Legionella Risk Factors in London Rental Properties: What Makes Some Properties Higher Risk

Why London Properties Have Specific Risk Factors

London has a distinctive housing stock that creates specific Legionella risk factors not found as commonly in newer housing. The majority of rental properties in inner London are Victorian or Edwardian terraced houses, converted into flats or let as HMOs. These buildings were constructed with gravity-fed water systems using large cold water tanks in the loft and large copper hot water cylinders in airing cupboards — systems that were standard until the widespread adoption of combi boilers from the 1990s onwards. Many of these older systems remain in use today, and they present a significantly higher Legionella risk than a modern combi boiler system.

Cold Water Storage Tanks

A cold water storage tank in the loft is one of the highest-risk factors in a domestic Legionella assessment. The tank stores cold water at mains pressure but at whatever temperature the loft space reaches. In a London summer, an uninsulated loft tank can warm to 25 or 30 degrees Celsius — well within the Legionella growth range of 20 to 45 degrees Celsius. The tank also provides nutrients for bacterial growth: scale, rust from older tanks, and debris that enters through inadequate covers or through the air vent.

A tank that is oversized for the property — a common finding in Victorian conversions where the original household has been subdivided into flats — compounds the risk because water sits in the tank for longer before being used, extending the time it spends at warm temperatures. The combination of warm water, extended residence time, scale, and biofilm makes an oversized loft tank the most likely point of Legionella proliferation in a London rental property.

Hot Water Cylinders Set Too Low

A hot water cylinder thermostat set at 50 degrees Celsius or below creates the ideal growth temperature for Legionella in the stored volume. Energy efficiency advice over the years encouraged landlords to lower thermostat settings to reduce energy bills — this is directly contrary to Legionella control requirements. The cylinder must be set to store water at 60 degrees Celsius. At this temperature, Legionella are killed within two minutes. Below 50 degrees Celsius, the bacteria can survive and multiply within the stored volume.

Cylinder thermostats in older properties are sometimes inaccurate — a dial set to 60 degrees may actually deliver 50 degrees at the cylinder. The risk assessment should include a temperature check at the cylinder using a calibrated thermometer. Where the temperature is below the required level, the thermostat should be adjusted and re-checked.

Dead Legs and Redundant Pipework

London's Victorian properties have been adapted repeatedly over their lifetimes — bathrooms added, kitchens moved, ensuite facilities created in bedrooms, properties subdivided and recombined. Each adaptation leaves a legacy of redundant pipework: branch pipes that were capped rather than removed when an outlet was relocated. These dead legs are invisible from outside the walls but create significant Legionella risk because the water within them stagnates at room temperature, which is typically within the Legionella growth range for most of the year.

Dead legs are identified during the risk assessment by tracing the pipework layout and noting any capped branches or outlets that are no longer in service. Removing a dead leg requires cutting out the redundant section and capping the active main — a minor plumbing job that eliminates a persistent Legionella risk point. Where dead legs cannot be removed (for example, embedded in original Victorian brickwork), they should be documented and included in the control regime with regular flushing.

Void Periods

A rental property left empty for more than four weeks presents an elevated Legionella risk because all outlets become infrequently used simultaneously. Water in every branch pipe and outlet gradually reaches room temperature. During a summer void period, water in a loft tank can exceed 25 degrees Celsius for extended periods. Without regular flushing, biofilm establishes itself in showerheads and at tap outlets.

The control measures during a void period are straightforward but require active management: flush all outlets at least weekly if the system remains charged, or drain the system entirely if the void will exceed four weeks. Before re-letting, carry out a full flush of all outlets, check all temperatures, and document the actions taken. This pre-letting flush and documentation is part of a compliant Legionella control regime and should be recorded in the landlord compliance file.

HMOs and Shared Systems

HMOs present a higher Legionella risk than single-let properties because of shared water systems serving multiple users. A shared bathroom on a landing used by five or six tenants will see regular use of the shower and taps — this is protective. However, an HMO where one bathroom is allocated to a tenant who travels frequently will see extended periods without outlet use. The variation in outlet use across an HMO makes regular monitoring and flushing more important than in a single-let property where all outlets serve the same household.