Why hot water is stored hot and cold kept cold, and how dead legs and stagnation raise bacterial risk.
Hot water systems carry two opposite risks that have to be balanced: scalding from water that is too hot, and bacterial growth in water that is too warm and too still. Understanding the temperature picture is core revision.
Legionella and similar bacteria multiply in warm, stagnant water, broadly in the range from about 20 to 45 °C. Under the conventional temperature-control regime, store hot water at 60 °C or more, distribute it so the unblended hot supply reaches 50 °C within one minute at outlets (55 °C in healthcare premises), and keep cold water below 20 °C where possible. Check the hot supply before any mixing valve; its cooler outlet has a separate scald-protection purpose.
Stagnation makes the problem worse. A draw-off branch has flow only when its outlet opens; this is one meaning of dead leg. The term is also used for redundant or little-used pipework. A necessary branch is not automatically defective, but infrequent use can leave its water warm and undisturbed for long periods. Warm temperatures, poor turnover and deposits can favour bacterial growth.
Remove redundant pipework back to the live connection, keep necessary branches short and manage infrequently used outlets. Insulation slows heat transfer; it does not indefinitely hold unused water at a safe temperature. Verify temperatures and maintain water turnover as part of the hygiene controls.
The apparent conflict with scald safety is resolved by storing water hot but delivering it blended down at the tap, for example through a thermostatic mixing valve, so what reaches the user is safe to touch.
This is safety-critical revision. For real risk assessment and control of bacteria in water systems, follow the dedicated public health guidance and the current Part G, not memory alone.
Secondary circulation and the remaining branches
A secondary loop keeps hot water near distant outlets and returns it to the store. It needs suitable insulation, balancing and temperature control. The short branches from the loop to the taps still need turnover: running the circulation pump does not flush an unused outlet. Remove redundant dead ends, manage rarely used outlets and verify the temperatures rather than assuming that circulation alone controls Legionella.