New research by London Water Services finds that eco-retrofit installations are creating conditions ideal for Legionella growth in the capital’s rental stock, while most landlords remain unaware of a compliance obligation the law has required since 2001.
Analysis of UKHSA case data alongside an assessment across the capital’s rental sector identified a significant and largely unacknowledged risk: the green retrofit boom is inadvertently creating conditions ideal for Legionella growth in properties where landlords already carry a legal duty to manage it.
Legionella bacteria thrive between 20 and 45 degrees Celsius. Traditional hot water cylinders store water at 60 degrees, a temperature at which the bacteria cannot survive. Heat pumps and solar thermal systems, optimised for efficiency rather than heat output, routinely deliver water in the 45 to 55 degree range, and temperatures can drift lower during cooler weather or periods of low demand.
“We carry out risk assessments across London’s rental market, and the pattern we’re seeing is consistent,” said a spokesperson. “Landlords who have done the right thing environmentally are creating a water safety problem they’re entirely unaware of. A heat pump installation without a follow-up legionella assessment is an incomplete job.”
London’s older buildings compound the risk
London’s housing stock presents a particular challenge. Older properties carry the features most associated with legionella risk: cold water storage tanks, long pipework runs with dead legs, and distribution systems that allow water to sit stagnant.
When a heat pump is installed in a building with Victorian or Edwardian plumbing, the interaction between new heating technology and old infrastructure creates conditions neither was designed to produce.
“Older London properties with original storage tanks and a new heat pump are one of the highest-risk combinations. The tank keeps water at a temperature the bacteria love, and no one has flushed the system properly since the retrofit went in.”
A compliance gap landlords are not spotting
Under the Health and Safety at Work Act 1974 and COSHH regulations, every landlord is already required to carry out a legionella risk assessment on their rental property. The HSE’s Approved Code of Practice L8 has made this a duty since 2001. The HSE does not proactively audit domestic landlords, but if a tenant contracts Legionnaires’ disease and the source is traced to the property, the landlord faces prosecution with unlimited fines and, in serious cases, imprisonment.
The specific issue the retrofit boom creates is a review gap. HSE guidance requires risk assessments to be revisited whenever significant changes are made to a water system. A heat pump installation qualifies. London Water Services estimates the majority of landlords completing eco-retrofits under current government schemes are unaware this obligation exists.
“The legal duty is not new, but the retrofit trigger is catching people out. If you’ve changed your heating system, your previous risk assessment is out of date. That’s not an opinion, it’s what the guidance says.”