Increased importance for safe Domestic Hot Water systems due to Covid-19

Wednesday, October 14, 2020

Despite a lot of attention and measures to prevent legionella, periodically a new outbreak with sometimes fatal outcome reaches the news. According to European Centre for Disease Prevention and Control (ECDC) there is an alarming trend of growing cases per 100.000 population. According their latest information, the number increased from 4.6 in 2016, to 5.8 in 2017 and 7.7 in 2018.   

As commonly known the root cause of people suffering from the Legionnaires' disease, a severe form of pneumonia caused by inhalation of aerosols containing legionella, is the Legionella pneumophila bacteria. These bacteria can often be found in poorly designed, balanced or maintained domestic hot water systems in residential, commercial and public buildings.

“The ongoing COVID-19 pandemic has a significant potential to increase the risks of waterborne infections, including Legionnaire’s disease, over and above the inherent public health impact of the pandemic.“ 

Source: ESGLI Guidance for managing Legionella in hospital water systems during the Covid-19 pandemic

Increased risk due to Covid-19 pandemic

Without proper measures the number of Legionella infections will grow even more when the numbers for 2020 and 2021 will be published in due time. The expected growth highly depends on the development of Covid-19 pandemic in 2020 and likely continuing into 2021. Let’s look at how Covid-19 increases the risk of Legionella infections.

1. Lockdowns and limited use of buildings

First of all, the risk increases due to global lockdowns of countries, cities and buildings. The domestic hot water systems in closed, or partially used buildings will decrease the water consumption and cause an increase in the time that water remains in the system. Due to the decreased consumption it is also unclear what happens with the water temperature.

All of this means an increased risk of Legionella infections once the buildings are slowly being opened again. It is crucial for building owners and facility managers to take responsibility and e.g. flush the system with sufficiently hot water for an adequate period of time.

“The temporary shutdown or reduced operation of a building and reductions in normal water use can create hazards for returning occupants“

Source: Centers for Disease Control and Prevention (CDC)

CDC also recommends concrete steps such as establishing a comprehensive water management program, flushing of the water system, maintenance of the water system etc. to minimize Legionella risk before your business or building re-opens.

2. Existing risks in occupied buildings

Second, there is also existing Legionella risk in many occupied buildings. Periodic inspection of the hot and cold-water systems is important. A risk assessment should define the frequency of inspection and monitoring. The risk areas that support growth of microorganisms, including Legionella, are controllable with good design, operation and maintenance of the system. Areas which pose an increased risk of contamination and colonization of Legionella if not properly managed are, for example, areas where optimum temperatures for microbial growth and stagnation occur e.g. dead-legs, capped pipes (dead ends), infrequently used outlets and any areas of the system where there is poor circulation. Or where there are cool areas of the hot water system so the temperatures fall within the range of 20-45°C.

Source: European Centre of Disease Prevention and Control (ECDC): European Technical Guidelines for the Prevention, Control and Investigation, of Infections Caused by Legionella species.


Since temperature is one of most critical factors for Legionella growth, many DHW regulations highlight the control measures for water temperature to include the following:

  • For recirculating hot-water systems, the temperature of the water leaving the heater should be not less than 60°C and the temperature of the return should be not less than 50°C. Very small differences between the temperature at the outlet of the heater and the returning water may indicate shortcuts in the circulation.
  • The temperature of hot water at the tap should reach its maximum value within one minute and not be less than 50°C, except where thermostatic mixer valves are installed.
  • In systems in which water temperature at the tap cannot be maintained at 50°C because of the risk of scalding a susceptible population (e.g. in an old people’s home), alternative means of control should be implemented. Alternative measures include the use of biocides or periodic flushing (superheating) of the system with a return (and tap water) temperature of at least 60°C. This measure requires stringent safety measures to prevent scalding.

Source: World Health Organization (WHO): Legionella and the prevention of legionellosis

3. Risk of Co-infection

Finally, an additional risk that will occur in these times of Covid-19, is co-infection. This means there will be people who are infected with both Covid-19 and Legionella bacteria. A co-infection of Legionnaire’s disease which is caused by bacteria, and COVID-19, which is caused by the Corona virus, can be fatal.

“In this study we established that COVID-19 can cause co-infections with bacteria, viruses and fungus.“

Source: Journal of Microbiology, Immunology and Infection (2020) 53

So, the medical fact is when a person is infected with the Corona virus and exposed to Legionella bacteria as well, the co-infection causes aggravated COVID-19.

Therefore all measures to ensure safe water systems during normal operation times, hence Legionella preventive measures, need to be taken.

Thermal balancing, a safe solution

A highly effective and safe solution to reduce risk of Legionella in DHW systems is achieved by applying thermal balancing valves. They are set to a safe water temperature that doesn’t allow the legionella bacteria to grow, e.g. at 50°C. The valves are installed in every riser of the system, close to the circulation pipe. The valves ensure hydronic balance, safe and constant water temperatures. Besides the safety benefits, they also make the system run energy-efficiently, while providing user comfort with short waiting times at the same time.

Expand with electronic disinfection

The use of thermal balancing valves will provide a safe DHW system, but some systems require a higher degree of reliability and safety. E.g. in public buildings such as hospitals and hotels there are regulations to monitor system temperatures, apply periodic water temperature increases and other measurements to have full control of the system and insights to possible legionella threads.

For such buildings it is recommended to establish an electronic controlled temperature registration, monitoring and thermal disinfection solution. Such a system reduces the risk of legionella contamination even further by periodically increasing the water temperature to a level it kills the legionella bacteria. At a temperature of 65°C it takes only 10 minutes to disinfect the water. Electronic disinfection is not only a good solution to kill the legionella. It also lowers operational costs and increases the reliability of both the DHW system and required data collection.

Danfoss thermal balancing & electronic disinfection solutions

Danfoss has been offering thermal balancing and electronic disinfection solutions for DHW systems for about two decades. The solution consists of two main products:


MTCV is a thermal balancing valve that establishes safe water temperatures and dynamic hydronic balance throughout the system. Optionally it can be equipped with a disinfection module (MTCV-C), Pt 1000 temperature sensor and TWA thermal actuator to temporarily open and close the disinfection module mounted inside the valve. Once the thermal actuator is activated, the by-pass in the disinfection module is fully opened to – temporarily – allow full water flow capacity. By that the DHW system riser is flushed at an extra high temperature to kill any bacteria. Flushing every riser periodically keeps the system safe.


CCR2+ is an electronic controller to register and collect temperature data and to automate a disinfection process where the water temperatures are temporarily increased.

By combining MTCV-C and CCR2+ an advanced and cost-effective solution to reduce the risk of Legionella, increase the energy-efficiency while providing a higher user comfort is established.

Both products have been significantly upgraded to perform even better. The MTCV has recently been improved by using a special POM polymer main cone and lead-free materials used for all components that are in direct contact with the water.

And CCR2+, for example, now includes remote control and monitoring of DHW system temperatures on a mobile device connected to WiFi and has the option for full integration with a Building Management Systems (BMS). All temperatures are logged into the controller, and e.g. the facility manager can access the data on his mobile phone or receive alarms and notifications automatically.

You can find more information about thermal balancing and electronic disinfection of DHW systems here:

> MTCV datasheet

> CCR2+ datasheet

> MTCV and CCR2+ page