When a building has a Legionella problem, the first thing most people reach for is a dead leg.

It is an easy diagnosis. Dead legs are visible on schematics, they feature prominently in risk assessment checklists, and they give the responsible person something concrete to point at. Dead legs are a genuine risk, and ACoP L8 paragraph 80 recommends designing water systems without them wherever possible, or limiting their length.

The problem is not that they are unimportant. The problem is that they are frequently cited as the sole cause of a positive result without first asking whether their position in the system could physically account for it. A short dead leg at the furthest sentinel point, for example, is unlikely to explain positive results across several outlets upstream of it.

In our experience, where that attribution is made without the question being asked, the real cause can be elsewhere in the system, harder to find and harder to fix without the right data.

This is the first article in our series looking at what causes ongoing Legionella problems in building water systems. The framework we use is MOD: Mechanical failure, Occupant behaviour and Design. We suggest that troubleshooting by category, rather than treating symptoms, is the quickest route from a persistent positive result to the actual cause.

This article covers Mechanical, including faults that routine checks can miss. The next two articles cover Occupant behaviour and Design, followed by a final article on continuous monitoring.

Dead Legs and Legionella: Why Location Matters

A dead leg is a section of pipework where water sits without regular movement. ACoP L8 paragraph 59 identifies avoiding water stagnation and temperatures between 20°C and 45°C as important precautions, because these conditions can encourage Legionella growth.

In practice, the significance of a dead leg depends on where in the system it sits and how it connects to the wider distribution network.

A dead leg close to the mains point of entry, where incoming cold water feeds the building's distribution, may present a different risk from one at the furthest sentinel point. Its position can influence whether contamination could affect other parts of the system and whether it offers a plausible explanation for the sampling results.

This distinction is not always made clearly in risk assessments. Dead legs can be listed, rated and recommended for remedial work without enough consideration of their position in the system. Yet the presence of a dead leg does not, on its own, establish that it caused a particular positive result.

ACoP L8 paragraph 80 recommends avoiding dead legs where possible, and HSG274 Part 2 paragraph 2.77 advises that redundant supply pipework should be cut back as close as possible to a common supply.

Those recommendations stand. The point is not that dead legs should be ignored. The point is that attributing a Legionella problem to a dead leg without establishing whether it could account for the result is not a diagnosis. It is an assumption.

TMV Failure and Legionella Risk

Thermostatic mixing valves (TMVs) are installed to deliver blended water at safe outlet temperatures, protecting building occupants from scalding whilst allowing the hot water system to operate at the higher temperatures needed for Legionella control.

HSG274 Part 2 paragraph 2.6 sets out the usual temperature control parameters: hot water stored at a minimum of 60°C, distributed to reach 50°C at outlets within one minute, and cold water maintained below 20°C where possible. In healthcare premises, the hot water outlet temperature requirement is 55°C.

When a TMV fails or begins to pass, the consequences depend on the nature of the fault. Hot water may migrate into the cold supply, raising cold water temperatures at affected outlets. Cold water entering the hot side can reduce temperatures and compromise the thermal control measures the system depends on.

Because a passing TMV can affect either system, attribution is not straightforward. A temperature exceedance on the cold side near an outlet with a TMV may be investigated as a cold water distribution problem when the actual cause is the valve.

HSG274 Part 2 paragraph 2.74 also notes that blended water downstream of a TMV may provide conditions in which Legionella can multiply. Paragraph 2.76 recommends positioning TMVs as close to the point of use as possible to minimise the volume of blended water held in the system.

Table 2.1 calls for regular TMV maintenance by competent persons, with associated strainers and filters inspected, cleaned, descaled and disinfected where needed, annually or at a frequency determined by the risk assessment and manufacturer's recommendations.

In our experience, these checks are not always carried out as thoroughly as intended, particularly where valves are difficult to access.

Passing Check Valves: Another Cause That Can Be Missed

Check valves are another mechanical fault worth investigating when temperature readings suggest hot and cold water are mixing unexpectedly.

A check valve is intended to prevent water flowing in the wrong direction. Where a valve is passing, hot water can move back towards the cold feed and raise temperatures at affected outlets.

