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 Keep Hot Water Hot To Beat Legionella

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​​​Keep Hot Water Hot To Beat Legionella ​
3D animation of Legionella

​​The legionella bacterium is deadly and tepid water is its breeding ground. Keeping water hot doesn’t just save energy – it can save lives too.

​​It’s one of those frustrating things that everyone has encountered – you turn the hot tap on and the water runs cold for a minute or more. What most people aren’t aware of is that this can be more than just frustrating – domestic hot water (DHW) is designed to be maintained at temperatures of more than 60°C for the specific purpose of preventing bacterial growth.

​​BS 8558:2011, the British Standard for the design, installation, testing and maintenance of services supplying water for domestic use within buildings, sets a very clear target for DHW temperatures. According to BS 8558:2011 the hot water tank must be maintained at a temperature no less than 60°C and the return temperature must be no less than 50°C.

If these temperatures can’t be ensured then the risks of bacteria, including legionella bacteria, increase dramatically.​

When hot water runs cold

​​Held in small diameter pipes near to the taps, hot water can cool to room temperature in just a few minutes. Since water runs only intermittently it’s inevitable that the hot water in the pipes will cool over time and, upon turning a hot water tap, the first water out will not be genuinely hot.

Legionella can’t multiply at temperatures below 20°C but is destroyed by prolonged exposure to temperatures above 50°C with the bacteria unable to withstand even a few moments at temperatures above 70°C. Preventing legionella in a DHW system is largely about ensuring that the temperatures remain above 60°C for as long as possible.​

The role of insulation

​​Insulation can’t stop hot water in pipes cooling down to room temperature – all that insulation can do is slow the descent - but it does have an important role to play in preventing diseases like Legionella.

​​A very common and convenient arrangement for DHW services is to route the pipework alongside the cold water pipework. This can be doubly problematic – the hot water pipework heats the cold water and the cold water pipework cools the hot water meaning that both services quickly end up at a temperature within the ‘danger zone’.

Pipe insulation slows the rate of heat loss/gain and insulating hot and cold water pipes that are close together mitigates against the effect of accelerated cross heating/cooling.​

Meeting BS 8558:2011 and saving energy

​​BS 8558:2011 is an essential standard to satisfy when looking to ensure a reliably safe water supply. One clause within BS 8558:2011 is that DHW should reach the tap at a temperature of no less than 55°C within a minute of the tap being turned and a second is that any circulation water is returned to the tank at a temperature no less than 50°C.

​​Achieving these goals is largely a matter of good plumbing practice – engineering a system that doesn’t rely on long pipe runs and has a ready supply of appropriately hot water ready to go quickly. On a long pipe run the heat losses in transit can, however, be very significant and good use of insulation can deliver DHW to the taps a couple of degrees hotter than would otherwise be the case.

Of course insulating the pipes not only reduces the risks of microbial growth – it also saves energy and has a remarkably short payback period! With the financial investment for pipe insulation often being repaid within just a few months there’s really no reason not to insulate and reduce the risks of Legionella growth within the DHW system.​

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The legionella bacterium is deadly and tepid water is its breeding ground. Keeping water hot doesn’t just save energy – it can save lives too.


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