Combi Boiler, Hot Water Cylinder or Water Tank: Why Does It Matter for Legionella?
Combi Boiler, Hot Water Cylinder or Water Tank: Why Does It Matter for Legionella?
Understanding how a property's hot and cold water system works is an important part of a Legionella Risk Assessment.
Two rental properties can look very similar but have completely different water systems behind the taps.
One may have a combi boiler supplying hot water on demand, another may store hot water in a cylinder, while an older property may also have a cold-water storage tank.
That matters during a Legionella Risk Assessment because the assessor needs to understand where water comes from, whether any of it is stored, how it is heated and where there may be opportunities for water to remain unused.
What about a combi boiler?
A property with a combi boiler will commonly have cold water supplied directly from the mains, with hot water heated as it is required rather than being stored in a domestic hot-water cylinder.
From a Legionella point of view, having less stored water can make the system relatively straightforward to assess, but it does not mean there is nothing to consider.
Showers, little-used taps, flexible hoses, redundant pipework and outlets that are rarely used can still allow water to sit for extended periods. The way the property is occupied also matters, particularly if it is sometimes left empty.
That is why the assessor looks at the complete accessible system rather than simply recording that a combi boiler is present.
What changes when there is a hot-water cylinder?
A hot-water cylinder means that domestic hot water is being stored rather than produced only when a tap is opened.
During the assessment, that stored-water arrangement becomes part of the risk picture. The assessor may need to consider the cylinder, its insulation, temperature control and the associated pipework, alongside the temperatures being achieved at outlets around the property.
Not every property with a cylinder has a cold-water storage tank. Modern unvented cylinders, for example, can be supplied directly from the mains.
This is one reason simply asking whether a property has a boiler or cylinder does not always tell the whole story. The actual arrangement needs to be identified.
And what if there is a cold-water storage tank?
Cold-water storage can introduce another part of the system that needs to be considered.
Where a domestic cold-water storage cistern is present, its location, condition, insulation and general arrangement may all be relevant. The assessor will also consider whether the stored water is being turned over normally rather than remaining unused for long periods.
Tanks are more commonly associated with traditional vented systems and older properties, although every property should be assessed on what is actually installed rather than assumptions based on its age.
It isn't about one system being automatically "safe" or "unsafe".
A combi boiler, unvented cylinder or traditional stored-water system simply presents a different arrangement for the assessor to understand. The important part is identifying how the system works and whether there are factors within that property that could allow Legionella risk to increase.
The outlets still matter
Whatever type of boiler or storage system is installed, much of the assessment still comes back to what happens throughout the property.
Showers, taps and other outlets need to be considered alongside water temperatures, little-used areas and possible stagnation points.
A perfectly modern boiler does not prevent water sitting in a guest shower that is only used a few times a year. Likewise, a well-maintained hot-water cylinder does not tell you whether redundant pipework exists somewhere else in the system.
We showed some of these points in our recent article, What Actually Happens During a Legionella Risk Assessment?, where example report pages show how the system, little-used outlets and redundant pipework are recorded.
Vacancy can change the picture too
How a property is used is just as relevant as the equipment installed in it.
An occupied property normally has regular movement of water through taps, showers and toilets. Once it is empty, that normal pattern stops and water can remain sitting in parts of the system.
We covered this separately in Why Empty Properties Can Create Legionella Risks, including some of the practical things landlords should consider before a property is occupied again.
Why we ask about the water system before an assessment
If you're arranging a Legionella Risk Assessment, you may be asked whether the property has a combi boiler, a hot-water cylinder, a cold-water tank or another arrangement.
It is not a test question and you do not need to be a heating engineer to answer it. The information simply helps build an initial picture of the property before the assessment.
The system is then looked at during the visit so that the final assessment reflects what is actually present rather than relying solely on information supplied beforehand.
You can find more information about what Thorn's assessments cover on our Legionella Risk Assessments Glasgow page.
Thorn provides Legionella Risk Assessments for landlords and letting agents across Glasgow and East Dunbartonshire, carried out by an Elmhurst Energy Approved Legionella Risk Assessor.
Not sure what type of water system your property has?
Ask Wendy can answer common questions about Legionella Risk Assessments, help explain what information is useful before the visit and provide guide pricing.
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