contact deutsch imprint
data protection
400 records of legionella testing in decentralized heating and hot water systems

Legionella in drinking water installations with decentralized heating has been underestimated as a health risk for years. Although the German Umweltbundesamt (UBA, Federal Environment Agency) issued a paper on significant legionella growth in these systems in December 2018, statistically relevant and valid data on water samplings and tests for legionella is scarce. Therefore, we decided to make a contribution to this health topic and scanned our comprehensive drinking water data bank. We found 400 records of water samples taken form decentralized systems. The results are not only interesting for experts.    

On December 18th, 2018 the Umweltbundesamt (UBA) warned the public in a so called expert statement not to underestimate the health risk of Legionella infections coming from decentralized heating and hot water systems within buildings.

Decentralized drinking water heaters are defined by the German technical rule W 551, Drinking water heating and drinking water piping systems, technical measures to reduce Legionella growth; design, construction, operation and rehabilitation of drinking water installations, issued by the German Association for Gas and Water (DVGW) as continuous-flow heaters and heaters with small hot water tanks.

Just a few years ago, decentralized heaters were deemed as relatively safe in terms of legionella contamination. Recent research projects imply that a significant legionella population can occur in the heating appliances and the connected piping system, UBA states.

UBA recommends to include decentralized heating systems in the testing scheme if legionella infections have been reported to the authorities in direct vicinity in order to locate the source of the outbreak.

As far as we can tell, the risk seems to be higher in corona times. Exposure to legionella and other potentially pathogenic agents is now particularly problematic, especially after building closures or reduced operations with fewer people returning to full-time building operations. The reduced consumption of water can cause water stagnation in building water systems, increasing the risk for growth and spread of Legionella.

Due to the lack of legally obliged routine testing for legionella in decentralized heating systems sufficient data on sampling and analysis for legionella is yet not available. The biggest set of relevant drinking water data was collected by the university of Kiel in a research project on legionella growth in decentralized water heating 4 years ago. Cold and hot water samples were drawn from 84 apartments in a huge residential complex. In 54% of all samples the German threshold value for legionella, 100 colony forming units in 100 ml potable water (100 CFU/100 ml) was exceeded. In 12% of all samples a extrem high contamination above 10,000 CFU in 100 ml water was detected.

This evident "data economy" lead us to the decision to take a closer look at our own set of data concering decentralized water heating systems. After the lockdown many plant operators and house owners contracted us to carry out water samplings in buildings before re-opening often including decentralized water heating.

From June 2nd to October 31st, 2020 we carried out 414 water samplings. In 370 cases samples were successfully drawn and tested for legionella. In 30 cases the agarplates could not be evaluated. In 10 samples the accompanying flora was too massive to achieve a valid result on legionella CFU. 4 samples were rejected by the lab.

75 samples of 370 samples with valid results exceeded the threshold value of 100 CFU/100 ml. That is a contamination rate of 20.3% and, therefore, significantly higher than the yearly average rate of central heating systems tested (depending on the year it varies between 5.6% and 8.0%).

The distribution of the contamination classes in accordance with with German technical rule W 551 is quite interesting: 48 samples showed a medium contamination from 101 to 999 CFU, at 26 sampling point a high contamination of 1,000 to 9,999 CFU was detected, a alarmingly high value. In one case the contamination was extremely high meaning more than 10,000 CFU were detected necessitating immediate remedial action in the heating system affected.

One reason for the surprisingly high contamination rate is that apparently many building water systems were not well maintained during the lockdown phase. One customer reported that he implemented a plan for regular flushings of the drinking water installations when they are not in operation, but the janitor commissioned to do the job went on vacation, afterwards fell ill and was quarantined for two weeks - and a substitute was not appointed giving Legionella 4 weeks time to grow in stagnant water.