Heatwaves and cooling towers: why hot spells test your controls

When the weather turns properly hot, everyone notices the air conditioning. Fewer people think about what's happening on the roof. But a heatwave is one of the toughest tests a cooling tower faces all year, and it's when small weaknesses in a treatment programme tend to show up.

Here's why hot spells put pressure on Legionella control, what tends to go wrong, and how to get ready before the next one, whether that's next week or next summer.

Why heat matters

Legionella bacteria grow best in water between roughly 20 and 45°C, which is the range ACOP L8 and HSG274 highlight. Cooling tower water sits in or near that range for much of its working life. In a heatwave, the water returning from the plant is warmer, the pond is warmer and the whole system spends more time in the zone bacteria like.

At the same time, the tower is working harder than usual. That changes almost everything about how the treatment programme behaves.

What a heatwave does to your system

• More evaporation. The tower evaporates more water, so it needs more make-up water and the water concentrates faster. The bleed valve works harder to keep conductivity in check.
• Chemicals get used faster. More make-up and more bleed mean more inhibitor and biocide going in. Drums that normally last a month can empty much sooner.
• Higher biocide demand. Warm water and more organic material mean bacteria grow faster and use up biocide more quickly.
• More dirt. Dry weather means more dust and pollen in the air, and the tower's moving more air through it. More debris ends up in the pond.
• Fans at full speed. Towers running flat out move more air, so the drift eliminators are working harder too. Damaged or poorly fitted eliminators let more aerosol escape.
• Standby units called into service. On sites with spare capacity, towers that have been sitting idle may be switched on to cope with the load. If they've been standing full of untreated water, that's a real risk.

What tends to go wrong

Put those together and the classic heatwave problems are:

• A chemical drum running dry earlier than expected, often over a weekend
• Bleed not keeping up, so the water over-concentrates and scale starts to form
• Dip slide counts stepping up as biocide levels struggle to keep pace
• A standby tower switched on without the checks it needed first

None of these is unusual, and all of them can be prevented or caught early. We cover the common faults in our post on dosing failures, and how to read rising counts in dip slides and TVCs explained.

A pre-heatwave checklist

Ideally, do this in late spring, before the first hot week. If the forecast is already looking warm, do it now:

1. Check chemical stock. Make sure there's enough on site to cover heavier use, stored safely in line with the safety data sheets.
2. Check the dosing kit. Pumps primed and delivering, lines clear, controllers calibrated, bleed valve working.
3. Inspect the tower. Pond clean, fill in good condition, drift eliminators intact and properly seated.
4. Check when the last clean was. If a clean is due soon, consider doing it before the hot weather rather than in the middle of it.
5. Plan for standby units. If a standby tower might be needed, make sure it's been kept circulating with treatment, or cleaned and disinfected, so it's ready to run. See bringing a tower back after a shutdown.
6. Agree extra checks. Your risk assessment may call for more frequent monitoring during periods of high load. Agree with your treatment contractor what that looks like.

During the heatwave

• Check drum levels more often than usual, and before weekends.
• Keep weekly dip slides going without fail, and look at the trend, not just the number.
• Watch conductivity and make sure the bleed is coping.
• If something looks wrong, act on it quickly and record what you did.

If a result comes back above an action level, follow your written scheme. Our guide to the first 48 hours after a positive result walks through it.

After the hot spell

Once things cool down, take a look back. Did anything run out or fail? Did counts move? Is there more scale or sludge than before? If so, it might be time to bring a clean forward or review the treatment programme and risk assessment before next summer. With hot spells becoming a regular part of British summers, it's worth building heatwave planning into your written scheme rather than treating it as a one-off.

Our water treatment and dosing services include pre-summer checks as standard. If you'd like us to look over your system before the next heatwave, give us a ring.