Why do scale and corrosion matter for legionella?
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Key points
- Hardness minerals come out of solution faster as water is heated, so scale concentrates on the hot side: elements, cylinder walls, TMVs and shower heads.
- Scale is a legionella problem because it anchors biofilm and shelters it from heat and disinfectant; corrosion adds pitted crevices and releases iron, a nutrient legionella needs.
- Scale and sediment collect where flow is slowest: the base of calorifiers and tanks, dead legs and little-used branches.
- The effective sequence is descale and clean first, then disinfect. Either step alone leaves the problem in place.
- Shower heads and hoses are typically descaled quarterly or as the risk assessment sets; tanks and calorifiers are cleaned when inspection finds deposits.
Scale and corrosion matter for legionella because they build and maintain the habitat the bacteria depend on. Limescale gives biofilm a rough, sheltered surface to anchor to; corrosion pits metal and releases iron, a nutrient legionella needs. Neither introduces legionella to the system, but both make it far easier for the bacteria to establish and far harder for your controls to reach them. Much of England has hard water, so for a large share of duty holders this is a routine maintenance issue rather than an edge case.
Why the hot side scales up
Hard water carries dissolved calcium and magnesium, and heat drives those minerals out of solution. Every time a calorifier or immersion heater raises the temperature, another thin layer of hardness is deposited on the hottest surfaces: the element, the cylinder walls, and the fittings downstream. That is why scale is overwhelmingly a hot-water problem, and why it collects in predictable places. It settles where water is hottest, and it accumulates where flow is slowest: the base of the calorifier, the bottom of storage tanks, dead legs, the strainers inside thermostatic mixing valves, and the spray plates of shower heads.
The calorifier inspection exists largely because of this. The base of the vessel is the cold spot where sediment and scale gather, and the drain-valve sample is the honest look at what has settled there.
What scale and corrosion do for legionella
The deposits help legionella in three distinct ways:
- Anchorage. A smooth pipe wall sheds biofilm; a rough, crystalline scale surface holds it. Once biofilm anchors, the whole food chain it supports establishes with it.
- Shelter. Scale is porous and layered. Bacteria lodged in its crevices are shielded from heat circulating in the open water and from disinfectant, so a treatment that clears the flowing water can leave the population under the scale intact.
- Nutrients. Corroding steel releases iron, and iron is a growth requirement for legionella. Scale also traps organic debris that feeds the biofilm. A corroding, scaled system is simultaneously housing and feeding the bacteria.
The signs are practical ones: reduced flow at outlets as strainers and spray plates choke, rumbling or kettling from a scaled calorifier, rust-coloured water after stagnation, and heavy sediment when a tank is inspected. Each is the system telling you the habitat is being built.
Scale does not bring legionella into a building, but it builds the shelter legionella hides in. Descaling is not cosmetic maintenance; it is removing the bacteria's cover so heat and disinfectant can work.
The control regime: descale, clean, then disinfect
The sequence matters more than any single product. Descale and physically clean first, stripping the deposits from elements, tanks and outlets so the sheltered surfaces are gone. Then disinfect, because heat and chemicals can now reach the bacteria that were hiding behind the scale. Reversed or halved, the process fails: disinfecting a scaled system leaves the shelter intact, and descaling without disinfection leaves the bacteria in place.
On frequencies, there is no single legal interval for the whole system. Shower heads and hoses are typically descaled and disinfected quarterly, or at whatever frequency the risk assessment and manufacturer set, because they produce the aerosol that matters most; our guide to cleaning shower heads covers the method. Tanks and calorifiers are inspected on a planned basis and cleaned when the inspection finds sediment and scale, which in hard water areas is sooner rather than later. All of it belongs in the written scheme of control with a named frequency, and every clean goes in the log book.
Free legionella risk assessment template
A structured Word document following the five-step approach in ACOP L8. Covers risk identification, written scheme, monitoring, and records. If it isn't written down, you can't evidence it.
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Prevention: softening and temperature discipline
Two measures slow the rate at which the habitat rebuilds. In hard water areas a water softener removes the hardness minerals before they can deposit, cutting scale formation downstream, an indirect but real benefit for legionella control, though the softener itself then needs maintaining and the temperature regime still applies in full. And sound temperature control does double duty: storing hot water at 60°C and keeping cold below 20°C suppresses growth directly, and a temperature check confirms the targets are actually being met. A system that is kept at the right temperatures and descaled on schedule denies legionella both the climate and the cover it needs.
Free legionella risk assessment template
A structured Word document following the five-step approach in ACOP L8. Covers risk identification, written scheme, monitoring, and records. If it isn't written down, you can't evidence it.
Follows ACoP L8 and HSG274 Part 2. Free. No spam.
Frequently asked questions
Does limescale cause legionella?
Scale does not cause legionella, but it creates the conditions the bacteria exploit. A scaled surface is rough, so biofilm anchors to it more easily; the layers and crevices within scale shelter bacteria from heat and disinfectant; and scale traps organic matter that feeds the biofilm food chain. A heavily scaled calorifier or shower head is therefore a friendlier habitat for legionella than a clean one, which is why descaling is a standard part of the control regime rather than a cosmetic task.
Why does scale form faster in hot water systems?
Hard water carries dissolved calcium and magnesium, and those minerals come out of solution faster as water is heated. Every time a calorifier or immersion heater raises the temperature, a little more hardness is deposited on the hottest surfaces: the heating element, the cylinder walls, and the downstream fittings. That is why the hot side of a system scales up while the cold side largely does not, and why the deposits concentrate exactly where the water is hottest.
Does corrosion make legionella worse?
Yes, in two ways. Corrosion roughens and pits metal surfaces, creating protected crevices where biofilm establishes and disinfectant cannot reach. It also releases iron and other metals into the water, and iron is a nutrient legionella needs to grow. A corroding system is therefore both sheltering and feeding the bacteria. Rust-coloured water, pinhole leaks and heavy sediment in tank or calorifier bases are all signs the system is handing legionella an advantage.
Does descaling kill legionella?
Descaling removes habitat rather than killing bacteria, but that is precisely why it matters. Stripping the scale takes away the sheltered surfaces biofilm and legionella hide behind, so that heat and disinfectant can reach what remains. The effective sequence is to descale and clean first, then disinfect. Descaling alone leaves bacteria in place; disinfecting alone leaves the shelter in place; the two together reset the system.
How often should a hot water system be descaled?
There is no single fixed interval for the whole system. Shower heads and hoses are typically descaled and disinfected quarterly, or at the frequency the risk assessment and manufacturer set, and tanks and calorifiers are inspected on a planned basis and cleaned when sediment and scale are found. In hard water areas the intervals shorten. The right answer for your system comes from the risk assessment and the inspection findings, recorded in the written scheme of control.
Will a water softener stop limescale and legionella?
A water softener removes the hardness minerals that form scale, so it reduces the habitat available downstream, which is an indirect benefit for legionella control. It does not kill or remove legionella, and softened water is not disinfected water, so the temperature and flushing regime is still required in full. The softener itself also needs maintaining, because its resin bed and brine tank are wet vessels that can grow biofilm if neglected.
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