domestic water systems

Domestic Water Systems

Landlords are not legally obliged to test for Legionella. Landlords are, however, required to ensure that the domestic water system in their properties is safe. A risk assessment will advise whether testing is a useful part of the domestic water system management.

We can’t do a risk assessment at long range, so we leave this for local agents to carry out. Domestic rentals can be divided into two types: those that use a water storage tank, and those that use mains water. The former carries higher risks. Water sitting in a cold water tank causes a build-up of biofilm on the tank walls. Legionella breeds in the biofilm and can then infect and re-infect showers and taps which are fed from the tank.

Hot water tanks are less of a problem if they keep the water at higher than 55°C which kills the bugs. Cold water has to be kept below 20°C to control Legionella growth. In summer this can be very difficult to achieve and flaws in the combined hot and cold water system design often mean that hot water pipes heat the cold water – we have all noticed this effect in houses we have visited. This is bad news for Legionella control and it can be useful to carry out a test to check if the Legionella is under control.

Showers are the main source of Legionella dispersal in domestic water systems. If showers are not being used regularly e.g. between tenancies or holidays, this can present a moderately high risk scenario. Showers which are used infrequently are indeed a likely source of Legionella infection. The bacteria grow in the biofilm which forms when the shower is not being used, and the shower itself is a perfect means of dispersal of the aerosols that spread the Legionella bacteria.

The showerhead should be cleaned between each occupancy and tested either quarterly, to ensure that the cleaning is working, or at the same time as the showerhead cleaning. It is important to understand that the whole water system can harbour Legionella and that while the shower head is the main source of dispersal the Legionella can grow anywhere in the system, particularly cold water tanks.

To test that a domestic water system is free of Legionella we suggest that the first outlet fed from the tank is tested – this also checks whether the tank is harboring Legionella – and the last outlet in the system – this checks whether there is Legionella in the water pipework.

A positive in either case indicates where the problem is and treatment can be planned accordingly. Repeat testing after treatment is recommended to ensure the system is clear of Legionella. A subsequent test after a week or two can be useful to ensure that there is not an internal source of re-infection. Once clear, revert to the planned testing and assessment cycle.