Advice on Legionellosis (Legionnaire’s Disease)

Following an enquiry from one of Cumbria Tourism's Members on legionellosis, we have summarised an extract of the correspondence for the benefit of all our members.  The summary of points is listed below.

  1. It is only the shower outlets that present a risk as aerosolisation is required to enable droplets of infected water to be breathed into the lungs.

  2. The control measures relate to temperature of boilers/calorifiers and 60 Centigrade will provide effective pasteurisation against legionella bacteria pretty much instantly, and 50 Centigrade over a period of minutes will achieve the same (therefore the requirement to ensure you store hot water in boiler or storage tanks at 60 C or higher and achieve at least 50 C at shower outlets within a minute of running the shower).

  3. So long as showers are used or flushed regularly (at least once per week for 5 minutes or so) with water of this temperature, then the risk of legionellosis is deemed to be controlled.

  4. A common problem with total reliance on the above regime is the design coupled with the capacity of some boilers. In respect of boiler design/capacity, the problem arises if the boiler can allow water in the base to remain below 50 C during normal operation; therefore this water never becomes pasteurised w.r.t. legionella. Then – typically at times of heavy or unusual demand – such water may pass from the boiler to the showers without being heated to temperatures the boiler would usually achieve at its topmost parts (where the thermostat is situated).
    In recognition of this problem, a recirculatory loop is recommended in the Code if the boiler design and capacity are not adequate to prevent this from occuring. (A recirculatory loop is designed to enable water to circulate fully within the boiler and typically is run on an overnight setting for 1 hour at a minimum of 60 C). An easy rule of thumb to apply is to measure the outlet temperature (either directly at the shower or at a representative tap or pipework) during times of peak demand. If your guesthouse never runs hot water below 50 C, the capacity of your heating plant is considered adequate. If you find you run water cooler than 50 C on some occasions, then either adjustment of the thermostat, alteration of the timing settings or the addition of a recirculatory loop are options.

  5. The only other considerations are
    a) installation of a thermostatic mixing valve (tmv) which limits the outlet temperature of the shower and
    b) cold water stored in tanks or pipework.
    So long as the distance from tmv to shower outlet is only a few feet or so then compliance with the Code is achieved. If cold water in pipework exceeds 20 C (which is possible in heated rooms) then so long as it is regularly flushed compliance is achieved (again, once a week would be considered appropriate). However, cold water storage tanks should be monitored and maintained below 20 C….this is usually only a challenge to achieve if the tank is in a warm area, badly insulated tank with low usage (as incoming cold water tends to keep the temperature below this if usage is regular).