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The dangers of inappropriate design of water systems in buildings

Water systems are often more complex than people realise. This is especially true with old buildings and those that occupy large sites. But what effect does the design of a water system have on the Legionella risk? Quite a significant one, in our experience.

System design

The ideal is a system that:

  • Is closed off from external contaminants.
  • Allows the flow of water through the system without delays.
  • Is temperature controlled throughout. The temperature must be below 20°C for cold water and above 50°C for hot water continuously to prevent the growth of Legionella bacteria.

Inappropriate design can hinder the working of the system, either because of unsuitable components or lack of consideration for maintenance requirements or if it is designed to serve a large number of users but this is no longer the requirement.


We have seen Thermostatic Mixing Valves (TMVs) intended to regulate water temperature that are not producing the correct temperature but are located in inaccessible locations. They can remain uncalibrated because the building owner has an unenviable choice of whether to skip the maintenance or dismantle part of their premises each time a check or recalibration is required. The one we mentioned had been in place and untouched for 10 years,

Another very common reason for TMVs not to be serviced by the duty holder is the cost, as they should be serviced every 6 months.

TMVs are a key piece of equipment in water systems, not just because they regulate temperature, but because of the settings in which they tend to be used. They are typically used where there are a large number of outlets. Where are large numbers of outlets commonly found? Schools, leisure centres, and health and care settings, public houses.

Opportunity for bacteria growth

One of the things about water is that it contains minerals (calcium etc). Over time, these minerals can be deposited at various points in a system. As well as potentially restricting flow, the deposits form an effective breeding ground for bacteria of all kinds. What are the three things that bacteria need to thrive? Food, heat, and time. Calcium will serve as a more than adequate food source.  Other potential food sources include organic contamination.

Location of pipes and the nearby environment

Unbelievable as it may sound, we have seen instances in which hot water pipes have been sited close enough to cold water pipes that they could heat the cold water. It may be that those pipes previously had insulation, but it is, in any case, a design flaw.

Loss of, or damage to insulation is of course an ever-present danger. Any unexpected change to a system can render unexpected results from what was originally foreseen.

The temperature of the environment in which pipes are located must also be considered. Does this vary throughout the day and night? Are there any opportunities for extremes to occur which are enough to adversely affect the water within the pipes?

Header tanks

These are used to provide gravity pressure into a system to deal with variations in demand. Header tanks provide all manner of complexity when it comes to regulating the flow and safety of water. They are generally an anachronism found in legacy systems. As such, we would always try to design a system that did not require one or design them out of existing systems.

Though they are commonly used within central heating systems nowadays, we have found them feeding hot or cold water systems in use and we have found them in various states of repair. Damaged lids for example will typically cause contamination from bacteria growth.

One issue that we have found applies even if they don’t directly feed the water system. If damaged or if a lid is missing, the evaporating particles can give out Legionella, which can be inhaled and infect people.

Header tanks are usually located at the highest point to allow gravity to pressurise the water system.  This is usually the loft, which is great in winter months but not so great in the summer months when the water will naturally heat up to its natural surrounding temperature.  One way to control this is to make sure the header tank is well insulated to reduce the natural radiation heating of the tank and water it contains. It is then essential to ensure that this insulation remains in good condition.

The potential for stagnant water and the resulting bacteria growth is to be carefully avoided if tanks of any kind are present.

Dead legs

A dead leg is simply a section of pipe that doesn’t go anywhere.

Why are there dead legs? Sometimes plans are changed during construction. Sometimes they are caused by changes that are made during the operation of a building. Perhaps a section of existing pipework is used in a redesign and a section not used.

The solution to a dead leg for Legionella safety is to (if possible) flush the system regularly or remove the section.

In summary

Legionella risk assessments can play a vital role in meeting your Health & Safety obligations. The more complex a system, the less likely you will deal with Legionella simply by running the taps. And your system may be more complex than you think.

Get in touch with us if you require any Legionella advice.

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