What is water age and why does it drive legionella risk?

Water age is simply how long water has been sitting in your system before anyone draws it. It matters because almost everything that makes building water risky develops with time: the chlorine residual the mains arrived with decays, the water warms or cools toward the temperature range legionella grows in, and still water gives biofilm the undisturbed conditions it needs. A young system turns its water over in minutes or hours; an old one holds it for days, and the difference is a design choice you can measure and change.

Measuring water age: the turnover calculation

You can estimate the age of your stored water with one division: tank volume ÷ daily water use = average turnover time. Take a 500-litre cold water storage tank serving a building that uses about 250 litres a day. On average, every litre spends two days in that tank before it even leaves for the distribution pipework. Add a long run to a distant wing, or a branch feeding a little-used outlet, and the true age at some taps is longer still.

Run the same calculation on a tank that is oversized for the demand, say 1,000 litres against the same 250-litre daily use, and the answer is four days. Four days is long enough for the chlorine residual to have decayed and, in a warm roof space, for the stored water to climb toward 20°C, the threshold cold water is supposed to stay below. The calculation turns a vague worry about an old tank into a number you can act on: if the turnover comes out in days rather than hours, the storage is part of the problem.

What happens to water as it ages

Three things happen to standing water, and all three favour legionella. First, the disinfectant residual decays. Mains water arrives with a chlorine residual that suppresses bacterial growth, but that residual is consumed over hours and days, so old water has lost the protection it arrived with. Second, the temperature drifts. Cold water in an uninsulated tank or a pipe run through a warm ceiling void warms toward ambient; hot water standing in a long, uncirculated run cools. Both directions head for the same destination: the 20°C to 45°C band where legionella grows. Third, the biology gets time to work. Still water lets biofilm establish on surfaces without being disturbed, and the food chain within it, covered in our page on legionella in water, multiplies undisturbed.

This is why temperature control and stagnation control are really the same strategy seen from two angles: both are about denying water the time and the climate to turn into habitat.

Where water age hides

The tank is the obvious reservoir, but age accumulates anywhere water stands. Dead legs hold water indefinitely by definition. Little-used outlets age the water in their branch between uses, which is why the weekly flushing rule exists. Oversized hot water cylinders and long dead-end hot runs let hot water cool to tepid between draw-offs. Expansion vessels and other tee-connected components hold pockets that barely turn over at all. A risk assessment is, in large part, a systematic hunt for these pockets of old water.

Reducing water age

The levers are a mix of design and discipline:

• Right-size storage. If the turnover calculation comes out in days, reduce the tank volume to match real demand, or bypass storage where the supply allows. A tank inspection is the moment to check both the condition and the sizing.
• Insulate the cold side. Lagging tanks and cold pipes keeps standing water below 20°C even in warm roof spaces, so aged water at least stays out of the growth range.
• Remove dead volume. Cutting out dead legs and redundant pipework removes water that would otherwise stand indefinitely.
• Keep hot water circulating. On larger systems, a return loop keeps hot water hot all the way to the last outlet rather than cooling in the run.
• Flush on schedule. Any outlet used less than about once a week gets a weekly flush to temperature, recorded in the log book. The temperature checker confirms the flush actually turned the branch over.

The extreme case: shutdowns

A closed building is the turnover calculation run to its limit: every tank, run and branch ages at once, for as long as the closure lasts. That is why holiday lets, schools over the summer, and void properties need a deliberate recommissioning routine, flushing the whole system through to temperature before reoccupation and disinfecting where the closure has been long. Our guide to making a system safe after a shutdown sets out the sequence. Water age is not an abstract plumbing concept; it is the single variable that quietly decides how much work a building's water needs when it comes back to life.