Problems can occur when some of the properties are empty or the tenants/owners are absent for extended periods. The lack of flow into just one property means that the hot water pipework cools sufficiently for legionella to proliferate, in time these bacteria can then affect the remainder of the building.
Testing for legionella from the hot water flow and return pipework can highlight low level contamination before the risk increases.
Most residential blocks just have communal cold water storage tanks which feed into each property, the responsibility then rests with individual tenants/owners to generate their own hot water requirements. In these cases, Landlords routinely test the cold water storage tanks for legionella bacteria as an ‘insurance’ against claims from tenants/owners who fall ill with legionnaires’ disease and are looking for someone to blame!