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.
If there is a problem, this cannot be caused by the heat pump itself. For single family buildings this is traditionally done with an additional in-tank electric resistance heater on the mono-bloc heat pump water heater. But, as a consequence of the existing regulations for legionella and the need for heat pumps suitable for retrofit, especially in collective systems, refrigerants and solutions for DHW applications with higher than traditional (>60oC) heat supply temperature are in demand. Modifications to the refrigerant cycle such as cascading refrigerant cycles can be used to increase the output temperature further up to 80°C with other refrigerants, like CO2 and in cascade R410A/R134a, but also gas driven heat pumps are in the market.
However based upon literature studies it is concluded that the domestic hot water temperatures can 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. This finding sets 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.
- Legionella disease is a lung disease originated by inhaling vaporized water from showers and not caused by drinking water from the tap.
- The risks of Legionella infection exists in locations of stored (stagnant) hot water at relatively low temperatures and in collective systems. The bigger and more complicated a water distribution network is, the greater the risk for the growth of Legionella bacteria. Systems in single-family houses with a water distribution system smaller than three litres are known not to be affected by legionella.
- 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.
- Increasing hot water storage temperatures to 60°C recommended for Legionella control decreases the energy efficiency, increasing CO2
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.