A storage thermometer near the top of a hot water cylinder can read a confident 60°C while the bottom third of the same vessel sits at 35°C — squarely in the band where Legionella breeds. That gap is stratification, and it is the reason a single dial reading can sign off a calorifier that is quietly growing bacteria at its base.

If you already know the 60/50/20 numbers and run your monthly checks, the question worth sharpening is narrower than the rule itself. Are you measuring the water that is actually at risk, or just the water that happens to sit next to the sensor?

Why hot water layers, and why the base is the problem

Hot water is less dense than cold, so it rises. In a vertical hot water cylinder or calorifier, the hottest water collects under the crown and the coolest, densest water settles at the base. Between them sits a transition band — the thermocline — where temperature falls away sharply over a few inches of height. None of this is a fault; it is physics, and a degree of stratification exists in any stored-water vessel.

The trouble is where the cold layer ends up. The cold feed enters low, so incoming mains water arrives at the base. Sediment, scale and corrosion debris settle there too, because that is where flow is slowest. HSE lists sludge, scale and biofilm among the conditions that let Legionella establish, since they shield bacteria and feed them [4]. Put a cool layer, slow-moving water and a nutrient bed in the same place and you have built a small incubator at the bottom of an otherwise compliant cylinder.

Stored hot water is generally expected to sit at or above 60°C, with distributed water reaching around 50°C at outlets (55°C in healthcare premises) and cold water held below 20°C [1]. Legionella multiplies fastest roughly between 20°C and 45°C and is progressively killed above about 60°C [2]. Stratification is what lets the bottom of a vessel drift into that growth range while the gauge at the top still shows the number you wanted to see.

A cross-section worth picturing

Picture a vertical cylinder cut down the middle and label four things.

  • The cold feed connects low on the body, near the base. Fresh, cool mains water arrives here.
  • The hot draw-off leaves at the very top, where the hottest water has risen.
  • The heat source sits in the middle-to-upper body: an immersion element in a directly heated cylinder, or a primary heating coil fed by flow and return in a calorifier. Water below the heat source is warmed slowly and indirectly — it is the last to come up to temperature and the first to cool.
  • The storage thermometer pocket is usually fitted high, in or near the hot draw-off zone. It reports the best-case temperature in the vessel.

Now lay those facts over one another and the failure mode draws itself. The base is the cold corner — coolest water, slowest movement, and the settling ground for scale and sludge. It is also the one part of the cylinder the routine thermometer never touches. A drain or test point at the base tells a very different story from the gauge at the crown: a top reading of 60°C sitting above a base that has drifted into the 20–45°C growth band is an everyday finding on a neglected calorifier.

Where stratification turns into a control failure

A normal temperature gradient is not a problem in itself. It becomes one in a handful of recognisable situations.

The reading is taken in the wrong place. A monthly check that records only the flow temperature, or only the top-of-cylinder gauge, confirms the easy number and misses the base entirely. Logging the calorifier flow and return temperature together is what reveals whether the whole stored volume is hot, not just the surface. The benchmark values behind those checks are covered in UK water temperature guidance for Legionella control.

Low draw-off lets the base cool. Every time hot water is drawn, cold feed pushes up from the bottom and the contents mix; the vessel partly destratifies and then reheats. In a busy building that churn keeps the base turning over. Over a quiet weekend, an under-used wing, or a building running below its design occupancy, the base can sit untouched for days and cool steadily into the growth band — the same lukewarm trap covered in the danger zone.

The cylinder is oversized. A vessel specified for a demand that never materialised stores more water than the building turns over, so a large cool reservoir lingers at the base between draw-offs. Generous sizing chosen for “resilience” can quietly enlarge the very zone you are trying to keep hot.

Setpoints get lowered to save energy, without reassessment. Dropping the stored water temperature to cut gas or electricity is tempting, but a couple of degrees off the top reading can pull the already-cooler base well down into the danger range. Any change to storage or circulation temperature is a change to the control scheme and should be assessed as one, not dialled in at the boiler on a cold Friday.

Monitoring that actually proves control

The point of all this is a sharper routine, not anxiety about your plant room. A few habits separate a check that proves control from a check that merely produces a number.

Take more than one temperature. Flow and return at the calorifier together describe whether the stored volume is hot through, not just on top. Where a base drain or test point exists, a periodic reading there is the most honest measurement you can take of the cold corner.

Deal with the sediment. The nutrient bed at the base is removable — draining off sediment and inspecting internal surfaces on a sensible cycle keeps the bottom of the vessel from becoming a reservoir. HSG274 sets out the inspection and monitoring expectations for calorifiers; the exact frequency is one your risk assessment fixes for the system in front of you [3]. If a vessel turns out to be heavily fouled, cleaning or disinfection belongs in the remedial plan rather than the routine (Cleaning and disinfection after remedial work: thermal and chemical approaches).

Record the decision, not just the digit. “Base drain 41°C, inside the 20–45°C concern band, sediment flushed, recheck next visit, escalate if unchanged” is a managed control. “60°C — pass” against the top gauge alone is a number that may be hiding the problem it was meant to catch.

Treat the cylinder as one of many control points

Stratification is general physics applied through a competent, site-specific risk assessment — not a fixed rule you can lift from a guidance note and bolt onto any plant room. The temperatures, test points and frequencies that suit a small directly heated cylinder in a low-rise office are not the ones for a large healthcare calorifier serving vulnerable users. Where to measure, how often, and what counts as an exceedance are decisions for the responsible person and the assessment behind the scheme, not for a web page to settle.

Common questions

Does deliberately mixing the cylinder to break stratification improve Legionella control?

It can, but only if the whole contents genuinely reach storage temperature. A well-mixed, fully heated vessel has no cold base to worry about. A destratification or shunt pump that merely moves water around without bringing the base up to temperature can spread cooler water rather than fix it. The aim is a hot base, not just a stirred one — so verify it with a base reading, not an assumption.

If the top of the cylinder reads 60°C, do I still need to worry about the bottom?

Yes. The top gauge tells you the hottest water in the vessel is hot, which is the least informative thing you can know about it. The base is where cool water, slow movement and sediment coincide, so it is the reading that actually tests control. A pass at the crown and a failure at the base is the classic stratification picture, and the gauge will never show it to you.

Sources

[1] HSE, “Hot and cold water systems”. https://www.hse.gov.uk/legionnaires/hot-and-cold.htm [2] HSE, “Legionnaires’ disease. The control of legionella bacteria in water systems — Approved Code of Practice and guidance (L8)”. https://www.hse.gov.uk/pubns/books/l8.htm [3] HSE, “Legionnaires’ disease: Technical guidance (HSG274)”. https://www.hse.gov.uk/pubns/books/hsg274.htm [4] HSE, “Systems most likely to create legionella risk”. https://www.hse.gov.uk/legionnaires/risk-systems.htm