Mehrfamilienhaus – Trinkwassererwärmer als Gefahrenquelle für Legionellen

No mandatory testing = safe for Legionella? Decentralised water heaters as a health risk in residential buildings

Decentralised water heaters are used in residential buildings as part of an individual or group supply system. Heaters are either tankless or come with mini tanks. For many landlords and housing associations, these heaters are an attractive option because they are exempt from mandatory Legionella testing (section 31 of the German Drinking Water Regulation (TrinkwV)). Yet are these systems really as ‘Legionella-proof’ as building operators have come to expect, based on building codes and the absence of mandatory testing?

Clear-cut standards for circulating water heaters

DIN 1988-200 on decentralised water heaters

How ‘Legionella-proof’ are decentralised water heaters really?

Summary

Differences in the mandatory testing of water heaters

Legionella are rod-shaped bacteria that naturally occur at low concentrations in drinking water. At higher concentrations, however, these bacteria can cause serious illness such as atypical pneumonia (legionellosis). Drinking water installations offer ideal growth conditions for these pathogens at temperatures of 30 to 45 °C. To effectively prevent the spread of Legionella, the kinds of circulating water heaters often found in larger buildings must maintain the temperatures that are clearly defined by TrinkvW and its citation of the relevant building codes. Water at the heater outlet must be at least 60 °C. After 3 litres have been drawn off, a minimum temperature of 55 °C must also be present at each tapping point in the building and in the circulation return line. A system is then considered safe if its ‘specified normal operation’ is simultaneously ensured at each tapping point at intervals of no more than 72 hours.

Test parameters for a systematic test procedure to confirm safe condition for handover/acceptance (transfer of responsibility)

Excerpt from table 1 from VDI 6023 Part 1.
Test parameter Requirements
Temperature of potable water cold Maximum 25 °C after 3 l drawn off, measured in 250 ml in a measuring beaker
Temperature of potable water hot As per DVGW W 551 (A) after 3 l drawn off, measured in 250 ml in a measuring beaker
Colony count, determined as per section 15(1c) TrinkwV As per TrinkwV, annex 3, part 1

DIN 1988-200 on decentralised water heaters

Specifications for decentralised water heaters are given in section 9.7.2.4 of DIN 1988-200. However, these should also be considered in terms of hygiene. This is especially important for the housing sector. DIN 1988-200 states the following: “Decentralised water heaters can be operated without being subject to any further requirements if the downstream flow volume of 3 l is not exceeded in the flow path.” Accordingly, while building operators are free to choose any hot water temperature, this does NOT exempt them from a regular exchange of water across all tapping points.

At the time the standard was written, its authors could not have foreseen the consequences of these specific provisions when applied in the context of initial and general duties of testing for Legionella. This is because the standard entered into force in 2011, shortly before the adoption of the amended TrinkwV (details of the current TrinkwV here). This amendment was the first to specify that residential buildings with more than two units and a large centralised hot water system must complete initial Legionella testing by 31 December 2013, and then perform repeat testing every three years. Conversely, this also meant that apartments with decentralised water heaters were not subject to mandatory testing. These heaters were wrongly considered generally safe – although the Federal Environment Agency published a bulletin addressing this error in 2018.

Risk of Legionella despite a lack of mandatory testing

Many building operators assume the lack of mandatory testing regulations for (semi-)detached buildings and apartment buildings with decentralised water heaters means that these drinking water installations do not involve any risks for hygiene.

While this may be true, it is not a hard and fast rule. This article cannot cover all of the form factors and operating modes of decentralised water heaters, although these aspects are particularly important in terms of the hygienic safety of these appliances. As a practical example, a decentralised heat exchanger that has a small volume of heating water running through it continuously should certainly receive a poorer hygiene rating than a plate heat exchanger that only has heating water running through it when requested, and which also cools down rapidly because the manufacturer has omitted insulation for this component for this reason. Key differences are also found in the cold water connection and the routing of the cold water line in the appliance. A cold water meter and one-piece pipe crossings shared with the hot water line are inadvisable here, as this can result in the cold water temperature rising to unacceptable levels. As a consequence, Legionella could then start to propagate excessively not only in potable water hot (PWH) that is too cold but also in the potable water cold (PWC) line. Ideally, the water meter should therefore be installed at some distance to the appliance.

Mikroskopische Darstellung von Legionellen vor blauem Hintergrund.

Another risk factor for decentralised water heaters is always present in cases where the PWC line is heated to temperatures significantly over 25 °C by adjacent heating lines, for example. This allows low concentrations of Legionella present in the cold water line to spread excessively even in core sections of the drinking water installation. This is a problem because the bacteria are not reliably killed by the plate heat exchangers in decentralised water heaters, as a result of the contact time between the water and the hot surfaces in the plate heat exchanger being too short. Even at 60 °C, a contact time of roughly 30 minutes (!) would be needed.

Summary

Legionella can also spread excessively in small-scale systems or decentralised water heaters not subject to mandatory testing, even if these have been installed according to applicable technical codes. The temperatures prescribed by DIN 1988-200, section 9.7.2.4 ‘Decentralised water heaters’, should be reconsidered, especially as these appear to be outdated in light of insights gained from contemporary practice. In part 2 of this article on water heaters, we introduce the measures that owners and operators of buildings with decentralised water heaters can utilise to properly protect the health of their users.

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