HSG274 Part 2 paragraph 2.32 notes that single check valves are commonly used to prevent hot water flowing back into the cold feed. It also advises that, where applicable, an anti-gravity loop should be fitted as a failsafe in case the valve fails.

The difficulty is that a passing check valve may not be obvious from a routine temperature reading. The cold water at an outlet may appear slightly warmer than expected, but the reading alone does not establish why.

Identifying the cause means looking at the relevant valves and pipework rather than treating the temperature exceedance as an isolated problem.

Hot Water Balancing: The Hidden Risk

Hot water balancing issues are confined to the hot system, but they are among the most common mechanical causes of persistent Legionella problems we encounter in commercial buildings.

A hot water system with a pumped secondary return circuit is designed to maintain circulation throughout the distribution network. HSG274 Part 2 paragraph 2.37 advises that the system should be designed to return water from each loop to the calorifier at a temperature of at least 50°C, or 55°C in healthcare premises.

The return circuit only delivers that outcome if it is correctly balanced. Where flow rates through individual loops are unequal, some sections of the circuit can run cooler than the system appears to be maintaining overall.

HSG274 Part 2 Table 2.1 sets out monitoring for both principal and subordinate loops of the hot water secondary return system. Principal return loops should be checked monthly, whilst subordinate loops should be checked quarterly, ideally on a rolling monthly rota.

In practice, the subordinate checks are not always carried out thoroughly.

A principal loop temperature reading at the plant room return can show a compliant system on paper. The subordinate loops, the smaller branch returns serving individual risers, wings or floors, may be running significantly cooler without that problem appearing in the principal reading.

A return leg that is not circulating properly can be particularly difficult to identify manually. A single temperature reading at an accessible point tells you little about what is happening elsewhere in that loop. A responsible person can have a documented monitoring programme, with readings within range at the measured points, whilst sections of the hot water return system remain at temperatures that support Legionella growth.

This is precisely the gap that Legionella Watch  addresses. Sensors on subordinate loop returns provide data on what those sections are actually doing between monitoring visits, not what the system appears to be doing at the point of measurement. Where a return leg has dropped out of circulation, the data shows it immediately. Manually, it can go undetected for months.

Identifying Mechanical Causes of Legionella

Mechanical causes of Legionella risk are, in many cases, relatively straightforward to resolve once they are correctly identified. A faulty valve can be replaced. An unbalanced return can be corrected. Redundant pipework can be removed or cut back appropriately.

The challenge is often finding the fault.

Routine inspections and temperature monitoring are important, and they should identify certain problems. But periodic readings taken at fixed points cannot show everything happening elsewhere in a system or between visits.

When a building keeps returning positive Legionella results despite apparently satisfactory routine checks, it is worth looking beyond the monitoring points.

That could mean checking temperatures at subordinate return loops rather than relying solely on the principal return, investigating TMVs and check valves for passing, and comparing results with the layout of the system to establish which faults could actually explain the findings.

Where the issue is intermittent, continuous monitoring can help show when temperatures begin to change and whether remedial work has corrected the problem.

What to Take From This

Finding a dead leg doesn't necessarily mean you've found the cause of a Legionella problem. Its position in the system matters, and it's important to establish whether it could actually explain the results.

Passing TMVs, check valves and poorly circulating hot water return loops can also cause problems that aren't always obvious from routine temperature checks.

In our experience, mechanical faults are often relatively straightforward to put right once they've been identified. Finding them is usually the difficult part, particularly when the problem develops between monitoring visits.

Dealing With Recurring Legionella Problems?

If you're experiencing repeated positive results and routine checks aren't identifying the cause, LCS can help you investigate further.

Our diagnostic monitoring service uses continuous data to investigate how water systems are performing, helping identify faults that periodic checks may be missing.

Discuss your Legionella issue with LCS

Next in the MOD series: Occupant Behaviour: how changes in building use and flushing practices can contribute to persistent Legionella problems.

Keith Elwell is Managing Director of Legionella Consultancy Services Ltd. LCS provides Legionella risk assessments, written control schemes and IoT remote monitoring across commercial, healthcare and industrial premises in the UK.