Decentralised water heaters in residential buildings – what operators can do to protect tenants from Legionella

Throughout Germany, decentralised water heaters are found in many apartment kitchens and bathrooms. Building operators often assume that the use of these appliances is not relevant for drinking water hygiene because they are not subject to mandatory Legionella testing (as per section 31 of the German Drinking Water Regulation (TrinkwV)). According to the Federal Environment Agency (UBA), however, insights from practice show that “Legionella can also spread in decentralised water heaters and the piping systems that they connect to.” This article discusses the actions that building operators can take to maintain drinking water quality and user safety.

Specific measures for maintaining drinking water quality

Legionella can lead to severe illnesses such as legionellosis (atypical pneumonia) and is a particularly serious health risk for people with weakened immune systems. To protect residents and users, landlords and housing associations should keep three important factors in mind when it comes to the hygienic operation of decentralised water heaters.

1. The temperature range 30 °C to 45 °C provides Legionella with ideal growth conditions. Accordingly, it is essential that potable water cold (PWC) remains at a temperature of 25 °C or below along its entire flow path to the decentralised water heaters.
2. Ideally, Legionella should be undetectable in 100 ml sampled from PWC. To ensure testing results are reliable, this sampling must be carried out by a professional. According to DVGW W 551, owners of any type of building must conduct Legionella testing if there is a suspicion that the water in the PWC line has become warmer than 25 °C.
3. Decentralised water heaters should also be designed and operated to be ‘hostile’ to Legionella. Illustrative examples are provided here, plus information about the key factors involved.

Our handy checklist can also be used to see if building operators are already using all of the available tools and strategies for maintaining drinking water quality.

Nor should we forget the tenants: even while on vacation (for example), tenants must take steps to ensure the exchange of water across all tapping points – paying particular attention to decentralised water heaters. During day-to-day use, this happens automatically. For prolonged periods of absence, tenants have two options. A neighbour could take charge and fully open the fittings every 72 hours. Or the tenant could simply turn on all the fittings when they return, and let them run until the cold water line is properly cold and the hot water line is properly hot.