There is an article circulating at the moment that attributes the root cause of Legionella risk in building water systems to something called Hunter's Curve, a 1940s American pipe-sizing method built around fixture flow rates that no longer reflect how many buildings use water.

The principles it describes are sound. But Hunter's Curve is not a method you would normally find named in a UK building services specification.

What we use is the loading units method, set out in guidance including CIBSE Guide G and BS EN 806-3. These methods use probability to estimate how many outlets will be used at the same time and the flow rates a system needs to accommodate. The difficulty is that calculated demand can be considerably higher than the demand a building actually generates.

Research from Northumbria University supports that concern. Jess Tindall and Jamie Pendle presented a paper at the 2015 CIBSE Technical Symposium examining whether domestic water systems were being significantly oversized. Their subsequent peer-reviewed research, published in 2017, compared three commonly used UK cold water sizing methods with measured flow rates in case-study buildings.

All three methods significantly overestimated the measured peak flow rates, although BS EN 806-3 produced the closest estimates. The research also identified the consequences of oversizing: water takes longer to pass through the system, which can lead to unwanted increases in cold water temperature.

This matters. But it is not the most important point.

This is the third article in our MOD series on troubleshooting persistent Legionella problems. The framework is 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.

The first article covered Mechanical: passing TMVs, return loops that have dropped out of circulation, and dead legs blamed without reference to their position in the system. The second covered Occupant behaviour: buildings used differently from how they were designed, and flushing regimes that depend on signatures rather than evidence.

This article is where those threads meet. Design is the letter we save for last because many of the conditions the first two articles described begin here, in decisions made long before anyone ran a temperature check.

The Cold Water Problem

Much of the discussion around Legionella risk focuses on hot water. Temperature control is relatively well understood: store at a minimum of 60°C and distribute so that water reaches at least 50°C at outlets within one minute, or 55°C in healthcare premises.

In commercial buildings with pumped secondary return circuits, circulation helps maintain those temperatures throughout the hot water distribution system, provided the system is correctly designed, balanced and maintained.

The oversized pipework problem can exist in hot water systems too, but a properly functioning return circuit helps manage the risk. Dead legs and poorly circulating sections remain a concern.

Cold water presents a different challenge.

It cannot rely on the same high-temperature control measures used in hot water systems. Cold water should be maintained below 20°C where possible, but it can warm while sitting in distribution pipework, particularly where the surrounding building is warm and demand is low.

In a plant room, a roof void or a south-facing riser, water that remains in the pipework for extended periods may gradually warm into the temperature range in which Legionella can multiply.

Water movement matters alongside temperature. Regular turnover helps limit stagnation and reduces the time water spends warming in the system. When demand falls, those protections become harder to maintain.

The question, then, is whether cold water in UK buildings is moving enough.

We would suggest that in many cases it is not. And changes in how buildings use water may be making the problem worse.

What We Have Done to Cold Water Demand

Over the past two decades, the UK has pursued a sustained and largely sensible programme of reducing water consumption at the point of use. Low-flow taps. Reduced-flush cisterns. Water-efficient showerheads. More recently, waterless urinals in commercial buildings.

Each of these interventions reduces the volume of water moved through the distribution system every time an outlet is used. A tap that once drew 12 litres per minute might now draw 4. A cistern that once flushed 13 litres might now use 6, or less on a reduced flush. A urinal that once flushed automatically may now use no water at all.

These are the right interventions from a resource perspective. Water is a finite resource, and the 2022 drought was a reminder of how quickly water shortages can become a practical problem. Reducing consumption per use event is logical and necessary.

The problem is that these fittings are often installed into pipework that was sized for the flow rates they replaced. The pipe bore has not changed. The pipe length has not changed. The volume of water sitting in the distribution system between use events has not changed.

Only the volume moved at each use event has changed: downward.

In buildings where the original pipework remains, that can mean water is spending longer in the cold water distribution system. It is a change that happens gradually, refurbishment by refurbishment, as fittings are replaced with more efficient ones without reviewing the demand the wider system now needs to serve.

The UK-Specific Compounding Factor

There is a further dimension to this that relates to the UK's building history.

Before unvented hot water systems became more widely adopted, many UK buildings relied on vented, gravity-fed arrangements. Cold water was stored in a tank, often at roof level, with hot water supplied from a cylinder below it.

