Legionella and hot water temperature: geysers, heat pumps and solar systems in South African buildings

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Legionella · Hot water · Geysers, heat pumps and solar

At what temperature does Legionella grow?

The guidance bodies use slightly different numbers. Know which body says what before someone on site argues the point.

The practical point is the gap between 45 °C and 60 °C. Water at 50 °C feels hot to the hand and a lot of maintenance teams assume it is safe. It is not a killing temperature. WHO puts the top of the growth range at 50 °C, and any stagnant or dirty part of the system runs cooler than the flow.

What temperature kills Legionella, and how fast?

In laboratory tests on 75 isolates from 19 species (Stout, Best and Yu, 1986), the thermal death times were 1.3 to 10.6 minutes at 60 °C, 0.7 to 2.6 minutes at 70 °C, and 0.3 to 0.7 minutes at 80 °C. Those are laboratory figures in clean water. In a real system the bacteria sit inside biofilm and inside amoebae, which shelter them, so you never rely on a short burst of heat.

For thermal disinfection of a system, the Australian health and aged care guideline (enHealth, 2015) runs each outlet at 70 °C or above for at least 5 minutes, or 60 °C or above for at least 10 minutes if 70 °C cannot be reached. That is a disinfection procedure with scald controls around it, done by people who know what they are doing, not a setting you leave the geyser on.

What temperature should a geyser or calorifier be set to?

60 °C or above for storage. Every Legionella guideline lands in the same place: the NICD recommends storing hot water above 60 °C and delivering it to taps above 50 °C. The WHO asks for water leaving the heater at or above 60 °C so that hot water stays above 50 °C, and cold water below 25 °C, ideally below 20 °C. The UK’s HSG274 Part 2 asks for stored hot water of at least 60 °C, calorifier flow never below 60 °C, outlets reaching at least 50 °C within one minute (55 °C in healthcare premises) and each circulation return at least 50 °C (55 °C in healthcare).

You may hear someone quote SANS 10252-1, the South African standard for water supply installations in buildings. A note in that standard says hot water temperature should be controlled in the 55 to 60 °C range and that running at 60 to 70 °C makes water more corrosive. That is a design note about pipework, not a Legionella control. The 55 °C bottom end sits below the 60 °C storage figure in every Legionella guideline. For Legionella control, store at 60 °C or above.

Is it safe to turn the geyser down to save electricity?

No. It saves power, but it moves the stored water into the range where Legionella grows.

South African buildings have spent years managing electricity: load shedding, tariff pressure, heat pumps, solar preheating and geyser timers. Each one is sensible on the electricity bill. Each one can push stored water into the growth range without anyone noticing.

• Geysers on timers or switched off overnight. At the hotel case in our courses, the staff change room geysers switch off from 22h00 to 05h00. At 06h30 the spa change room shower runs at 38 °C after a minute. With the element off, every litre drawn overnight and early in the morning is replaced by cold water, so the geyser can sit in the growth range by morning, as it did here.
• Heat pumps set low. Heat pumps are efficient at lower set points, so they get set to 50 or 55 °C. The same hotel’s calorifiers have run at 55 °C since an energy saving drive. Where a heat pump cannot hold 60 °C, the electric element backup should top it up.
• Solar preheat. Solar water that comes in warm and sits in a preheat vessel without ever being raised to 60 °C is warm, stored water. The final heater must bring it up.
• The cool bottom of a horizontal geyser. Stellenbosch University modelling found the lower surfaces of horizontal geysers stay below 45 °C, and the authors concluded that the pipework downstream is where growth is favoured (Stone et al., 2018). Sediment collects at the bottom of the vessel too.

None of this means you cannot save electricity. It means the saving comes from insulation, good loop balancing and a heat source sized to hold 60 °C, not from switching the stored water off or running the whole system at the temperature Legionella likes.

How do you prevent scalding without lowering the storage temperature?

This is the argument I hear most often, usually after someone has been hurt. A frail care centre turns its geysers down to 50 °C after a resident is scalded. The scald risk is real and the decision was made with good intent. It is still the wrong fix, because it trades one hazard for another in the most vulnerable people in the building.

The right answer is to store hot water at 60 °C or above and fit thermostatic mixing valves (TMVs) as close to the outlet as possible wherever vulnerable people wash: frail care, children, assisted bathing. The UK guidance calls for type 3 TMVs that are pre-set and fail safe if the cold supply is interrupted, checked at least annually. Fit and test the TMVs first, then raise the storage temperature. Raising it first puts very hot water at the outlets of the people the scald fix was meant to protect. A TMV outlet is mixed down on purpose, so it is not used as a hot sentinel when you check the loop. The healthcare routine is in Legionella in hospitals, clinics and frail care.

What temperature should cold water be to control Legionella?

Cold water should stay below 20 °C, and a cold tap should reach that within two minutes of running. In South Africa the usual culprits are roof tanks in the sun and too much stored water. After the water outages of recent years many buildings added polyethylene tanks that hold two or three days’ supply. The UK guidance says stored cold water should not normally exceed one day’s use (12 hours in healthcare), because water that sits warms and stagnates. Our guide to Legionella in water storage tanks covers JoJo tanks, outages and boreholes.

A 2025 study of residential high-rise buildings in Hillbrow, Johannesburg, found L. pneumophila in 14 of the 15 buildings sampled, all of which used storage tanks, most of them plastic (Buthane et al., Microorganisms, 2025). It does not prove that tanks cause the problem, but it is a strong reason to look inside yours.

How do you check your own hot water system for Legionella?

1. Use a calibrated probe thermometer and a timer. Not the dial on the calorifier.
2. Read the calorifier or geyser flow and the return on each loop. Flow at least 60 °C, returns at least 50 °C.
3. Pick hot sentinel outlets: the nearest to and furthest from the calorifier on each loop. Run the hot tap and record the temperature at one minute. At least 50 °C (55 °C in healthcare).
4. Run the cold sentinel taps, nearest to and furthest from the cold water tank, and record them at two minutes. Below 20 °C.
5. Write it down at the outlet: date and time, outlet, reading and run time, the limit, pass or fail, what you did, your name.
6. Do it monthly, and flush little-used outlets every week, following our Legionella flushing procedure.

Running a hotel or guesthouse? The full routine is in our hotel and guesthouse checklist. When temperatures cannot be held, sampling comes in: see Legionella testing in South Africa.

Frequently asked questions

Sources and further reading

• HSE HSG274 Part 2: hot and cold water systems
• HSE: Legionella in health and social care
• WHO fact sheet: legionellosis
• NICD recommendations for Legionnaires’ disease
• Stout, Best and Yu (1986), thermal stress and Legionellaceae
• Buthane et al. (2025), Legionella in Hillbrow high-rise buildings