WATER TREATMENT, DRAINAGE & PIPED SERVICES
A practical guide to temperature monitoring for Legionella control
Legionella control is often discussed in terms of water treatment chemistry, dosing regimes, disinfectants and filtration cycles. All of these play an important part, but one of the most fundamental and cost-effective tools for managing Legionella risk sits outside the chemistry conversation altogether: temperature control. Jason Webb, managing director, Electronic Temperature Instruments (ETI) explains
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sed well, temperature control remains one of the simplest and most reliable ways to keep a water system safe, and the principles behind it are well established. What tends to make
this difference is consistency: applying the right checks, in the right way, often enough to build a genuine picture of how that system is really performing. Legionella bacteria are present in low numbers in most water systems without posing any real risk. The danger arises when thermal conditions allow them to multiply. Broadly speaking, Legionella can proliferate between 20°C and 45°C, with growth slowing or stopping outside that range. This is the basis for the Health and Safety Executive’s well-established guidance: cold water should be stored below 20°C, hot water generated at 60°C or above, and distributed at no less than 50°C at the point of use – rising to 55°C in healthcare settings. Each of these figures marks the difference between a system that is biologically hostile to Legionella and one that allows it to grow, which is why precise control, rather than an approximate sense of ‘warm’ or ‘cold’, is so important.
Common mistakes to avoid
Given how clear this guidance is, it is worth asking why Legionella risk remains such a persistent issue across the built environment. The answer usually lies less in the science and more in how consistently it is applied day to day. A handful of recurring mistakes tend to show up again and again. The most common is treating temperature checks as a monthly formality rather than an ongoing discipline: a system that passes inspection on the day it is
28 BUILDING SERVICES & ENVIRONMENTAL ENGINEER AUGUST 2026
tested may still have drifted in and out of the safe range in between visits, particularly where occupancy or seasonal demand fluctuates. Inconsistent technique is another familiar culprit, whether that means taking readings at different points in a system, in different ways, or without allowing enough time for a sensor to settle and give an accurate result. The type of probe used should suit the point being measured, too. For example, a penetration probe designed to be dipped directly into running water is the right choice at an outlet or within a storage tank, while a surface probe, designed to sit against pipework, is better suited to checking flow and return temperatures on a calorifier. That being said, even the most advanced tools will become unreliable if they are not calibrated and maintained properly. Regular calibration, ideally to a UKAS-certified standard, must be a routine part of any monitoring regime.
The value of accurate, traceable data
What connects most of these issues is a lack of traceability. A single reading, however accurate, is only ever a snapshot. What protects both occupants and operators is a continuous, well- documented record: accurate, properly logged measurements that build into a clear picture of how a system performs over time. This kind of data does two things at once. It allows problems to be spotted and corrected early, often before they become serious, and it gives facilities teams a defensible audit trail to demonstrate due diligence if a system is ever scrutinised. Moving from paper-based records to digitally logged spot-checks is one of the most valuable steps an organisation can take. Bluetooth- enabled thermometers, paired with a digital logging platform, offer a practical way to
achieve this. Each reading is automatically time- stamped and uploaded to a secure archive, creating a verifiable record that proves checks were completed accurately and on time. This matters particularly for timed checks such as confirming that running water falls below 20°C within two minutes or rises above 50°C within one minute. Capturing this temperature at the start and end of the process provides clear, auditable proof of compliance.
Strengthening long-term strategy
Technology has an important part to play in making that consistency achievable. Many buildings already use a Building Management System to manage heating and ventilation. Extending that same infrastructure to water temperature monitoring brings automated alerts and instant access to historical data, supporting more targeted and informed manual checks on the ground. However, systems and sensors only deliver their full value when the people using them understand what the data means and how to act on it. Training is just as important as the technology itself. Even the most capable monitoring set-up depends on someone knowing what a deviation looks like and the steps needed to fix it. None of this requires reinventing how Legionella risk is managed. It is about applying well-established guidance with more consistency: precise temperature control, regular and properly calibrated checks, data that is logged rather than left to memory, and people trained to interpret what they see. Done well, the benefits extend beyond compliance, supporting the energy efficiency and wider sustainability goals now expected across the built environment.
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