New Legionella Cylinder Thermostat


Anti-Legionella Thermostat

Anti-Legionella Thermostat


I have recently been involved in the installation of the new hot water tank thermostats which are designed to reduce the risk of legionella bacteria building up in the system. This has led to a lot of confusion over the workings of the hot water system. Therefore I have put this page together to help people understand what’s happening and why.

Microbiological Growth

It should be understood that Legionella and other bacteria are always present in the water supply. If these bacteria are given favourable breeding conditions, then their levels can become unacceptable. The preferred breeding temperature range of Legionella is from 20°C to 50°C and it generally takes several days at this temperature to promote colonisation. Once temperatures exceed 50°C the bacteria start to die off. This is almost instantaneous at 70°C or above whilst over 90% of legionella are killed in less than 10 minutes at 60°C and in around one hour at 55°C. So therefore the new anti-legionella thermostat acts independently to pasteurise the cylinder contents by heating to 60°C or above and keeping it there fore an hour on a regular basis (at least once a week).

What the Thermostat ACTUALY does.

Every time the system is turned on, (regardless whether or not it has been on before) it WILL attempt to elevate the stored water to a temperature above 60°C and keep it there for a full hour (some have run on beyond 70 minutes). After this time the hot water system will function in the normal way with the normal levels of control. HOWEVER, one week later the cylinder stat will repeat this heating process.

So where is the confusion I hear you cry…… Well if the cylinder stat gets interrupted during this time it will just keep restarting the whole process over and over again.

The disruption could be:

  1. The power going off, So when it comes back on again the water will heat up regardless of the timer asking for it.
  2. The temperature dipping below 60°C, This could be caused by running a hot tap, The cylinder stat will call for heat again raise the temperature above 60°C and start the clock again.
  3. The boiler thermostat is set too low, The temperature of the stored water doesn’t reach 60°C and so the boiler is constantly running day and night trying to get to the magic 60°C and then keep it there for the 60 minutes.

I have had it where the customer has turned down the boiler thermostat to reduce the temperature of the water as they believe it is coming out of the taps too hot, this would be obviously counterproductive if they understood what was actually happening.

So in summary;

IF the boiler is constantly running… and the hot water seems unusually hot then pleas check that; the boiler thermostat is set to maximum; as all boilers can run at this setting. It just means it won’t be on for so long heating the stored water up, and the radiators will heat up quicker. But it will mean that the stored water will be able to reach the 60°C required when the anti-legionella cycle starts. All other controls can be set in the normal way to suit the individual home owner.

I also advise my customers to reset the system by turning the system off then on again (as early as they can – preferably over an hour before the house normally wakes up) on a Saturday after the system has been turned off for any reason. This is so the water temperature is at the highest on what is usually the busiest morning with any guests wanting a showers, so they add more cold water to the 60°C hot water therefore the hot water goes further….

I hope this helps somebody somewhere…..