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Returning to a building? What you need to do now about the Legionella risk

It’s about now that many of us are returning to offices after a period of working from home. But how safe is your building for you and your staff? Amongst the range of risks that exist in properties that have stood empty, have you considered the risk of Legionella?

It’s a question that’s worth asking.

As we always say, there is an ever-present risk of Legionella in all hot and cold water systems.

How is Legionella prevented?

The simplest way is to run the taps and other water outlets. That is, to ensure regular flow of water through the entire system and out of all taps, showers, etc.

Why is it different now?

In some cases this will be the first time in living memory that a building has been left idle for weeks, let alone months. Lack of use is a serious hazard because it can lead to the growth of Legionella bacteria within the water system. The larger and more complex the estate, the greater is the extent of the potential problem. For example, water might be pumped around the system, heating and cooling as it goes. The hazards in these environments will be numerous. But even the simplest of systems will grow Legionella if left unused.

In a typical building in normal use, every tap in every kitchen and bathroom and every other water outlet will be used by at least someone most days. Now, all of a sudden, not only have entire buildings been left empty for long periods, but we also have to consider the effect of staggered returns. For example, it may be that some departments have returned to work and some have not. Or that such a skeleton staff is operating that you can’t be sure that your whole system has been flushed with water regularly.

What should you do?

There are simple checks that any building manager can do themselves. Any checks that you carry out yourself should be documented.

Because places of work and places with public access need to be independently checked every two years anyway, it may be the ideal time to have this done. You can see what is included in ours here: If a building is left unused for some time, this could constitute a ‘material change’ in circumstances and therefore should trigger action according to your existing Legionella Management Plan. Residential landlords similarly have to conduct (usually simple) risk assessments and ensure effective management. So, a landlord will also have to take action if a property has been left unused for a week or more.

It’s useful to check the temperature of the water at various points in the system. Cold water has to remain below 20 degrees Celsius and hot water above 50 degrees Celsius throughout the system to ensure safety.

To flush the system, you need to run every water outlet for long enough for water to drain from the entire system of pipework (this could be seconds or minutes). If in doubt, you can drain it via the stop tap. Particular caution should be taken with any tanks that may have held stagnant water. In these cases, an inspection is recommended, including visual inspection and temperature checks. Disinfecting a tank should be considered. In our view, chemical disinfectant should be the last resort both for cost and environmental reasons.

What precautions should be taken when flushing a water system?

It’s important when doing so to take precautions. Assume that Legionella bacteria are present and avoid allowing these to become airborne and be inhaled.

If your building is being repopulated gradually, then continue to flush the system at least every seven days and continue to ensure that every part of the system is included.


We are happy to provide advice to any building owners or managers who have any concerns. Just contact us here. An initial phone conversation will cost you nothing. If subsequently, you need a risk assessment or other services we can quote for the work, but you’ll get no pressure from us.

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