Legionella in hot water systems

Heat pumping technologies for single-family buildings as well as in collective systems for multi-family buildings, sports centres, hospitals, etc. are well fitted and capable to deliver the required temperatures according to general legislation dominant in the participating countries, to fight Legionella.

Legionnaires’ disease is a form of serious pneumonic infection caused by inhaling (and in rare cases aspiration of) the bacterium Legionellapneumophilaor other Legionellaspecies. Legionella disease is a lung disease and not caused by drinking water.

Legionella can be found in distribution water systems in buildings, particularly in hot and cold distribution systems in large and complex buildings such as hotels, hospitals, office blocks, multi-occupancy accommodation buildings, commercial buildings, shopping malls and passenger vessels.

The risks for single family houses are much smaller compared to large collective systems in buildings, so that it can be debated if there is any risk at all as the piping systems, towards the tap with the highest risk, rarely contain more than 3 litres of stagnant water and the water in the system is refreshed regularly. The temperatures for domestic hot water can thus be below 60°C without increased risk of Legionella if the overall volume of domestic hot water in the distribution system is below three litres.

Principle of Volume Limitation Concept

This finding can set the rule for designing the domestic hot water systems supplied by low-temperature collective distribution systems in multifamily buildings, as well as for district heating systems.

The idea is based on the German technical rule W551 that in systems where the total volume between the point of distribution and the furthest tap does not exceed three liters there is no need for additional disinfection techniques. These are so called small systems and are usually only found in single-family homes but the principle can be applied in apartment buildings as well (Karlsson/Ottosson). This so called Volume Limitation Concept can only be applied in buildings where the distributed hot water in the circulation system is used as energy carrier with a heat transfer system at the end user point.

In a number of countries the legislation is still an obstacle in the acceptance of such concepts.

Controlling Legionella in Hot Water Systems

The risks of Legionella infection exists in locations of stored (stagnant) hot water at relatively low temperatures and in collective systems where the distributed hot water is directly used by the end-user in a shower-like situation.

Legionella can be safely and easily controlled with good design, engineering and management protocols. The most effective treatment against Legionella is to keep cold water cold under 20 to 25°C and hot water, above 55°C, as there is no growth. A benefit of thermal treatment is that it there are no additives required into the water.

The traditional solution to inhibit Legionella growth is to keep the DHW above a certain temperature. The threshold for the water temperature when reaching the tap point is for the legislation in the majority of countries 50°C. That is because it is above the limit for Legionella growth as at > 46°C Legionella bacteria are inactivated. A small number of countries, such as Netherlands, have decided on a higher system temperature to further ensure the absence of Legionella.

For smaller systems with hot water storage tanks legislation often also requires heating up the tank once a day or once a week to a higher temperatures (60°C or 70°C). Some studies show that this increases the danger of the growth of Legionella. Under the Annex these studies are noted and not further verified and investigated. Still there is a lot of uncertainty here.

Increasing hot water storage temperatures to 60°C and the temperatures in distribution systems recommended for Legionella control decreases the energy efficiency, increasing CO2-emissions. By end user such high temperatures are not needed. The desired usage temperature at the tap water with the highest risk (i.e. shower head) is however only 30 – 40°C. This means that the temperature difference between hot water outlet and the cold water inlet almost doubles, which has a negative effect on the energy efficiency.

The bigger and more complicated a water distribution network is, the greater the risk for the growth of Legionella bacteria. By taking advantage of the knowledge of how water- and heating installations ought to be designed, the growth of Legionella bacteria can be prevented. It is basically a question of keeping installations clean, the cold water cold and the hot water hot.

Harmonization in legislation does not exist, but is needed when the demands in legislation differ too much there will be no equal markets as the demands have consequences for the test procedures as well. Although legionella cycles are not included in test procedures because considered as “safety operation” and not “normal use “, the temperature demands affect the test procedures for heat pump water heaters.

It is not suggested to take a slack attitude to the problem, but the broad-brush turn-up-the-thermostat approach, given the energy penalty involved, can be debated.

Further study in this area is justified.

Annex Report on Legionella

The Annex has in its collaboration analysed the work done on Legionella and Heat Pump Water Heaters. The Final Report on this is available and can be downloaded.