Can legionella grow in cold water?

Not in water that is genuinely cold. Below 20°C, legionella survives but does not multiply; growth only begins when the water warms into the 20°C to 45°C range. That is why UK guidance sets the cold water target at below 20°C at the outlet, reached within two minutes of running.

The catch, and the thing that makes cold systems a genuine legionella risk, is that cold water in a real building is frequently not cold at all. "Cold" is a label on a pipe, not a measurement. A cold water storage tank in a loft can sit well above 20°C through the summer. A cold distribution pipe clipped to a hot one picks up heat along its whole length. By the time the water reaches the tap, the cold system may be delivering water squarely inside the growth range, with the temperature nobody checked because it was, after all, the cold tap.

Dormant is not dead

The second misunderstanding is about what cold does to the bacteria. Cold water does not kill legionella; it suspends it. Below 20°C the organism stops multiplying and lies dormant, fully viable, waiting. Warm the water again and growth resumes from exactly where it left off. This means the cold control has to be continuous. A system that is cold in winter but drifts above 20°C every summer is not controlled; it is seasonally colonised. It also means chilling is never a remediation method on its own: cooling a colonised system down stops new growth but leaves the existing population, and its biofilm, in place.

Where cold water picks up heat

Heat gain has a handful of usual suspects, and a walk of the system will find them. The cold water storage tank is the first: sited in a roof space that bakes in summer, uninsulated, sometimes with the lid missing or ill-fitting, so the stored water warms for hours every day. The cold water storage tank inspection exists precisely to catch this. Pipe routing is the second: cold pipes run through hot voids, above boilers, or touching hot pipes without insulation between them. The third is stagnation: a cold branch that is rarely drawn lets water sit in a warm building until it reaches ambient temperature, which in many buildings is above 20°C. Dead legs and little-used outlets are as much a cold-side problem as a hot-side one.

Measuring the cold side properly

Run the outlet for up to two minutes and take the temperature of the water once it has stabilised, so you are reading the supply rather than the water that was standing in the branch. Below 20°C is the target. A reading above that, or a cold tap that never runs properly cold, is a finding: note it, trace the heat source, and fix the cause, whether that is insulating the tank and its pipes, re-routing or lagging pipework, or putting the branch on a flushing regime. The legionella temperature checker walks through the technique, and legionella temperature control puts the cold target alongside the hot ones.

Log every cold-side reading with the date, the outlet and the result in your log book, and keep the records for at least five years. The written scheme in your risk assessment should name which cold outlets are checked and how often, so the cold side of the system is managed with the same discipline as the hot. Summer is the season to pay particular attention: the readings that pass in January are not evidence about July.