As stated on the Health & Safety Executive’s website, “For most domestic hot and cold water systems, temperature is the most reliable way of ensuring the risk of exposure to Legionella bacteria is minimised”. Simple as this sounds, there is more to monitoring the water temperature than meets the eye, so we’ve put together this handy how-to guide for duty holders.
Duty holders with responsibility for controlling the risk of Legionella should check water temperatures as part of the risk assessment process. As a minimum this should be done every two years, but if there are any alterations to the plumbing system in the meantime we recommend redoing the assessment.
Why is temperature important?
Legionella bacteria thrives at temperatures between 20°C and 50°C so one of the key control measures for minimising the risk is to ensure that your cold water is cold (i.e. below 20°C) and the hot water is hot (above 50°C). In cold water the bacteria is dormant and hot temperatures kill it, so either way it should not pose a risk if the temperatures are managed properly.
The first thing you’ll need is a reliable thermometer which you can easily hold under running water as well as insert into a water storage tank. If it doesn’t have a digital display, make sure that you can read the markings clearly. Have a pen and paper handy so you can write down your findings as it’s important to keep a record for future reference. You may also need a torch if your water tank is in the loft or a dark cupboard, as well as a watch or mobile phone to time the process.
Start with the sentinel tap
The tap situated at the furthest point from the boiler is the sentinel tap. Turn it on and hold the thermometer in the running water for a minute, making sure that the tip of it is in the flow of the water. The temperature reading should reach at least 50°C. Whatever the result, record your findings in your risk assessment. If the temperature doesn’t reach 50°C you need to take remedial action e.g. adjust the thermostat.
Mixer or blender valves
Mixer or blender valves ensure water temperatures at the tap do not exceed 45°C so, if you have such valves fitted, the temperature of the hot water should be checked at the calorifier (water heater) outlet instead, and should be at least 60°C. Again, record your findings.
Don’t forget the cold water
It’s also important to check the temperature of the cold water outlets. Starting with the sentinel tap again, let the tap run for two minutes with the thermometer in the water flow. The reading should be less than 20°C; if it’s warmer than this you should consult a plumber to take action to reduce the temperature.
Water storage tanks
If you have a cold water storage tank it’s also worth monitoring the temperature of the water as part of your risk assessment. Remove the tank lid (water storage tanks should have a well-fitting lid to prevent debris from entering the system) and insert the thermometer for a minute. Record the temperature, which should be lower than 20°C. If it’s above this level, the tank may require additional cladding or insulation to prevent it heating up unduly in hot weather. Make sure that you replace the lid properly.
All temperature records should include the location of the outlet or tank, the actual temperature, the name of the person carrying out the check as well as the date and time the temperature was taken. If any issues are identified or remedial action required, this should also be noted in the records.