Solar Water Heating & Legionella – What You Need to Know
In this article the water safety specialists at Legionella Control International take a detailed look at solar water heating systems and the implications for the control of legionella.
The article examines the social and economic drivers for such eco-friendly heating systems, how they may increase the risks from legionella, and how these risks can be controlled to keep people safe from harm.
Switching to solar water heating… but what about legionella?
The 21st century has seen significant changes occurring throughout the energy sector.
Renewable energy sources now provide us with a cost effective, eco-friendly alternative to fossil fuels.
Those fossil fuels have increased greenhouse gases, leading to global warming and a whole host of other unwanted environmental issues we are now trying to combat.
Meanwhile, renewable energy such as solar, wind, geothermal etc. is just that – renewable and without end.
One of the most popular examples of these renewable energy sources is solar energy.
We can now use the energy coming from the sun to heat our homes, offices, hospitals and other businesses.
The same energy can also be used to heat our hot water supplies.
If we can tap in to renewable energy sources effectively then the savings can be significant, but this system is not without potential issues.
Here in the UK, if you are thinking about switching to a solar water heating system, for example, you need to be aware of the risks from Legionella bacteria.
What is Legionella bacteria?
Legionella is a type of bacterium that naturally occurs in water sources such as lakes and rivers.
It causes Legionnaires’ disease which is a potentially fatal lung condition.
As well as occurring in natural water sources the bacterium can also find its way into man-made water supplies, such as those found in buildings, which is where it starts to become a problem if not dealt with properly.
If the conditions within such a buildings water system are found to be ideal, Legionella bacteria will begin to multiply.
This may lead to dangerous colonisation of the plumbing systems and the risk of illness for those using the system for their water needs.
Fortunately, there are some simple steps people can take to minimise the risks posed by Legionella bacteria and so stay safe.
In this article, we are going to cover these legionella risks as they relate to solar water heating.
It is an important topic to learn more about if you have (or are thinking of getting) solar panels or some other type of eco-friendly water heating system.
How can people become ill from exposure to Legionella bacteria?
As we’ve highlighted previously, Legionella bacteria can cause Legionnaires’ disease.
This disease is a serious type of pneumonia caused by this bacterium.
Legionnaires’ disease got its name from a major outbreak at a hotel in Philadelphia, USA in 1976, where a convention of the American Legion was taking place.
The bacteria must be inhaled, usually through exposure to a spray or fine mist containing the bacteria.
Taking a shower, using a hot tub, or even walking past a fountain generating spray can all pose a risk if the water being used is unsafe.
While many people survive Legionnaires’ disease, its fatality rate is known to be around 10% to 15% of those who are sickened.
However, this can rise to 30% in some cases, and if people in higher-risk categories are involved, the fatality rate has been seen to hit between 40% – 80%.
Clearly, anything we can do to limit the chance of people contracting the disease makes sense.
Legionella bacteria can also cause Pontiac fever and a number of other less serious conditions.
Pontiac fever is far less serious than Legionnaires’ disease and typically does not require hospitalisation or medical treatment.
Indeed, it is thought that some people who contract it assume they have a bad case of the ‘flu.
How does solar water heating increase the risks from legionella?
For a solar water heating system to work, you need to install solar panels, usually on your roof.
The panels are made from special light/heat sensitive tubes or plates, depending on the design, but the premise is the same.
The solar panels collect energy from the sun and use it to heat the water inside the property.
As with many regular water heating systems, a hot water cylinder is required to store the water before it is used.
With traditional hot water systems, any water that is stored in a cylinder must be kept at a safe temperature.
Legionella readily thrives when the water temperatures sit somewhere between 20 and 45 degrees Celsius.
Ideally, you should heat the water to at least 60 degrees Celsius to ensure it kills off any Legionella (and other) bacteria that could be in there.
The big problem in the UK is that water heated by solar energy alone does not always reach the desired temperature (60 degrees Celsius) to keep legionella at bay.
Keeping legionella under control in solar heating systems
Most standard hot water cylinders have hotter water sitting at the top of the cylinder – as hot water is less dense than cold water it rises.
Colder water is found at the bottom, and this is where fresh water supplies are typically added to the cylinder.
This is the reason why hot water cylinders usually include something known as an ‘anti-legionella cycle’.
The anti-legionella cycle mixes the water supply throughout the cylinder to achieve an overall temperature of at least 60 degrees Celsius.
Now we know how legionella can thrive in moderate temperatures, it’s easy to see how the risk could potentially arise in standard hot water cylinders too.
An important document to assist with managing and controlling Legionella bacteria in water systems is the Health and Safety Executives (HSE) ACOP L8 document.