The available pressure at outlets could be relatively low. To achieve acceptable flow rates, larger pipework was sometimes needed. That was rational engineering for the conditions the system was designed to operate under.

Over time, many buildings have been refurbished or converted to different water supply arrangements. But replacing the plant or changing the pressure source does not necessarily mean replacing the distribution pipework.

Some of that original pipework remains in walls, floors and ceiling voids. Where it was sized for higher flows or different operating pressures, it may now be serving fittings that draw considerably less water than the original design expected.

The result can be a distribution system that holds more water than its current demand is able to turn over regularly.

Into that system, we are installing fittings designed to use a fraction of the water their predecessors consumed.

The pipework may still be doing exactly what it was designed to do. The problem is that the demand it was designed around has changed.

Two Objectives, One Problem

We are trying to achieve two objectives that can pull in opposite directions.

The water efficiency agenda reduces consumption through low-flow fittings, smaller cisterns and other water-saving measures. Legionella control under ACoP L8 and HSG274 Part 2 requires us to manage stagnation and maintain cold water below 20°C where possible.

Both objectives are valid. Neither is wrong. But they interact, and that interaction is not always considered at the system design level.

What tends to happen in practice is that a building manager installs water-efficient fittings, reports the saving, ticks the sustainability box, and nobody considers what the change has done to the distribution system that fitting is connected to.

The fitting is assessed in isolation. The system is not reassessed at all.

The result is that risk assessments may be conducted against systems operating very differently from when they were last assessed, not because anything has failed, but because the demand profile has changed.

A cold water distribution network carrying substantially less water than it did five years ago presents a different risk from the one the previous assessment evaluated. The pipework has not changed. What has changed is how much water moves through it.

What This Means in Practice

This is not an argument against water efficiency. The measures are sensible and necessary.

The practical question is whether we are assessing the risks in the systems we actually have, or relying on assumptions based on how those buildings used to operate.

A cold water system with oversized legacy pipework, low-flow fittings, infrequently used outlets, branches to rarely occupied offices or building wings with seasonal demand warrants a different assessment from a new-build system sized for its expected use.

Where water-efficient fittings have been installed, it is worth reviewing their effect on demand across the wider system, particularly in areas that are already used infrequently. Changes in occupancy and water use should also be reflected in the risk assessment and monitoring arrangements.

In our experience, the outlets that show temperature exceedances and positive Legionella results are frequently not the ones that receive the most attention in a standard monitoring programme.

They are often the ones that nobody is looking at, precisely because nothing obvious has changed. The pipework is where it always was, the fittings look the same and the building appears to be operating normally.

What has changed is the demand, quietly and incrementally, as fitting by fitting the throughput has declined.

Legionella Watch provides continuous temperature monitoring data that helps show how water systems are performing, rather than relying solely on how they were designed to operate. With sensors positioned appropriately, changes in temperature and patterns of water use can be investigated between routine checks, particularly where building use and demand have changed.

Pulling M, O and D Together

The three letters are not three separate problems. They are three places the same problem shows up.

Mechanical: A component fails or underperforms, and the failure sits between the monitoring points. The fix is usually straightforward once the fault is found.

Occupant behaviour: The system is sound but the demand is not there, and records built on signatures cannot show whether the water responded. The fix is training, priority and evidence in place of recollection.

Design: The system was never matched, or is no longer matched, to the building it actually serves. Oversized pipework, low-flow fittings and occupancy patterns interact, and the risk sits in the gap between what the system was designed to do and what the building asks of it. This one cannot always be flushed away or swapped out. It has to be assessed, and then managed over the life of the building.

That is why we suggest troubleshooting by category. Treating the symptom, another flush, another sample, another dead leg on the removal list, keeps the cycle going.

Working out which of M, O or D you are actually dealing with is what turns a persistent problem into one that can be properly investigated.

Could Your Building's Design Be Contributing to Legionella Problems?

If you've changed your water-saving fittings, building occupancy or how parts of your property are used, it's worth understanding what impact this has had on your water system.

LCS can help assess these risks and use continuous monitoring to build a clearer picture of how your system is performing, helping you identify issues that routine checks may be missing.

Discuss your water system with LCS

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.