When people think about Legionella risk in a building’s water system, the focus is usually on disinfectant levels or recent testing results. While those factors are important, they tend to overlook two of the biggest drivers of risks: stagnation and temperature.
Stagnation and temperature aren’t new concepts, but they’re often misunderstood in how they show up in real systems. A system can look completely normal on paper: residuals are present, temperatures are within range at the source, even recent testing coming back fine, but unfortunately can still have conditions that support bacterial growth. Most of the time, this comes down to how water is moving through the system and what happens to temperature once it leaves the water heaters.
Stagnation is commonly thought of as water sitting completely still, it’s more often a low-flow issue. Water might still be moving, just not enough to keep conditions stable. When this happens, disinfectants start to decay, temperatures begin to drift, and the system then becomes much more dependent on how often that water is being turned over.
That area of stagnation is where things start to break down. If water is only moving intermittently, the system will end up in this cycle where fresh water enters, brings in disinfectants, and then sits long enough for the residual to disappear again. It never truly stabilizes. Instead, it keeps shifting between controlled and uncontrolled conditions. That inconsistency is what creates opportunity for growth.
At the same time, uncontrolled temperatures can add considerably to legionella growth. We rely on temperature as one of the primary ways to control Legionella, however maintaining a target temperature (>70°F and <115°F) across an entire system is not an easy goal. The temperature reading that appears at the source is rarely the same at the point of use. Hot water can lose heat as it moves through the system; especially if recirculation is not balanced properly or if the system is oversized for the current demand.
Temperature gauge and hot water piping.
Mixing valves can also play a role. This is especially true when they’re reducing temperature earlier than intended or feeding large portions of the system.
Examples of domestic piping.
On the other hand, cold water doesn’t always stay cold. It picks up heat from the environment, especially in mechanical spaces or during warmer months, and can end up sitting right in the range where bacteria thrive. When combined with low flow, the problem compounds quickly.
When discussing legionella, stagnation and temperature should not be viewed as separate issues but rather factors that feed into one another. Water that isn’t moving is going to equalize with its surroundings. That means hot water cools down, cold water warms up, and everything starts to settle into a temperature range where Legionella can grow. At the same time, whatever disinfectant was present is being consumed or breaking down, with nothing coming in to replace it consistently. This is really the core of it.
A system with good flow can tolerate minor temperature fluctuations. A system with strong temperature control can sometimes compensate for areas of lower flow. But when both start to slip at the same time, is when risk increases. Even while nothing looks obviously wrong during routine checks.
Testing may not tell the full story.
This is an example of how testing may not always tell the full story. Sampling gives a snapshot of conditions at a specific place and time. It does not capture what happens between users, or how long water has sat in the system before that sample was taken. Results can be acceptable and there can still be sections of the system that are spending a significant amount of time under conditions that support growth.
That is where a lot of systems fall into a false sense of security. Everything looks fine based on the data that’s being collected; however, the underlying behavior of the system has not fully been addressed. Managing this isn’t about chasing numbers as much as understanding how the system operates and how the factors all affect one another. Where is the water moving consistently, and where is it not? Where is temperature being maintained, and where is it drifting? Those answers frequently explain more than any single test result.
Approaching the systems in this mindset, the problem becomes clearer. It’s less about reacting to individual readings and more about being proactive and making sure the system is not creating the conditions that allow problems to develop in the first place.
Stagnation and temperature have always been part of water safety, but they’re often treated as background factors instead of primary drivers. They’re doing lots of heavy lifting by either helping maintain control or quietly harboring the perfect environment for legionella growth.
Brenden Nihart
ASSE Certified Legionella Water Safety and Water Quality Specialist
Brenden is a water safety and water quality specialist at Watertech of America, Inc. His work centers on preventing waterborne illnesses, particularly Legionella, and supporting clients in managing boiler and cooling systems to ensure reliable performance and regulatory compliance. Brenden has a Masters of Science in Freshwater Science and Technology from UW-Milwaukee, and an ASSE 12080 certification.