This is the HSEs Approved Code of Practice for managing and controlling Legionella bacteria in water systems.
ACOP L8 is supported by extensive guidance in HSG274 Parts 1, 2 and 3.
The L8 code reveals that hot water should be heated to sufficient temperatures of 60 degrees Celsius.
This process, called pasteurisation or thermal disinfection kills any Legionella bacteria within the water system – in this case, the hot water cylinder.
Why is thermal disinfection important when using a solar heating system?
Even the traditional wet, cold, and windy British summer can produce significant amounts of sunshine.
This sunshine supplies energy to your solar panels creating plenty of heat to give you the hot water you want.
You’ll likely find that reaching the desired temperature of 60 degrees Celsius at your hot water cylinder is achievable under such conditions.
However, the dark and damp British winter is another matter altogether.
While you can still get a portion of your energy needs met by the sun during the winter months, it’s unlikely you’ll be able to meet all of them.
This means the temperature of the hot water is likely to dip into the legionella danger zone of 20 – 45 degrees Celsius.
If you don’t do anything about this, Legionella bacteria are going to start developing and spreading within your water system.
This increases the chances of you or others who use the solar water heating system of being exposed to harmful Legionella bacteria and contracting Legionnaires’ disease.
Fortunately, there are steps you can take to make sure these risks are kept under control.
How can you heat your hot water to the required temperature?
There are two main ways you can increase the temperature of your hot water so as to achieve thermal disinfection:
- Via a conventional boiler
- By an immersion heater
Both these approaches are designed to work alongside and support the solar water heating system.
It may sound counterproductive to rely on a fossil fuel style heating system when you’ve gone to the trouble of investing in a solar heating system.
However, you’ll use the secondary system far less than normal – only relying on it when you need to, when the sun isn’t strong enough to heat the water to achieve the necessary temperature on its own.
Instead of relying 100% on a fossil-fuel heating system, you may only need to rely on it for say 20% of your hot water requirements… it still makes a massive difference.
Let’s say you install a boiler to make sure the water is always at the correct and safe temperature.
You won’t need to rely on it all the time.
During the summer months, you’re likely to get enough energy from your solar panels to heat the water, so the boiler isn’t needed.
However, during the winter the sun is far weaker and provides less heat.
During this time, the boiler is there as a top up… to make sure you can sufficiently and safely heat your water.
The same applies if you choose to install an immersion heater instead.
This gets its name because the heating element is immersed in the water and heats it from inside the cylinder.
If you think of your hot water cylinder like a giant kettle with the heater inside, you’ll get the idea. (Of course, in this case it won’t be boiling!)
Controlling legionella using your heating thermostat
Your backup boiler or other secondary heat source will have a temperature control thermostat, so it can turn on and off automatically when it is required to do so.
This means that if your solar heating gets enough energy from the sun to heat your water supplies to 60 degrees Celsius, the boiler won’t need to turn on.
If it doesn’t, the thermostat kicks in and heats the water for long enough to kill any bacteria present inside the system.
Commercial hot water systems are set up to run a regular pasteurisation cycle to get rid of Legionella bacteria inside water storage tanks and cylinders.
While this isn’t normally required in a domestic system, the nature of a solar hot water system does present challenges, as we have seen here.
It means that we must take extra care to ensure that while we are using a renewable energy source, we are not increasing the risks from Legionella bacteria spreading throughout a water system and exposing building occupants to additional health risks.
Becoming more familiar with your solar water heating system
You can see that familiarity with any new solar water heating system, what it can do, how it’s controlled, and its limitations, is vital if you are going to use it to save energy and to provide a safe source of hot water throughout the year.
If you are considering installing a solar water heater, make sure you seek advice from a professional company with experience in such installations.
It’s especially important to ask about the measures included to combat Legionella bacteria in the system.
Many people are surprised to learn that legionella is easy to control once the correct measures are in place and monitored.
It’s worth asking about protection against legionella before you agree to an installation.
If you already have a solar water heating system installed, it may already do what it needs to.
If not, find out whether you should be doing anything else to reduce the potential health risks.
Expert legionella and water safety solutions
Legionella Control International is a world-leading legionella and water safety specialist. Our teams of experts support business owners, landlords and those responsible for the control of waterborne pathogens including Legionella bacteria in the workplace, helping them to protect their staff, tenants and others and so meet their health and safety obligations in this specialist area.
We deliver a range of specialist risk management solutions including water quality analysis and legionella sampling, risk assessment, regulatory compliance auditing, training, expert witness support and other services that help keep people safe.