London is in the middle of a green retrofit wave. Heat pumps, solar thermal systems, and energy-efficient water heaters are being installed across the capital at pace, driven by government incentives and rising energy costs. For landlords and property managers, the environmental logic is sound. But water safety specialists are raising an alarm that is not yet getting the attention it deserves: many of these systems operate at temperatures that Legionella bacteria find ideal.
Legionella thrives in water held between 20 and 45 degrees Celsius. Traditional hot water cylinders are designed to store water at 60 degrees, a temperature at which the bacteria cannot survive. Heat pumps and solar thermal systems, however, are optimised for energy efficiency rather than heat output. They routinely deliver water in the 45 to 55 degree range, and on cooler days or during periods of lower demand, temperatures can drift lower still.
This is not a theoretical concern. The UK Health Security Agency confirmed 472 cases of Legionnaires’ disease in England and Wales in 2024, with just over half acquired in the community rather than through travel or healthcare settings. Domestic water systems are a plausible but largely uninvestigated source: a Public Health England study found Legionella present in 48 per cent of UK household showers sampled, with the clinically significant serogroup 1 strain detected in nearly one in 40 homes tested.
London’s building stock makes the risk worse
The capital’s housing presents a particular challenge. London has a higher proportion of older properties than most UK cities, and older buildings tend to carry the legacy features that create legionella risk: cold water storage tanks in loft spaces, long pipework runs with dead legs, and complex distribution systems that allow water to sit stagnant. When an eco-retrofit is applied to a building with Victorian or Edwardian plumbing, the interaction between the new heating technology and the old infrastructure can produce conditions that neither was individually designed to create.
The problem is compounded by the broader shift in property use. Hybrid working patterns have reduced water demand in residential buildings that once had predictable daily flow. Void periods between tenancies, common across London’s buy-to-let sector, leave water sitting in pipework for days or weeks. Both factors increase stagnation risk, and stagnant water in the 20 to 45 degree range is where legionella colonies establish and grow.
The legal exposure landlords are not thinking about
Landlords already carry a legal duty to assess and control legionella risk under the Health and Safety at Work Act 1974 and the Control of Substances Hazardous to Health Regulations 2002. The HSE’s Approved Code of Practice L8 is clear: every rental property requires a legionella risk assessment, regardless of size. Failure to comply does not trigger a proactive inspection. The HSE does not routinely audit domestic landlords. But if a tenant contracts Legionnaires’ disease and the source is traced to the property, the landlord faces prosecution and must demonstrate in court that they fulfilled their legal duty. The penalties are unlimited fines and, in the most serious cases, imprisonment.
What the retrofit boom adds to this picture is a new compliance gap. A landlord who commissioned a legionella risk assessment two years ago, before a heat pump installation, may now be operating a materially different water system that has never been reassessed. HSE guidance requires risk assessments to be reviewed whenever significant changes are made to a water system. A new heating technology qualifies. Many landlords completing eco-retrofits under current government schemes are unaware that the installation triggers this obligation.
What responsible landlords and managing agents should do now
The good news is that the risk is manageable. For properties with heat pumps or solar thermal systems, the key controls are temperature verification, flush protocols for infrequently used outlets, and a post-installation risk assessment carried out by a competent assessor familiar with both the building’s plumbing and the characteristics of the new heating system. Some heat pumps can be configured with a periodic thermal pasteurisation cycle that briefly raises stored water to 60 degrees to kill any bacteria that have accumulated. This should be verified, documented, and factored into the risk assessment.
For London landlords managing older stock, the combination of pre-war plumbing and a modern heat pump represents the highest-risk scenario. These properties warrant professional assessment rather than a landlord self-assessment, and the documentation should be updated to reflect the current system, not the one that existed before the retrofit.
The green transition in London’s housing stock is happening faster than the compliance conversation. Water safety specialists and letting agents are beginning to see a pattern: landlords who have done the right thing environmentally are inadvertently creating a new health and safety liability. Getting ahead of it requires nothing more than treating a new heating system the same way you would treat any other significant change to a property. In this case, that means a risk assessment before anyone turns on the tap.