The Legionella bacteria can attach itself to the biofilm on the inside of water pipes, where it multiplies if the conditions are right.

The wrong type of plumbing can increase the risk of Legionnaires’ disease

Researchers have tested different types of water distribution cabinets. 

Pipe-in-pipe plumbing systems on the Norwegian market are tested and approved through the SINTEF Technical Approval System. However, plumbing installed in connection with so-called combi-cabinets are not approved. These cabinets combine the distribution of both domestic tap water and water for floor heating. 

This is due to the risk of the Legionella bacterium, which can cause serious lung infection that claims the lives of an unknown number of people each year.

The Legionella bacterium can attach itself to the biofilm that forms on the inside of water pipes, where it multiplies if conditions are favourable. Once the bacterium has settled in the plumbing system, it can be difficult to get rid of.

“A commonly used method is to flush the plumbing system with hot water. Unfortunately, recent research indicates that this has little effect,” Dag Fredrik Nedberg says. He is an adviser at SINTEF Community.

About the Legionella bacterium

The Legionella bacterium is potentially fatal. Because it is difficult to get rid of, this makes it all the more important to identify water distribution systems that prevent its growth. 

The bacterium relies on favourable temperature conditions for its effective growth and reproduction, and temperatures of between approximately 30 and 45°C are thought to be optimal. The bacterium can reproduce rapidly at these temperatures, especially if there is standing water in the plumbing system. 

The prevention of bacterial establishment is thus all about maintaining the correct temperature conditions in the water pipes. 

Several factors in addition to temperature influence the risk of Legionella growth, and there is still a lot we do not know. The bacterium can also establish itself in cold water.

Tested different types of distribution cabinets

In a pipe-in-pipe system, it is usually in the distribution cabinet that hot and cold installations affect each other. To investigate how different cabinet solutions affect the temperature in stagnant cold water, researchers have compared the results of field measurements carried out in different types of distribution cabinets.

The poorest performers are the combi-cabinet systems, in which tap water and water for underfloor heating are installed side by side. Not surprisingly, the underfloor heating system emits a lot of heat. This causes the temperature in the cabinet to rise to as much as 40 °C, which is right in the most favourable temperature range for the Legionella bacterium. 

Tap water is heated rapidly to the same temperature, and if the water remains standing in the system for a few days, this will provide excellent growth conditions for the bacterium. 

Other types of distribution cabinet may also provide favourable conditions for bacterial growth, such as those connected to riser pipes, which are commonly installed in tall buildings.

In tall buildings, water rarely stands still in the riser pipes, and there is a constant supply of new hot water that heats up the distribution cabinet. 

The researchers' measurements showed that this type of cabinet maintained  temperatures over 30 °C throughout a 24-hour period, providing ideal conditions for the Legionella bacterium. 

An example of temperature development in a water distribution cabinet with riser pipes. Based on measurements taken by SINTEF.

Keep the water as cold as possible

The prevention of bacterial growth is thus all about maintaining the correct temperature conditions in the water pipes. Cold water should be kept at low temperatures, ideally below 20 °C. In practice, this quickly becomes complicated, since water that stands still in the pipes over time will reach the same temperature as its surroundings.

According to SINTEF, cold water should be kept as cold as possible and never be allowed to become warmer than room temperature, regardless of how long it remains standing in the plumbing system. 

In order to achieve this, we must ensure that the cold water is not heated by other installations in the building. The best approach is to make sure that systems required to supply hot water are isolated as much as possible from cold water installations.

‘Standard’ water distribution cabinets, which supply only hot and cold tap water, performed surprisingly well during the SINTEF investigation. Even if the hot water system releases some heat, the temperature in the cabinets remained only a few degrees above room temperature.

“If we are to completely avoid heat transfer in our tap water systems, the best approach is to install the hot and cold water systems in separate cabinets. However, we believe that standard distribution cabinets accommodating both hot and cold tap water represent an acceptable solution,” Dag Fredrik Nedberg says. 

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