Collective hot water: temperature, legionella and network upkeep

It is the only installation in a building where a wrong setting can make someone seriously ill.

Collective domestic hot water is an ordinary installation, until you realise what it involves: lukewarm water sitting in pipework at a temperature that favours bacterial growth, distributed to the showers of every flat.

The legionella risk is not theoretical. It is well documented, it concerns collective networks in particular, and it is controlled by settings and upkeep that cost almost nothing — provided somebody checks them.

The temperature question

The principle is simple and counter-intuitive: water must be produced and stored hot enough to prevent bacterial growth, then controlled at the outlet to avoid scalding. Lowering the cylinder temperature to save energy is a false economy.

• Production and storage temperature, which must stay above the safety threshold.
• Temperature at every point of the distribution network, including furthest from the plant room.
• Mixing or limiting devices at the outlets, against scalding.
• Regular readings, and their recording.

The second point is the one most often missed: a correctly set cylinder can still deliver markedly cooler water at the end of the furthest riser. That is where the risk concentrates.

A network’s weak points

• Dead legs: sections of pipework where nothing circulates, left after an outlet was removed.
• A failing circulation loop, leaving water to stagnate in the risers.
• A scaled cylinder, or one never drained.
• Flats vacant for months, whose pipework is no longer flushed.
• Degraded lagging, letting water cool in the circulation areas.

The first is invisible and frequent: every alteration to the network over decades leaves abandoned sections. They are found during an inspection and removed during works — see cleaning the stacks and network alterations.

What the board can ask for

Three documents show where the building stands: the hot water network maintenance contract, temperature readings for recent years, and intervention reports on the cylinder.

If those readings do not exist, that is already information: nobody is watching. Same check as in collective heating settings, and the contract is analysed as in replacing the collective boiler.

What to do after an interruption

After a prolonged shutdown, a drain-down or a recommissioning, water has stagnated. Restarting means flushing the outlets, beginning with the furthest, and checking temperature before normal use.

The board can simply ask for that procedure to be written into the contract and documented after each intervention — a line to add, not an extra cost.

The link with energy savings

Domestic hot water represents a significant share of a building’s consumption, and the temptation to lower temperatures is real. The savings lie elsewhere: lagging the pipework, reducing flow rates at the outlets, removing dead legs, regulating the circulation loop.

These works are cheap and immediate: see the steps of an energy renovation and spotting excess consumption.

Lowering the cylinder temperature to save money moves an energy cost onto a health risk.

Track your readings with CoproHarmony

CoproHarmony keeps contracts, intervention reports and temperature readings from one year to the next, and shows immediately what was not done. Create your free space.