The Conflict: Legionella or Burns?
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 can cause severe scalding injuries in a matter of seconds but water that is heated and stored at safe non-scalding temperatures 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 may cause pneumonia, and is contracted from small droplets of water that are contaminated with legionella bacteria and which have become suspended in the air. The time from infection to start of the illness (the incubation period) is between two and ten 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.
The effect of temperature on legionella
Turn the Temperature Down?
If temperature control is turned down to a completely safe level at the water heater, so that all the stored water is below 50ºC, the following will occur:
The risk of proliferation of legionella bacteria in the water heater will be greatly increased.
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.
The UK building regulations stipulate that hot water should be stored at no less than 60°C and circulated at no less than 55°C to help prevent the growth of legionella.
An option to help control Legionella is to use heat treatment, i.e. running water above 70°C for 30 minutes in the whole sanitary system but there must be a temperature control device on the outlet to ensure the user in subjected to only a ‘safe’ water temperature.
A solution to both problems is to fit a thermostatic mixing valve (TMV)
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
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.
This temperature is typically 38-44°C in a hospital
or up to 48°C in domestic applications,
to prevent scalding the end user.