Climate change doesn’t hand you a new bacterium. It quietly removes a control you have been relying on for decades, free of charge: cold water that arrives cold and stays cold. Most UK schemes pour their effort and energy into the hot side and lean on the cold side simply being below the temperature where the organism multiplies. Warmer, longer summers chip away at that free protection from the bottom up.

If you already run a compliant scheme, the work is not buying new kit. It is recognising which slow environmental shifts count as a material change to your system, and feeding them into the risk assessment before they surface as a failed temperature reading.

Where the warming actually bites

The growth band hasn’t moved. Legionella still multiplies fastest in roughly the 20-45°C range, sits dormant below about 20°C, and is progressively killed above around 60°C [1]. What moves is how easily a building’s cold water drifts up into that band. UK cold water systems are designed on the expectation that water is stored and distributed below about 20°C [3] — and that figure was comfortable when incoming mains and roof voids spent most of the year well beneath it. A run of thirty-degree days changes the arithmetic.

The failure modes are not new. They are the ordinary ones, happening more often and lasting longer. A storage tank baking in a flat-roof void. A cold main rising in a warm riser alongside hot flow and return. A long cold run sharing a south-facing ceiling void with warm plant. Incoming supply that itself arrives a few degrees warmer after weeks of heat. Each was a marginal concern in a temperate year and becomes a recurring one in a hot one.

Mapping where the heat gets in

The single most useful exercise here costs nothing but an hour and a floor plan. Sketch your cold water system as a line and mark every point where summer heat can enter it.

Start at the incoming main at the site boundary: note its depth and whether it runs warm before it reaches you. Follow it to the storage tank, then draw the space around the tank — roof void, plant room, cupboard — and write the likely summer air temperature of that space, not the water. From the tank, trace each distribution run and flag any length that shares a void, riser or duct with hot pipework or heat-generating plant. At every marked point, write one number: the warmest plausible water temperature there during a hot week.

Anywhere that number crosses about 20°C is a node that used to be safe and may not be. The map beats a single tap reading because it shows the trend across the whole system, and it tells you exactly where to fit a temperature logger before next summer rather than where to investigate after an exceedance. Most buildings have two or three of these nodes; you rarely have dozens. Find them, and you have found where climate is actually acting on you.

The risks that don’t show up on a thermometer

Temperature drift is the headline, but several climate effects work through other routes.

Emptier buildings in the heat. Heat and absence increasingly arrive together — a sweltering Friday with half the staff at home, a summer university shutdown, a seasonal site closure. Stagnation and warmth are the two conditions Legionella needs, and a hot, quiet building supplies both at once. Flushing regimes written for a normal occupancy pattern can quietly fail in a heatwave, which is the moment they matter most. Neglected water systems and stagnation is the mechanism behind a surprising share of incidents.

Flooding and supply disruption. Floods, pressure loss from bursts, and emergency repairs can draw contamination into systems, interrupt treatment dosing, and leave you recommissioning under time pressure. The cheap fix is to build the water-safety steps into your emergency response plan now, so they exist before the event rather than being improvised during it.

Water efficiency and reuse. Drought summers push water-saving measures: lower flows, usage restrictions, rainwater harvesting, greywater reuse. Each can slow turnover, lengthen residence times, or introduce a non-potable system that needs its own assessment. Saving water and controlling Legionella both depend on how the design handles stagnation — they are not automatically in conflict, but they are not automatically aligned either.

Cooling under load. Cooling towers and evaporative condensers run hardest in the heat, generating the most aerosol exactly when they run longest. HSE treats them as among the systems most likely to create risk [4]. As more buildings add evaporative cooling to cope with hotter summers, there are simply more of these systems to notify, maintain and control.

The decarbonisation tension worth flagging

Here is the awkward one. Some of the measures that cut carbon can collide with thermal Legionella control. Heat pumps typically deliver hot water at lower flow temperatures than gas boilers, and thermal control depends on storing hot water hot — around 60°C — and getting it to outlets above roughly 50°C [3]. A low-temperature retrofit that overlooks this can lower a control you quietly rely on.

The answer is not to refuse low-carbon plant. It is to design the Legionella control in alongside it — pasteurisation cycles, point-of-use measures, or supplementary treatment, decided through the risk assessment rather than bolted on afterwards. The practical discipline is to treat any net-zero retrofit as a system change that triggers a review [5]. That single habit catches most of the conflict before it is built into the pipework.

What to actually do

Don’t stand up a separate “climate programme”. Fold this into the discipline you already have. The trigger that matters is the one already in your duty: review the risk assessment when the system, its use, or the conditions it operates in change [5]. Slow climate drift is a change. It just arrives without an obvious start date.

For the year ahead, four moves earn their place:

  • Take the weakest cold-water nodes from your heat-gain map and log their temperature across a full summer, not as a single spot check.
  • Add a hot-spell clause to the assessment: what happens to flushing and monitoring when the building empties during a heatwave.
  • Check that any decarbonisation or water-efficiency project on the horizon has Legionella control written into its design brief, not its snagging list.
  • Confirm the incident plan covers loss of supply and flooding, including how you would recommission safely.

When you take this to whoever holds the budget, frame it honestly: the controls themselves haven’t changed, but the safety margin you used to get for free is thinning, and the cheapest place to act is the risk review.

A note on the figures

The temperatures here are the familiar guidance thresholds, and they are starting points a competent risk assessment confirms for your building — not numbers to apply blind. The climate links are reasoned forward from how this organism behaves, not a forecast about any single summer. Treat all of it as a prompt to test your own system’s assumptions, and let a competent person, your risk assessment and current HSE guidance settle what each figure should be on your site [1] [2].

FAQ

Does a warmer climate mean I should test for Legionella more often?

Not automatically. Sampling frequency follows your system and risk assessment rather than the weather, and testing verifies control rather than creating it [2]. The more useful response to hotter summers is better temperature monitoring on your cold-water weak points, so you see drift coming before it becomes an exceedance.

Is the cold water side really the issue, when control has always focused on hot water?

The hot side still matters and the duty to keep it hot is unchanged. The point is that the cold side has historically protected itself simply by being cold; HSE guidance expects cold water below about 20°C [3], and that is the assumption a warming climate pressures most. It now needs attention it has rarely had to earn.

Will switching to a heat pump increase our Legionella risk?

It can, if the design ignores the issue, because low-carbon hot water systems often run cooler than the temperatures thermal control depends on [3]. Done properly, the risk is managed by building Legionella control into the system design through the risk assessment, rather than treating it as a separate afterthought once the plant is in.

Sources

[1] 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 [2] HSE, “Legionnaires’ disease: Technical guidance (HSG274)”. https://www.hse.gov.uk/pubns/books/hsg274.htm [3] HSE, “Hot and cold water systems”. https://www.hse.gov.uk/legionnaires/hot-and-cold.htm [4] HSE, “Systems most likely to create legionella risk”. https://www.hse.gov.uk/legionnaires/risk-systems.htm [5] HSE, “Legionnaires’ disease - what you must do”. https://www.hse.gov.uk/legionnaires/what-you-must-do/index.htm