Most people think of Legionella as a hot water problem. In summer, it's often the cold water that gives us the most trouble. When lofts, ceiling voids and plant rooms heat up, the "cold" water passing through them warms up too, sometimes well into the range Legionella likes.
It's late June, the first proper warm spell of the year has probably arrived and monthly temperature checks across the country will be showing cold taps running warmer than usual. Here's why it happens and what you can do about it.
Why 20°C matters
Legionella bacteria grow best in water between roughly 20°C and 45°C. Below 20°C they don't really multiply, which is why the HSE's guidance for hot and cold water systems, HSG274 Part 2, says cold water should be kept below 20°C.
The usual check is at your sentinel outlets (the nearest and furthest taps on each part of the system). Run the cold tap and the water should drop below 20°C within two minutes. If it doesn't, something's warming it up.
Where the heat comes from
Cold water rarely gets warm for just one reason. These are the usual suspects:
- Hot lofts and roof spaces. On a sunny day a loft can get very warm, and a cold water tank sitting in it, or pipes running across it, will warm up with it.
- Warm ceiling voids and risers. In offices and blocks of flats, cold pipes often share space with heating pipes, hot water pipes and electrical equipment.
- Poor or missing insulation. Lagging that's been knocked off, crushed or never fitted in the first place.
- Hot and cold pipes run side by side. Even with insulation, a cold pipe clipped right next to a hot one picks up heat over a long run.
- Plant rooms. Tanks, booster sets and pipework near boilers and calorifiers.
- Low turnover. An oversized tank or a quiet building means water sits still and slowly warms to the temperature around it.
- Warm incoming mains. In a long hot spell, the water coming in from the street can itself be warmer than usual.
What if the mains is already warm?
That last one is outside your control, and it's worth knowing about. If the incoming mains is warm, you can't make your cold taps colder than the water arriving at the building. In that case the aim is to keep the cold water as close to the incoming temperature as you can, record the mains temperature alongside your outlet readings so the picture's clear, and make sure the other controls are spot on. Your risk assessment should recognise it rather than pretend it isn't happening.
Quick wins for this summer
Some fixes are simple and cheap, and can be done straight away:
- Check the insulation. Make sure tanks and cold pipes in lofts and voids are properly lagged, and replace anything damaged or missing.
- Check the tank lid. A properly fitted, insulated lid helps keep heat out as well as dirt. Our article on cold water tank inspections covers what to look for.
- Keep the water moving. Flush little-used outlets weekly, as always, and consider flushing more often during a heatwave where readings are borderline. Our flushing guide explains how.
- Ventilate. Improving airflow in a loft or plant room can bring the temperature down a little.
- Record and compare. Fit a max/min thermometer in the tank if there isn't one, so you can see how warm it gets on the hottest days.
Longer-term fixes
If warm cold water turns up every summer, it's usually a design problem rather than a maintenance one. Typical remedial works include:
- rerouting cold pipes away from heating and hot water pipes, or separating them properly
- replacing an oversized tank with a smaller one that turns over more quickly
- relocating a tank out of a hot loft into a cooler space
- getting rid of stored cold water altogether by feeding outlets from the mains or a booster set, where pressure and flow allow
- removing dead legs and long runs to outlets nobody uses
A typical scenario
Imagine a small office building on three floors, with a cold water tank on the roof and pipes running up through a riser next to the heating flow and return. In winter the cold sentinels are fine. In June the top-floor kitchen tap won't drop below about 23°C even after two minutes.
Our first move would be to check the tank temperature against the incoming mains. If the tank's much warmer than the mains, the tank and roof space are the main problem: insulation, lid, possibly shading or relocation. If the tank's fine but the tap's warm, the heat is coming from the riser, and the answer is insulation or rerouting. Either way, extra flushing would help in the short term while a permanent fix is planned.
Don't forget the hot side
Heatwaves can tempt people to turn the hot water down. Please don't. Stored hot water should still be at 60°C or above and reach outlets at 50°C or more, whatever the weather outside.
When to get help
If your monthly checks show cold water above 20°C, record it, follow what your written scheme of control says to do, and find out why. A single warm reading on a hot day might just mean more flushing. Repeated warm readings mean something needs fixing, and possibly that your risk assessment needs a review.
We can investigate the cause, carry out the fixes and keep an eye on things through our Legionella maintenance visits. If you're worried control has slipped, our water sampling service can give you a clearer picture.