Storing hot water at 48–50 °C is defensible only in a low-risk system: a single-family home whose cylinder turns over daily, has no dead legs, and runs a regular anti-legionella cycle. Legionella multiplies between 20 and 45 °C and dies only slowly at 50 °C, so classic guidance says store at 60 °C. The 48–50 °C practice trades a small, managed risk for roughly 20–30% better heat pump water-heating efficiency.
Why is 60 °C the standard storage advice?
Sixty degrees is the temperature at which stored water is reliably self-disinfecting. Legionella grows in the 20–45 °C band, survives but stalls up to about 50 °C, and dies quickly at 60 °C — the full time-and-temperature picture is on our page on what temperature kills legionella in a heat pump system. Guidance documents such as the UK HSE's HSG274 and the WHO's legionellosis guidance were written for the whole building stock — hospitals, hotels, sports centres, complex pipework — so they anchor on a temperature that is safe even in unfavourable systems. The caution is not academic: legionellosis notifications in Europe have climbed to record levels, exceeding 10,000 reported cases a year in recent ECDC surveillance.
What actually determines the risk at 48-50 °C?
Temperature is only one of four factors; the others are time, nutrients and exposure. A cylinder at 48 °C in a busy family home is a very different proposition from the same cylinder in a guest annexe used twice a year.
| Factor | Low risk | Elevated risk |
|---|---|---|
| Turnover | Full cylinder volume drawn off daily | Water stagnant for days or weeks |
| Pipework | Short, direct runs; no dead legs | Dead legs, capped tees, long recirculation loops |
| Temperature profile | Whole store reaches target; cold feed below 20 °C | Lukewarm zones 20–45 °C at the cylinder base |
| Occupants | Healthy adults | Elderly, immunocompromised, respiratory illness |
| System size | Single dwelling, one cylinder | Multi-occupancy, storage cascades, roof tanks |
Note the temperature-profile row: even a 60 °C set point leaves the bottom of a stratified cylinder cooler, which is why cylinder design and stratification behaviour matter as much as the number on the controller.
Why do heat pump owners run 48-50 °C at all?
Because storage temperature is the single biggest lever on water-heating efficiency. A heat pump loses roughly 2% of efficiency for every extra degree it must deliver, so heating to 48 °C instead of 60 °C improves water-heating COP by roughly a quarter and can keep an older unit inside its comfortable envelope instead of leaning on resistance heating. Several large heat pump installers therefore commission single-family systems at 48–50 °C storage with a scheduled disinfection cycle — a practice that has provoked genuine debate precisely because it departs from the blanket 60 °C rule.
What makes low-temperature storage acceptably safe?
Four measures, used together. First, a periodic anti-legionella cycle — typically weekly — that lifts the store to 60 °C, using the heat pump's built-in program or its backup heater. Second, hygienic system design: eliminate dead legs, keep the cold feed cold, and insulate so no pipe section idles in the growth band. Third, high turnover — the household should draw the cylinder's volume most days. Fourth, and most robust, avoid storing drinking water altogether: flow-through designs heat potable water instantaneously through a plate heat exchanger or coil fed from a thermal store, so the stored volume is heating water, not water anyone drinks. Solimpeks' hygienic combination tanks use exactly this principle: the domestic water lives only in a corrugated coil, heated in transit, with no stored potable volume to manage.
When should you simply store at 60 °C?
Whenever any elevated-risk factor from the table applies. Multi-occupancy buildings, holiday homes and rental properties with irregular occupancy, systems with recirculation loops or long dead legs, and households with elderly or immunocompromised members should follow the conventional 60 °C storage and 50 °C-plus distribution regime — accepting the heat pump efficiency penalty, or engineering around it with a flow-through store. The 48–50 °C practice is an informed optimisation for tightly defined domestic conditions, not a new universal rule.
Frequently asked questions
Is 50 °C hot enough to kill legionella?
Only very slowly — at 50 °C Legionella stops multiplying and dies off over hours rather than minutes, so 50 °C storage controls growth but does not disinfect. Reliable kill times start around 60 °C, which is why low-temperature regimes pair 48–50 °C storage with a periodic 60 °C cycle.
How often should an anti-legionella cycle run at 48-50 °C storage?
Weekly is the common commissioning default for single-family systems, scheduled for a cheap-electricity period. The cycle should verifiably bring the whole stored volume — including the cylinder base — to 60 °C, not just the sensor pocket.
Does a combi boiler or flow-through cylinder have the same legionella risk?
No. Instantaneous water heaters and flow-through stores hold no stored potable water at bathing temperature, so the classic cylinder risk largely disappears. This is why hygienic combination tanks with freshwater coils are the standard low-risk pairing for low-temperature heat pumps.
Is legionella actually a real risk in ordinary houses?
The risk in a busy single-family home with mains-fed, high-turnover hot water is low — most reported cases trace to complex or poorly maintained systems. It is not zero, though, and rising European case numbers keep regulators cautious, so the safeguards for low-temperature storage remain worth taking seriously.