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Combi Boiler and Legionella - The Temperature Conundrum

Hot water is an essential element for any business, particularly those in the healthcare industry. It serves multiple purposes, such as handwashing, cleaning, and disinfection procedures. Staff, visitors, patients, and their relatives, regardless of age, all rely on hot water.

To control bacterial growth, it is crucial to maintain specific temperature requirements for the hot water in your building. The guidance is straightforward: ensure a temperature of at least 50°C within 1 minute or at least 55°C for healthcare settings. However, there are some variations to this recommendation, depending on how your water is heated.

There are various methods to produce hot water, including storage calorifiers, localised water heaters, instantaneous water heaters, plate heat exchangers, solar/ground source heat pumps, and pre-heated vessels. One popular option is the use of gas-fired combi boilers without storage vessels. These compact, cost-efficient units are commonly found in domestic properties, small healthcare premises, retail units, and small to medium-sized commercial properties.

But first - what is a combi boiler and how does it work?

A combi boiler is a type of boiler that provides both hot water and central heating. Modern combi boilers utilize condensing technology, which efficiently heats the system water by extracting latent heat from the flue gases. When you need heating or hot water, a gas burner in the combi boiler ignites. The heat from the flame is transferred via the main heat exchanger to the primary circuit, which then supplies heat either directly to the radiators or to your hot water taps via a plate heat exchanger.

Combi Boiler Legionella Testing
Credit to HSE - HSG274 Part 2

The concept seems simple—no water tanks or storage vessels taking up valuable space that could be used for other purposes. Water is produced on demand, and the temperature can be easily adjusted.

However, when taking monthly temperatures, you may have noticed some patterns. Some may claim that the heater does not reach the correct temperature or takes longer than 1 minute to reach the minimum requirement. Additionally, temperature fluctuations can make it challenging to determine compliance.

Legionella Testing - So what temperature should the boiler achieve to be compliant?

Combi boilers produce hot water instantaneously, without a large volume of water heated and stored for later use. As a result, they have certain limitations:

A. The hot water flow rate is limited and is dependent on the heater’s power rating;

One disadvantage of a combi boiler is that the amount of water it can provide at any given time is limited to the size of the boiler; this typically means that on the average combi boiler, only one tap/outlet can be used effectively at a time. If multiple taps are used simultaneously, the temperature may noticeably decrease.

B. The water in instantaneous water heaters is usually heated to about 55°C at its lowest rate, and its temperature will rise and fall inversely to its flow rate.

You may observe this temperature variation when testing your water. Initially, it may start at a high temperature, then drop before gradually rising again. Sometimes it may take more than 1 minute to reach the required minimum temperature. This is generally acceptable as long as the time is not excessive. If it takes more than 2-3 minutes, it indicates that either the boiler is too small and unsuitable for your premises or that it requires repair or servicing.

C. They are susceptible to scale formation in hard water areas, where they will require frequent maintenance;

At least an annual service should be completed on all combi boilers.

D. This form of hot water heating should be considered only for smaller premises or where it is not economically viable to run a hot water circulation to a remote outlet;

Does size really matter?

Legionella Testing - Combi Boilers
Combi Boiler Size

Yes, the size of the combi boiler is key; anything smaller than 36kW will be struggling to provide hot water at the required temperature.

So how hot should my water be?

Ensuring the appropriate temperature of hot water is crucial to control bacterial growth, particularly for Legionella bacteria, which thrives between 20°C and 45°C. To effectively manage the system, your water should be maintained above 50°C. Although it may take slightly longer than 1 minute, the temperature at the furthest tap should never be lower than this threshold.

For healthcare facilities, the guidance recommends achieving a temperature of 55°C within 1 minute. However, it's important to note that this guidance primarily applies to larger water systems with hot water storage and recirculation. With a combi boiler, the aim is to produce water within the recommended range, but keep in mind that when testing the temperature at the furthest outlet, you may need to wait a bit longer. Monitor the temperature using a thermometer and record the highest reading. Remember that there will naturally be some heat loss in the distribution pipework, so a temperature between 50-55°C is still considered satisfactory.

When conducting temperature tests, choose an appropriate time of day, avoiding periods when the system experiences heavy usage, such as during dishwashing or cleaning the decontamination room. Increased usage can lead to readings that fall outside the recommended range.

Lastly, it is advisable to seek guidance from experienced professionals who possess a deep understanding of your specific system, rather than relying solely on those who merely quote general guidelines. It's important to recognise that more than 55°C temperature requirement is not always achievable with a combi boiler system. To meet this specific expectation, significant alterations to the system would be necessary, resulting in considerable expenses for the business. However, it is important to note that even with these changes, the risk of legionella proliferation may not be significantly reduced. Pursuing such modifications would impose unnecessary financial burdens without providing additional risk control benefits.

Remember - The First Principle team are always here to assist you with any further questions or concerns.

Click here to get a free no obligation quotation


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