As landlords across the capital rush to install heat pumps and solar thermal systems, a water safety risk is quietly building inside thousands of properties. Experts say the problem is real, largely undetected, and already a legal obligation to address.
A report published last week by The London Property News, drawing on research by London Water Services, has put a largely ignored problem back on the agenda: London’s green retrofit boom may be making its rental homes less safe from a water hygiene perspective, even as they become more energy efficient.
The issue centres on Legionella bacteria, the pathogen responsible for Legionnaires’ disease, a form of pneumonia that kills around 10% of those it infects. The bacteria proliferate in standing water held between 20 and 45 degrees Celsius. Traditional hot water cylinders have always run hot enough to suppress that risk, storing water at around 60 degrees. Heat pumps and solar thermal systems, designed to maximise efficiency rather than heat output, regularly deliver water in a temperature range that sits uncomfortably close to the bacterial danger zone.
That creates a problem that neither installer nor landlord is necessarily thinking about. The heating system gets signed off. The energy performance certificate improves. And somewhere in the plumbing, conditions shift in a direction no one intended.
An old building problem meets a new technology problem
The story is more acute in London than almost anywhere else in the country because of the age and complexity of its housing stock. Victorian and Edwardian terraces, the most common form of rental accommodation across inner London, come loaded with legacy infrastructure: cold water storage tanks in loft spaces, long pipe runs prone to stagnation, and distribution systems that were designed for a different era of heating entirely.
When a heat pump is bolted onto that kind of plumbing, the combination can create pockets of water sitting at exactly the wrong temperature for extended periods. The retrofit improves the carbon footprint of the building. It does not improve the layout of the cold water tank from 1908.
The report showed that landlords completing eco-installations under current government schemes are largely unaware that a follow-up legionella assessment is required. Its spokesperson put it directly: a heat pump installation without a water safety review is an incomplete job.
What the law already requires
This is not a situation where the regulation has not caught up. Under the Health and Safety at Work Act 1974 and COSHH regulations, every private landlord in England is already required by law to conduct a legionella risk assessment on any property they rent out. The HSE’s Approved Code of Practice L8 has placed this duty on landlords since 2001. Most landlords are unaware it exists.
More specifically, HSE guidance requires that risk assessments be revisited whenever a significant change is made to a water system. Changing the primary heating source qualifies. A landlord who had a risk assessment carried out five years ago and has since had a heat pump installed is, in the eyes of the guidance, operating with an out-of-date assessment.
The HSE does not proactively audit domestic landlords. The compliance gap therefore, goes largely undetected until something goes wrong. If a tenant contracts Legionnaires’ disease and the source is traced to the property, the landlord faces prosecution, unlimited fines, and in the most serious cases, a custodial sentence.
The retrofit incentive is outpacing the safety education
There is a reasonable case that the government’s push to accelerate heat pump adoption through the Boiler Upgrade Scheme and other incentives has moved faster than the guidance reaching landlords about what an installation actually changes. Landlord bodies have focused heavily on EPC compliance deadlines. Water safety obligations have attracted far less attention.
The result is a growing population of recently retrofitted rental properties where the energy performance has improved but the risk profile of the water system has quietly worsened. Tenants in those properties are unlikely to know. Landlords in those properties are unlikely to know. And until someone gets sick, there is no mechanism that brings the two facts together.
