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.