Hot water sources

If the hot water vessel/system has been used less than normal for a week or more, it's likely to contain Legionella. The first step is to take expert advice on how to disinfect the vessel and system. Once disinfected, hot water should then be drawn systematically through the system (nearest to furthest outlets), until fresh hot water is drawn through for at least a minute at each hot outlet.

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