Not much of a choice, but this conflict is at the heart of every plumbing system. Water heated and stored at sufficiently high temperatures to control and kill bacteria such as legionella, but it can also cause severe scalding injuries in a matter of seconds. If water is heated and stored at safe non-scalding temperatures, this provides the ideal medium and temperatures for bacteria growth.
Legionnaire’s Disease comes from a naturally occurring organism that can be found in low levels in the water supply. It is a bacterial disease that is contracted from small droplets of water that are contaminated with legionella bacteria and which have become suspended in the air and may cause pneumonia. The time from infection to start of the illness (the incubation period), is between 2-10 days and the disease can be particularly deadly to the very young or old, especially if infirm.
Aerosol droplets that allow transmission of legionella are found in: whirlpool spas, showers, cooling towers, taps with sprays etc. Legionella can grow in any water system that is not properly maintained.
Elimination of legionella from a system
Methods to eliminate legionella include chlorination. This method of treatment is used when a system is cleaned prior to commissioning and has real disadvantages if a system is being used. Another option is to use heat treatment (i.e. running water above 70°C for 30 minutes in the whole sanitary system), though this can also have serious drawbacks if no temperature control is used on the outlets.
If temperature control down to a completely safe level is exercised at the water heater (i.e. turning the cylinder down to a non-scalding temperature so that all the stored water is below 50ºc), the following will occur:
- The system will not comply with building regulations,
- Water usage will increase as users run taps for longer periods, in the hope of getting hot water,
- Users will not get a hot bath unless the water heater is close to the bath because of the temperature loss from the pipework between the water heater and the point of use,
- Washing up becomes a problem as lukewarm water will not shift grease.
Building regulations state that the circulation of hot water must be at temperatures sufficiently high to stop the legionella that naturally occurs in the water supply from multiplying to a level that will cause health problems to susceptible people. In the UK building regulations stipulate that hot water should be stored at no less than 60ºC and circulated at no less than 55ºC to prevent the growth of legionella.
The Burns Issue
Every year 23 people are killed by being immersed in hot water by mistake by a carer or nurse or by falling in to a bath and not being able to get out quickly enough. These are sobering statistics when you consider that the burns suffered by scald victims are every bit as painful and destructive as those suffered by victims of fires or explosions.
Almost 90% of the 570 people who suffer serious scalds each year, which require hospitalisation, are children. Other groups considered to be at high risk are the elderly and disabled. While children are normally scalded because they do not identify or understand the risk, the elderly and disabled are more likely to be injured or killed as a result of not being physically able to remove themselves from the scalding situation when they find themselves in danger.
A typical scenario is a carer or nurse filling a bath of hot water and leaving the person to get in by themselves; quite often the bather will sit on the side of the bath and swing their legs over and into the water. At 60 degrees an adult will suffer third degree burns after less than six seconds of immersion, with an elderly person this time is likely to be even less due to the more sensitive nature and reduced thickness of their skin. Regardless of skin sensitivity, however, it is clear that anyone who is even marginally impeded in their movements is going to suffer a serious scald injury at such temperatures. 90% of all people who are killed each year by scalding are the elderly aged 65 and over.
Legionella or Burns? The Solution
Water below 50°C can be considered ‘safe’ as even for a child to receive a second degree burn it would take 45 seconds, however water stored below 50°C creates a breeding ground for legionella bacteria to breed.
The best solution to both problems is to fit a thermostatic mixing valve at the point of use (i.e. local to the taps). This will allow the hot water to be stored at a sufficiently high temperature in the water heater to prevent bacteria growth but the TMV will mix cold water and hot together and discharge it out of the tap at a controlled and stable temperature, typically 38-44°C in a hospital or nursing home, to prevent scalding the end user.
I addition, Reliance UK also offer an ‘Anti-Legionella Valve’ designed to convert a standard single connection expansion vessel into a ‘flow through’ type so water is continually renewed to prevent water stagnation & bacteria growth.