Legionella and Plumbing Systems

What is Legionnaires’ disease?

Legionellosis is the generic term used to describe two distinct clinical conditions caused by legionella pneuomophila.

  • Legionnaires’ disease takes the form of a severe, potentially fatal, form of pneumonia.
  • Pontiac fever is a non-fatal, mild and self-limiting illness similar to influenza.

Legionnaires’ disease usually affects those who are old, have pre-existing conditions, such as diabetes, lung or heart disease, have problems with immunosuppression or have other risk factors such as smoking.

Infection occurs when droplets or aerosols from a contaminated source, containing legionella bacteria are inhaled and deposited in the lungs.

Legionella bacteria thrive in warm, stagnant water environments such as hot water cylinders and cold water storage cisterns that are not kept at the correct temperatures. Rainwater harvesting systems, often containing water with other contaminants, are also environments in which the bacteria can thrive.

Water systems should be designed to avoid stagnation, water temperatures should be monitored and methods of disinfection administered regularly.

Leigionaires’ in hot and cold water systems

Legionella bacteria occur in water systems at temperatures in the range 20–45ºC. These temperatures are commonly present in the lower part of hot water cylinders and also in uninsulated cold water storage cisterns in roof spaces and locations where they can be affected by solar energy such as in tank rooms on flat roofs.

Controlling legionella bacteria

Legionella bacteria can multiply to dangerous concentrations in five days. A simple and effective way of controlling the bacteria in hot water systems, is to increase the water temperature. This will start to kill the bacteria at around 50ºC and if the water temperature reaches 60ºC and is held at that temperature for sufficient time (usually about 10 minutes), the bacteria will be killed. Satisfactory routine control of legionella bacteria will be achieved if a temperature of 60ºC is maintained throughout the cylinder for a period of one hour each day.

Heating the water to 60ºC will kill the bacteria but there is an increased risk of skin burns (scalding). Fitting a thermostatic mixing valve (TMV) allows the water to be stored and distributed at high temperature while blending it with cold water before it reaches the tap. (Building Regulation Part G required the fitting of TMVs as standard on baths in new homes.)

Water in storage cisterns should not be allowed to exceed 20ºC.

If you need advice on controlling Legionella in your plumbing systems call Steven Banks on 01462 636376

Water systems risk areas

  • Hospitals and healthcare premises house persons who might be susceptible to respiratory problems.
  • Sports clubs and changing rooms, which are often used intermittently and usually have a number of showers.
  • Spa baths are a particular problem because the water is at an optimum temperature for them to grow, dirt,dead skin cells provide food for the bacteria to thrive, and the agitated water forms aerosols and spray through which the bacteria can be breathed in.
  • Hotels invariably have as many showers as they have bedrooms and these can all produce aerosols.
  • Car washes at garages, windscreen water in cars
  • Dental suites.
  • Reducing water temperatures at draw-off points to prevent scalding, this should be done at the point of use rather than at the source.
  • Dead legs in old systems left after pipework alterations.
  • Cold water storage cisterns often contain organic deposits which collect as a sludge in the bottom of the cistern, and these can support legionella bacteria.
  • Incorrectly positioned outlet pipes in relation to the cold mains inlet in cold water cisterns can result in stagnation, particularly where two or more cisterns are linked together.
  • Inadequate thermal insulation will allow the water temperature to increase in warm roof spaces. Cold water cistern should have a purpose-made, tight-fitting cover and adequate thermal insulation to protect against freezing in winter and heat gain in summer.
  • Oversized water cisterns reduce the risk of the building running out of water but can lead to water stagnating.

System design problems

  • Reducing water temperatures at draw-off points to prevent scalding.
  • Dead legs in old systems left after pipework alterations.
  • Cold water storage cisterns often contain organic deposits which collect as a sludge in the bottom of the cistern, and these can support legionella bacteria.
  • Incorrectly positioned outlet pipes in relation to the cold mains inlet in cold water cisterns can result in stagnation, particularly where two or more cisterns are linked together.
  • Inadequate thermal insulation will allow the water temperature to increase in warm roof spaces. Cold water cistern should have a purpose-made, tight-fitting cover and adequate thermal insulation to protect against freezing in winter and heat gain in summer.
  • Oversized water cisterns reduce the risk of the building running out of water but can lead to water stagnating.

Cleaning and disinfection

Hot water services, and if necessary cold water services, should be cleaned and disinfected if:

  • a routine inspection shows it to be necessary.
  • the system or part of it has been substantially altered or entered for maintenance purposes in a manner which may lead to contamination.
  • During or following an outbreak or suspected outbreak of legionellosis.

Disinfection of the water services can be carried out in two ways:

  • By chemical disinfectants such as chlorination when it is necessary to disinfect the whole system including storage cisterns.
  • By thermal disinfection by raising the water temperature to a level at which legionella will not survive.

Maintenance and Inspection to reduce legionella risk

Hot water systems:

Monthly

  • Check the temperatures of secondary flow and return at calorifiers.
  • Check the water temperatures up to one minute to see if it has reached 50ºC in the taps furthest from the calorifier.

Annually

  • arrange for samples to be taken from hot water calorifiers, in order to note the condition of the drain water
  • make a visual check on internal surfaces of calorifiers for scale and sludge.

Cold water systems:

Monthly

  • Check that the temperature is below 20ºC at the taps furthest from the cistern after running the water for up to 2 minutes.

Six monthly

  • Check cistern water temperature remote from float operated valve and mains water temp at float operated valve.

Annually

  • Visually inspect cold water storage cisterns and carry out remedial work where necessary.
  • Check taps for temperature as above on a rotational basis.

Shower heads

Quarterly

  • Dismantle, clean and descale shower heads and hoses.

Little used outlets

Weekly

  • Flush through and purge to drain, or purge to drain immediately before use, without release of aerosols.

Spa baths

For spa baths to remain safe to use they need to be checked and maintained regularly:

  • After an initial test, the residual disinfectant levels and pH value should be checked every two hours.
  • The water should be treated three times a day or continually.
  • A daily check of the water’s clarity, plus a check on automatic dosing systems, disinfectant levels in reservoirs, filters, and strainers and backwash sand filters.
  • On a weekly basis drain, disinfect and clean strainers.
  • Every month take samples for bacteriological testing, clean the input air filter and pipes and all automatic systems.

Showers

Showers are the most obvious danger points because they produce aerosol sprays, which have a high risk of legionella infection. Shower heads should be cleaned and disinfected on a regular basis.

So in summary control legionella by:

  • Prevention and control of legionella are achieved by good design, installation and routine maintenance.
  • Hot water storage temperatures should regularly be taken to 60ºC.
  • Cold water should be stored at not more than 20ºC.
  • Keep to a minimum the amount of water stored.
If you need advice on controlling Legionella in your plumbing systems call Steven Banks on 01462 636376