Why Legionella Risk Rises in Winter for Commercial Buildings

November 28th, 2025

Last updated: November 17th, 2025

Winter changes how water behaves inside commercial buildings. Temperatures drop, usage patterns shift, and those changes increase Legionella risk in ways that aren’t always obvious.

On the surface, a site might look stable. Heating runs longer. Staff still use the building. Nothing appears out of place.

Behind the scenes, however, the water system tells a different story.

Hot and cold supplies react sharply to the colder weather. Flow slows in areas that see less use. Temperatures drift in parts of the network that were steady throughout summer. Equipment that felt reliable a few months ago now works harder just to keep up.

Many facilities managers focus on frozen pipework or plant breakdowns during winter. Fair enough, but the water distribution network inside a building often faces more pressure than the visible assets. Legionella thrives in quiet, lukewarm areas of pipework, and winter produces more of those conditions.

That’s why the colder months demand closer attention. Commercial buildings need a clearer view of how their water systems behave as temperatures fall, because the risks grow long before the problems surface.

1. Colder mains water disrupts hot water temperatures

The moment the outside temperature drops, the incoming mains supply cools. As a result, hot water systems need more effort to compensate.

If the system is well designed and well maintained, it usually stays within safe limits. Older plant often fails to keep up, and that’s where the trouble starts.

A tap might still produce a safe reading, yet deeper in the system the water sits at a level that encourages bacterial activity.

One small stretch of pipework losing heat becomes the weak point.
Once that happens, Legionella has a foothold.

This problem grows through winter because the system is fighting colder feed temperatures every hour of the day. Even slight drops in output create conditions that raise the Legionella risk.

Read our blog, Legionella Temperatures – Understand What Temperature Kills Legionella for more. 

2. Lower building occupancy causes stagnation

Winter brings long closures for schools, quieter periods in offices, and reduced shift activity for many industrial sites. With fewer people using water, outlets sit idle.

Showers stay off.
Sinks barely run.
Wash-down points remain untouched for days.

When outlets don’t run, water stops moving. Once it becomes static, temperature control becomes unreliable, and that stillness is exactly what Legionella needs.

A section that held safe temperatures in September can drift into the danger zone by December.

Even buildings that remain open can face stagnation. One wing operating on reduced occupancy can influence how water behaves across the wider network, creating unpredictable flow patterns and weaker circulation.

3. Tanks and calorifiers lose heat faster

Cold weather punishes hot water storage. Tanks lose heat more quickly, especially overnight. If insulation has slipped or degraded, heat escapes even faster.

You might start the morning with water already below target temperature.
That puts more load on the heating plant and slows recovery.

Winter also encourages stratification.
A tank might look compliant at the top, yet the lower half sits far cooler.

Without intervention, this uneven profile compromises the entire system. A tank with steady temperatures in spring and autumn can behave very differently in winter.

These issues occur quietly. There’s no warning light. By the time someone notices abnormal readings, the system may have drifted for weeks.

4. Heating plant faces higher strain and becomes less stable

Winter is the toughest season for heating plant. Boilers run for longer periods. Pumps work harder to maintain circulation. Weak spots appear quicker than at any other time of year.

If a pump slows even slightly, circulation suffers. And as circulation falls, temperature stability follows.

That shift raises Legionella risk throughout the network.
Boilers struggling with higher demand often create the same effect.

A small drop in return temperature exposes long stretches of pipework to ideal growth conditions.

Commercial buildings feel this more acutely because their pipework covers large distances. Heat loss across these long runs increases as the cold sets in, and older insulation rarely holds up well.

5. Commercial buildings have hidden weaknesses winter exposes

Many commercial buildings have been altered again and again over the years. Old pipework is left in place. Redundant branches remain connected and insulation varies in quality. Winter highlights every weakness.

Water inside unused or poorly insulated sections cools quickly and stays idle for long periods.

Legionella doesn’t need a full system failure. One neglected section is enough and, once growth begins, it spreads through connected areas.
This is why winter conditions demand a closer look at the whole system rather than relying on a single temperature check.

Inspections during winter should reach further into the network and focus on areas that don’t normally raise concerns during warmer months.

Reducing Legionella Risk During Winter

Each building behaves differently, but winter preparation follows a simple principle: keep water moving, keep temperatures steady, and deal with weak spots early.

Here’s what helps:

• Increase flushing schedules in low-use areas
• Check temperatures at multiple locations
• Review tank and pipework insulation
• Confirm pumps are operating at the required performance
• Book water sampling during shutdowns or periods of low demand
• Arrange a winter hygiene review if the site has struggled with temperature control before
  

These actions don’t take long, but they make a significant difference.

A Proactive Approach Beats a Reactive One

Commercial buildings face more stress in winter than at any other point in the year. Temperature loss, reduced occupancy, and plant strain all push systems into riskier territory.

Legionella multiplies fastest when water systems drift out of their normal operating patterns, and winter produces exactly those conditions.

A proactive approach prevents expensive remedial work, keeps the building compliant and protects staff, visitors and contractors.

If your site is preparing for a reduced winter schedule or has areas that rarely see use, early action avoids far bigger problems later.

Robinsons Facilities Services supports commercial clients across Yorkshire with comprehensive Legionella Risk Assessments. We also perform Legionella testing, water sampling, and tank cleaning. Addressing the risk before temperatures drop helps keep the building safe through the coldest months.