What are the first signs of a legionella problem?

The first signs of a legionella problem in a water system are not signs of the bacteria at all, because legionella is invisible. They are signs that the conditions it needs have appeared: hot water that is slow to reach 50°C or never does, cold water running above 20°C, outlets that sit unused, scale and debris at taps and shower heads, and routine checks that have quietly stopped happening. Each one is a loss of control. Any of them, left alone, is how a well-run system becomes a colonised one.

This page is about the system, not about health symptoms. If you are worried someone is already ill, that is a medical matter and a different question. What follows is how a duty holder reads the warning signs a building gives off before legionella ever shows up in a sample.

You cannot see legionella, but you can see loss of control

Legionella has no colour, taste, or smell. Water from a heavily colonised system can look and taste completely normal. So the honest answer to "what does a legionella problem look like" is that it looks like nothing, which is exactly why the control regime under ACOP L8 and HSG274 is built on measurement and routine rather than on inspection by eye.

What you can observe are the three conditions legionella depends on: water standing still, water sitting in the 20°C to 45°C growth range, and the presence of scale, sediment, and biofilm to feed and shelter it. The warning signs below are those three conditions becoming visible. Our page on what causes legionella explains the mechanism behind each one.

The six warning signs, in the order you will usually meet them

1. Temperature drift at sentinel outlets. This is normally the first measurable sign. Run the hot tap furthest from the calorifier and time how long it takes to reach 50°C: it should get there within a minute. A hot outlet that warms slowly, or peaks in the 40s, means the water is spending time in the growth range. The same logic applies to cold outlets, which should drop below 20°C within two minutes of running. The temperature checker lets you log these readings against the correct limits.
2. Outlets that have gone quiet. A guest bathroom, a cleaner's sink, a shower in a disused room: any outlet used less than about weekly is stagnating, and stagnation is the condition legionella starts with. If you cannot say when an outlet was last run, that is a sign in itself.
3. Scale and debris at outlets. Limescale on tap aerators and shower heads is more than cosmetic. Scale and the sediment behind it are nutrients and shelter for biofilm, and a scaled outlet is one where the biofilm has a foothold. Heavy scale on shower heads matters most, because the shower turns whatever is in the water into breathable aerosol.
4. Discoloured or debris-carrying water. Rust-coloured or cloudy water usually means corrosion or disturbed sediment, not legionella itself, but corrosion and sediment are the raw material biofilm is built from. Treat it as a system-health warning and investigate the source.
5. Slime or biofilm where you can reach it. A slippery film inside a cold water tank lid, around a tap insert, or on a shower hose is biofilm: the layer that shelters legionella from heat and disinfectant. Visible biofilm anywhere means invisible biofilm elsewhere in the same system. See biofilm in water systems.
6. A monitoring log with gaps in it. The least physical sign is often the most telling. If the monitoring schedule has slipped, temperatures have not been taken for months, or flushing is not being recorded, then control has already been lost whether or not the water shows it yet.

What the signs cannot tell you

Two cautions run in opposite directions, and both matter. First, none of these signs confirms legionella. A slow hot tap might be a failing thermostatic valve or a long pipe run, and scale can build in a system that is otherwise well controlled. The signs tell you where to look and what to fix; they do not diagnose the water.

Second, and just as important, the absence of every sign does not prove the system is safe. A system can carry legionella with temperatures in range and outlets gleaming, particularly downstream of a dead leg you have not found. Good conditions are a control measure, not a guarantee, and no checklist, including this one, can certify a system as safe. That is what the risk assessment and the routine monitoring are for.

When suspicion becomes confirmation: sampling

The only way to confirm legionella is to send water to a laboratory. Sampling cultures the bacteria and reports a count in CFU per litre, which is read against the action levels in HSG274: broadly, below 100 needs no action beyond maintaining control, 100 to 1,000 triggers review and resampling, and above 1,000 requires immediate action. Our guide to sampling and action levels sets out the detail.

Remember what a sample is: a snapshot of the water at that outlet on that day. A clear result does not certify the system, and a positive one points at conditions that need fixing, not just water that needs treating.

What to do when you spot a sign

Each warning sign is a corrective action waiting to be taken and recorded. Restore temperatures to range and find out why they drifted. Put little-used outlets on a documented weekly flush, or remove the branch entirely. Descale and disinfect outlets. Get the monitoring schedule running again. Then write down what you found, what you did, and when, because the record is the evidence the control happened.

If the signs persist after you have corrected them, or the risk assessment says the people exposed are vulnerable, sample rather than guessing. And if a sample comes back positive, stop treating it as a maintenance issue and follow the escalation steps in what to do if legionella is detected.