Training

Cold water suppresses Legionella growth. The guidance states that organisms do not appear to multiply below 20°C and can remain dormant in cool water, multiplying only when temperatures reach a suitable level. UK incoming cold water usually arrives in a cool range, varying by season and source — in the order of 5–10°C in winter and up to 20°C in summer. As long as cold water stays below 20°C, temperature is doing its job as a control. The moment it warms above that, the control weakens.

Heat does kill Legionella, and the relationship between temperature and kill time is not gradual — it's exponential. In one isothermal laboratory model, a 90% kill at 51°C took over 25 minutes. At 55°C it took about 3.5 minutes. In a separate study of 75 isolates, thermal death times at 60°C ranged from about 1 to 11 minutes. At 70°C the range was 0.7 to 2.6 minutes. At 80°C, under a minute. A few degrees make an enormous difference. These are laboratory values — actual performance varies by strain and conditions — but the steepness of the curve is the point.

The approved code of practice identifies water temperatures between 20°C and 45°C as suitable for the growth of Legionella, provided other conditions are also present. That temperature band is the warm middle: too warm to suppress growth, too cool to achieve meaningful kill. Controls aim to avoid prolonged storage in this range.

The guidance states that hot water should be stored at least at 60°C, and that this temperature should be achieved throughout the vessel for at least one continuous hour a day. In a small building, the hot water cylinder is the one place designed to reach and hold a temperature capable of thermal inactivation. Whether it actually achieves that depends on the condition of the vessel, the position of the thermostat, and whether the full volume of water — including the base — reaches the required temperature. If it does, thermal inactivation can occur. If it doesn't, downstream sections are not themselves a substitute kill step.

The thermostat on your cylinder is typically fitted near the top of the vessel. It regulates heat supply based on the temperature it reads at that position. But calorifiers are thermally stratified — the guidance notes that the water temperature at the base will usually be much cooler than at the top. The thermostat may read 60°C while the base sits well below that. That's why the guidance requires a separate measurement pocket in the base of the calorifier — the control thermostat alone does not tell you what's happening at the bottom.

The cooler base of the cylinder is also where sediment and loose deposits accumulate over time. Sludge settles. Scale builds. And those deposits create a sheltered environment — a niche where conditions can differ from the water above. The combination of lower temperature and accumulated material at the base of the vessel is exactly the kind of condition that can favour Legionella persistence.

In laboratory experiments using water-heater loose deposits, no inactivation of Legionella was observed during 4 hours at 50°C. At 55°C, a few bacteria were still detectable after 24 hours. In those same deposits, when amoebae were present, a 4-hour exposure at 55°C was followed — during subsequent storage — by a roughly 5-log amplification of Legionella. The organism didn't just survive: it amplified markedly with the help of its host. Only at 60°C, in this experiment, did concentrations remain at zero with or without amoebae. An underperforming cylinder can leave the base in this sort of range.

Where a scald risk has been identified, the guidance recommends thermostatic mixing valves to blend hot water down to a safe delivery temperature. The legal basis differs by premises type: in care homes and dental surgeries, CQC standards require that care is provided safely, which includes controlling delivery temperature; in new or converted dwellings including guest houses, building regulations require that bath water does not exceed 48°C; in standalone offices, the general duty under health and safety law applies through risk assessment. A TMV blends the hot supply with cold water to produce an outlet temperature typically in the range of 38–46°C, depending on outlet type and setting. The practical result is that where scald protection is required, the water reaching the user has been reduced to a temperature that no longer provides reliable thermal inactivation.

The pipe between the blend valve and the tap now carries water at a blended temperature that typically falls within, or close to, the 20–45°C range the approved code of practice associates with conditions suitable for growth. No chemical residual remains — that was consumed upstream, as S02 explained. And if the tap is underused, that water sits still. The guidance is explicit: TMVs should be sited as close as possible to the point of use, and mixed water pipework should be kept as short as possible. That instruction exists because the post-TMV section is recognised as a zone requiring management — water there is not exposed to high temperatures or biocides.

If the cylinder has always reached the required temperature throughout, the water entering the blend valve has always been thermally treated. But if the cylinder ever underperformed — the base stayed cool, the full volume didn't reach 60°C, the control scheme wasn't maintained — the guidance treats that as a system failure requiring action. The response isn't simply restoring the temperature. The guidance states that the whole system should be cleaned and disinfected if it has not been managed in accordance with the control scheme. Peer-reviewed evidence shows that distal points in systems with temperature failures had significantly higher colonisation rates, and that restoring temperature alone did not guarantee clearance — viable-but-non-culturable forms and biofilm-associated persistence remained concerns.

If the system has been compromised, the building manager's day-to-day controls are limited. The guidance states that regular use of peripheral outlets — weekly flushing for several minutes — can significantly reduce the number of Legionella discharged from the outlet. That regime must be sustained and logged, because lapses can result in a critical increase. But peer-reviewed research has shown that flushing had little to no effect on bacteria in pipe wall biofilms, and that any reduction rebounded within days. Flushing changes the water in the pipe. It does not strip what lives on the wall. The guidance treats fouling with slime and scale as a separate maintenance problem, not something resolved by flushing alone.

This module teaches one sequence. The guidance requires hot water stored at 60°C or above, achieved throughout the vessel. The blend valve then reduces it for safe delivery. The pipework after the blend must be kept short and the outlets kept in regular use, because that section is no longer protected by high temperatures or biocides. If those controls are not consistently achieved, the guidance is clear: the system should be cleaned and disinfected. That is the control scheme doing what it is designed to do — identifying when the problem has moved beyond day-to-day controls.