A facilities team under pressure to cut carbon eventually arrives at the hot water. Drop the calorifier setpoint a few degrees, the reasoning goes, and the gas bill falls with it. It does — and it can also slide stored water into the temperature band where Legionella multiplies. That one proposal is where water safety and the net zero plan look like they are pulling in opposite directions.

They mostly are not. Take away the headline argument over temperature, and a surprising amount of what counts as good Legionella control is also good environmental practice. The skill worth having is telling the genuine clashes from the false ones — and having a defensible answer ready for the moment the energy lead asks why the cylinder has to stay hot.

The two agendas share one enemy

Wasted water and wasted energy tend to live in the same places: oversized cold tanks, hot water that loses its heat down a long uninsulated run, dead legs feeding nothing, outlets nobody has used since a refurbishment. Stagnant and redundant capacity is exactly what HSE flags as raising Legionella risk [1], and it is also a quiet, permanent drain on resources. Remove the waste and you usually remove the risk at the same time.

That overlap is the part most sustainability conversations miss. The framing of “safety versus efficiency” assumes every green change costs you control. In practice the relationship is closer to: a few measures genuinely lower temperature or slow water down, and you scrutinise those hard; everything else is either neutral or actively helps both columns.

Where the goals reinforce each other

These are the changes to reach for first, because they cut consumption and tighten control in the same move:

  • Decommissioning dead legs and redundant outlets. Less stagnant volume to colonise, and less to flush every week. Pulling a redundant branch once beats flushing it for a decade — the stagnation case is covered in detail in Neglected water systems: the danger of stagnation.
  • Right-sizing cold storage and hot generation. A tank matched to real demand turns over faster, so water stagnates less and wastes less standing energy. Oversized storage is a sustainability cost and a risk in one.
  • Insulation and lagging. Keeping hot pipework hot and cold pipework cold is a temperature-control measure and an energy measure simultaneously. It is rare to find a change that scores so cleanly on both.
  • Leak detection and repair. Water efficiency, fewer damp warm voids, and a system you actually understand.
  • Replacing paper monitoring rounds with sensor or digital records. Fewer wasted call-outs and tighter, faster evidence that controls are holding.

None of these asks you to trade safety for carbon. They are the easy wins, and they are worth recording as safety improvements when the carbon team claims them, so the contribution is visible on both ledgers.

Where they genuinely conflict — and how to hold the line

A smaller set of measures does pull against control. Each needs a competent assessment and a sign-off before it goes near the system.

Lowering hot water temperature for energy savings. This is the headline conflict. UK guidance generally expects stored hot water around 60°C, distributed so it reaches outlets at roughly 50°C or above, with cold water held below about 20°C — because Legionella multiplies fastest in the rough 20–45°C window and is progressively killed at higher temperatures [2][3]. Cooling the system to save gas walks straight into that growth band. If energy is the real driver, the answer is better insulation, heat recovery and smarter timing — not a lower setpoint, unless an alternative control has been designed in and signed off through the risk assessment.

Heat pumps and low-temperature heating. Efficient, and increasingly the default for retrofitted plant, but they run at lower flow temperatures. Reaching the temperatures that control Legionella may need a specific cylinder design, supplementary heating or a periodic thermal treatment built into the scheme. Do not assume the standard temperature target falls out of a heat pump for free; design for it.

Cutting flushing to save water. Flushing low-use outlets is a core control. Reducing it to hit a consumption target swaps a measured water saving for an unmeasured rise in risk. If an outlet is barely used, the better answer is almost always to remove it, not to stop flushing it.

Low-flow and aerating fittings. They save water, but they can produce a finer spray and, on a rarely used outlet, hold a small volume that sits and stagnates. They are fine where there is genuine throughput and a control regime; they become a quiet hazard when fitted and forgotten on a low-use tap.

Rainwater harvesting and greywater reuse. Real sustainability gains that also introduce new stored, often untreated water systems. Each needs its own risk assessment and controls before commissioning — treated as a new hazard, not bolted onto the existing scheme as an afterthought.

Running a green proposal through the filter

When an energy or water-saving idea lands, send it through this before it touches the system. Take the first branch that applies:

  • Does it lower water temperature anywhere — storage, distribution or return?
    • If yes → it does not proceed on energy grounds alone. Either an alternative control is designed and signed off in the risk assessment, or the proposal is declined.
    • If no → go to the next question.
  • Does it slow water down or add stagnant volume — less flushing, more dead legs, fit-and-forget low-use outlets?
    • If yes → proceed only if the affected outlets are removed, or a flushing and turnover regime keeps the water moving. Otherwise decline.
    • If no → go to the next question.
  • Does it add a new water source or system — rainwater, greywater, recirculation?
    • If yes → it gets its own assessment and controls before commissioning, never after.
    • If no → go to the next question.
  • Does it remove waste without touching temperature or movement — insulation, leak repair, right-sizing, decommissioning, digital monitoring?
    • If yes → this is an easy win that helps both goals. Proceed, and log the safety benefit next to the carbon one.

The filter stops the dangerous changes being waved through by someone who only sees the kilowatt-hours, while letting the genuinely shared wins move quickly.

Get the right people in the room early

Most avoidable conflicts come from sequence, not bad intent: a setpoint gets dropped or a greywater system specified, and the water safety lead only finds out at commissioning. The fix is governance — put the energy or sustainability lead on the water safety group, so green proposals are tested against the written control scheme before procurement, not after. A water safety plan is the natural home for that joined-up thinking [4]; Developing a comprehensive Water Safety Plan covers how to build one.

A note on the numbers

Treat the temperatures here as orientation, not a setpoint sheet. The exact figures, flushing intervals and treatment options that are safe for your building come from a competent, site-specific risk assessment — and an energy change that looks marginal on a spreadsheet can be material on a system serving vulnerable users. Any temperature or treatment change should be reviewed and signed off before it goes live, not explained after an exceedance.

FAQ

Can we lower the hot water temperature to save energy without raising Legionella risk?

Not safely on energy grounds alone. The standard hot and cold temperatures exist to keep water out of the growth range [2][3], so a lower setpoint is one of the few green measures that directly weakens control. Look first at insulation, heat recovery and timing. A genuinely lower-temperature regime only works if an alternative, validated control is designed into the risk assessment and signed off — see Hot water temperature guidelines to prevent Legionella and Best practices in water treatment for Legionella control.

Does a rainwater or greywater system fall under our Legionella risk assessment?

Yes. Reusing rainwater or greywater introduces new stored water that can present a Legionella hazard, particularly if it ever feeds anything that produces a spray. It needs its own assessment and controls before it is commissioned, rather than being added to the existing scheme afterwards.

Is reducing flushing a legitimate way to cut water consumption?

Rarely, if the outlet stays in service. Flushing keeps water moving through low-use parts of the system, so cutting it to hit a water target trades a known saving for an unknown risk. The better move is to remove outlets you do not need — that cuts the flushing burden and consumption for good, without losing control.

Sources

[1] HSE, “Systems most likely to create legionella risk”. https://www.hse.gov.uk/legionnaires/risk-systems.htm [2] HSE, “Hot and cold water systems”. https://www.hse.gov.uk/legionnaires/hot-and-cold.htm [3] HSE, “Legionnaires’ disease. The control of legionella bacteria in water systems - Approved Code of Practice and guidance (L8)”. https://www.hse.gov.uk/pubns/books/l8.htm [4] BSI, “BS 8680:2020 - Water quality. Water safety plans. Code of practice”. https://knowledge.bsigroup.com/products/water-quality-water-safety-plans-code-of-practice