Legionella in water heaters and storage tanks: where the conditions concentrate

Water heaters and storage tanks come up again and again when people ask where legionella grows in a building. This page is about why: storage is one of the places building programs most often find the conditions they are trying to control.

Why storage matters

A tank holds water at a temperature that is set, but not necessarily uniform, for as long as demand allows. Where use is low, water can sit for a long time. Disinfectant residual arriving with the supply decays over that period. None of this is a fault in the equipment; it is what storage does.

Stratification and the bottom of the tank

Hot water rises. In many tanks the water near the bottom, close to the cold inlet, is cooler than the water drawn from the top. Sediment and scale also settle there. That cooler, sediment-rich zone is the part of a heater most often discussed in guidance, and it is the part least likely to be represented by a reading taken at the outlet.

How a program deals with it varies: periodic draining or blowdown of sediment, measures that keep the whole tank at the intended temperature, or design changes. Which applies is for the team and the equipment manufacturer to decide. This page sets no temperatures and no frequencies.

The tension with scalding

Higher storage temperatures discourage growth; higher delivery temperatures scald. Buildings handle that tension with mixing valves near outlets and with delivery limits set by code for particular occupancies. Getting it wrong in either direction is a real harm, which is why the numbers belong to the building’s own program and local code rather than to a website.

Cold water tanks are not exempt

Cold water storage can warm up when it sits in a hot plant room or roof space, or when turnover is low. A cold tank that is not cold is a storage problem of its own, and it is one of the quieter findings of a careful walk-through.

Low use makes everything worse

A system sized for a full building and run for a half-empty one stores water longer than it was designed to. The same applies to heaters serving wings that are rarely occupied, and to the branches they feed, which can turn into dead legs in practice even though nobody capped them.

What a program writes down for each vessel

For each heater or tank: where it is, what it serves, what is measured and where, the limit the team has chosen, how often the reading is taken and by whom, and what happens when the reading is outside the limit. Plus the maintenance record: draining, cleaning, inspection. The background on why storage features so often is in how legionella gets into building water, and the structure of the program around it is on the water management programs page.

Domestic heaters

A home water heater raises the same physical questions on a smaller scale, but no building program applies to it. The manufacturer’s instructions and a licensed plumber are the right sources there. Anyone concerned about their health should speak to a doctor.