Green Star rating of buildings is becoming an important economic imperative in building construction. However the popular move towards energy and water conservation in building design and operation may have it’s drawbacks.

A number of green strategies for water and energy conservation may be employed throughout the building network. Water in buildings needs to be turned over. Water unlike food does not have a ‘use by date’. But it is common sense that the longer water remains in the building the more likely it is to become stagnant. Water is best and safest when it is flowing. Just water movement helps to reduce and control biofilms. This is one reason that the HACCP based approaches to managing building water systems are deficient. There is an inherent lack of consideration of variable usage at different outlets and water retention times. Stagnation is a major issue leading to Legionella and other bacteria contaminating and multiplying in the building supply.

Let’s take a look at a few water and energy conservation strategies and consider the microbiological impacts.

Solar Boosting

Using solar energy to pre-heat water before boosting the temperature to 60+⁰C can return significant energy savings. The water is stored ready for use. Commonly the water in the storages is kept between 20 and  45⁰C. This is the growth range for Legionella and other bacteria. The storages can inadvertently become a reservoir for microbial contamination. Whilst sitting in the storage residual disinfectant from the mains supply disperses. The storages are stagnant and it has been estimated that a solar boosted system increases the residence time (age) of water in a building by nearly 3 times. Failure of the booster to heat water above 60⁰C due to heavy water demand or mechanical failure can lead to contamination throughout the building.

Solar boosted systems need to have their storages cleaned and disinfected and the booster serviced by a licenced plumber at least annually. Alternatively, solar systems without storages can avoid this problem.

Instantaneous Water Heaters

By only heating water when it is needed instantaneous water heaters save energy. This has the added benefit of removing storages that may become reservoirs for microbial contamination and   reducing residency time of the water. The heaters can be set to a pre-determined temperature of 50⁰C. This is useful in reducing risks of scalding. However, the down side is that  the water is never heated to a temperature that would kill Legionella or other bacteria. There is a legal requirement for water heaters with storages to operate at minimum temperatures of 60⁰C. This is also recommended for Legionella control.

So installing instantaneous water heaters that do not heat water above 60⁰C is removing a barrier to Legionella entering the building. Installation of tempering devices after the heater can achieve two things: 1) protect against scalding by heating water to 60⁰C and then reducing it to 50⁰C or less after the device, and 2) put in place a thermal barrier to control bacteria entering the building. This will still save energy over conventional storage heaters.

Above left: Instantaneous Heater with Tempering valve fitted (bottom left of picture)

Flow Restrictors

Above: Flow restrictors (between the taps and orange tags) on a wash basin.

Flow restrictors may either be built into the outlets or fitted before the outlets to reduce flow through the outlets. Obviously reducing flow rates saves precious water. However the consequences can be longer residence time, temperature losses, slower flow rates and biofilm build up. So how do these three cause problems? Firstly, longer residence time reduces the effects of the disinfectant in the potable supply. Secondly, lower flow rates also gives more time for cold water to warm up inside the building. Most buildings operate well above 20⁰C especially outside air conditioned areas in summer

Once water rises above 20⁰C it is in the range for Legionella growth. Lower flow rates mean greater losses of heat from hot water and greater gains of heat to cold water. This means both the cold and hot supply can enter the growth range for Legionella. Often flow restrictors  contain polymer components that are conducive to biofilm growth. Like aerators they may also contain strainers that will collect debris and stimulate biofilm growth.

Sensor and Hands Free Taps

Above:knee activated hands free outlet

Sensor and hands free taps are more water saving devices that can cause problems. In many cases the taps are activated for less than 20 seconds. Usually this is not enough time to flush the hot water line to the mixer. As a result the hot water line can lose temperature and the water stagnate. These devices often have multiple plastic components creating a large surface area that may be conducive to biofilm growth. Some more detail on the downsides and maintenance requirements of sensor taps is available on our ‘sensor taps or traps?’ blog. Furthermore routine flushing of sensor and hands free outlets can be labour intensive. To flush the tap for 2 minutes a person must be present for the entire process. This is not the case with conventional tap ware.


Aerators may also cause stagnation and biofilm. One of our other blogs expands on how this can and does occur and other risks associated with them.

Above: aerator with trapped debris and biofilm


In conclusion a range of green approaches to conserving water and energy in buildings may result in unwanted problems. Stagnation, loss of temperature control, reduced disinfection effects and biofilm stimulation are the problems. This is an unhealthy mix of all the factors that contribute to Legionella growth. So appreciating the consequences of green interventions is necessary to effectively manage and control the water microbiology of the building. Understanding the hydraulics and plumbing of green buildings is an essential part of an effective risk management plan. Good design and consideration of microbiology before construction is essential.

Further Reading

Hong et al (2014) Effect of disinfectant, water age, and pipe materials on bacterial and eukaryotic community structure in drinking water biofilm. Environ. Sci. Technol. 48:1426-1435

Rhoads et al (2016) Survey of green building water systems reveals elevated water age and water quality concerns. Environ. Sci.: Water Res. Technol. 2:164-173