Focus on drinking water hygiene – selected options for optimisation when planning, installing and operating drinking water installations

Drinking water is essential for life itself – and must therefore be hygienically safe at all times. For planners and installers, this means that their work forms the starting point for the safe, economical and consistently hygienic operation of the drinking water installation.

Architecture: separate shafts for potable water cold and hot

Over a building’s life, its architecture will also determine the scope of activities that are needed to maintain water quality. To ensure the responsible handling of drinking water as a resource, and to reduce flushing activities over the next 50 years to preserving specified normal operation, potable water cold and hot piping must be routed in shafts that are separate from one another. This can only succeed if architects are aware of these issues and their own responsibilities. All too often, however, this is still not the case, although this has been required in codes such as VDI 6023 Part 1 for the last few decades. The client should therefore specify the use of separate shafts when awarding the contract to the architects. 

Hygiene, cost-effectiveness and sustainability can be addressed even at the planning stage. Designing drinking water installations with reduced flow volumes – e.g. showers with 6 instead of 9 litres a minute or wash basins with 3 instead of 4.2 litres – offers multiple benefits:

• Lower initial investment thanks to smaller pipe dimensions and less material
• Reduced operating and wastewater costs across the entire usage period of at least 50 years
• Improved energy efficiency thanks to less hot water consumption without compromising on user comfort and reduced thermal losses due to circulation by avoiding circulating hot water in pre-walls.

Drinking water installations should hold as little water as possible. Nor is this ‘nice-to-have’ but is a hard-and-fast rule, which is required by codes of practice for hygienic and economic reasons. As well as minimising the water volume, T-piece installations also reduce the surfaces that can absorb and dissipate heat, and thus work to cut operating costs over the next 50 years. 

Benefits: 

• Reduced risk of cold water being heated
• Improved water exchange during normal use, shorter flush times during interruptions in use
• Up to 25% less water volume and up to 29% less surface area compared with ring-in-ring systems
• Lower costs for installation and operation while improving drinking water hygiene

As already mentioned, cold water pipes should not be routed alongside circulating drinking water or heating pipework in shared-use spaces such as shafts, suspended ceilings and pre-walls. Only this approach prevents heat being transmitted to cold water despite adequate insulation and offers reliable protection against undesirable heating to over 25 °C – a key safeguard against Legionella in overheated cold water. At the same time, heat losses in hot water pipes are also reduced: this improves energy efficiency while cutting long-term operating costs.

Keeping pre-walls ‘lean’

Complex installations in pre-walls with hot water circulation and additional integrated cold water lines often result in undesirable heating of the cold water pipe – in some cases to more than 35 °C. This increases the risk of Legionella propagating in excessively warm cold water. This problem can only be tackled with the outdated method of high-volume flushing. A safer approach is to avoid these types of installation and instead make use of more straightforward T-piece installations. In this case, the tapping points are connected in compliance with the ‘max. 3-litre rule’ (DVGW W 551 and DIN 1988-200). This is easy to achieve, because that would be over 20 metres with a pipe diameter of 15 mm! As a result, these volumes are fully exchanged with each use. And a pipe from the ceiling manifold in a hall to the first tapping point in a sanitary facility is only about 4 m long (600 ml capacity). So, given this background and while considering the risks, why make any unnecessary connections? 
 

Pipes for potable water cold must not run through warm equipment rooms and other structures or be routed next to heating pipes. Even with adequate insulation, running alongside underfloor heating is especially problematic, as the insulation merely delays – but does not prevent – the thermal transfer process. Underfloor heating is also operated day and night for months at a time. DIN CEN-TR 16355 therefore requires minimum distances from heating circuits for cold water pipes. This ensures compliance with temperature limits and mitigates any hygiene risks.
 

Short hot water stubs without circulation or trace heating protect neighbouring cold water lines from becoming excessively warm. Importantly, insulation must be avoided here: this reduces costs while improving hygiene – for the next 50 years. This is because uninsulated piping cools down faster, and temperatures remain in the critical range between 25 °C and 50 °C only briefly. Although German energy-saving legislation (EnEV) and DIN 1988-200 (2012, table 9) have allowed the omission of PWH insulation for more than 10 years, knowledge of this fact is still not widespread in practice. Conversely, however, PWC lines that share the same space – e.g. in a pre-wall – should continue to be insulated, as this ensures that the (brief) heat emissions are not absorbed by PWC.

In the high-traffic areas of buildings such as airports or public swimming baths, saving water is easily achieved, because the regular exchange of water happens all the time. Electronic, contactless fittings are an especially effective option here when it comes to saving water and energy:

• Consumption of cold and hot water can be cut by up to 70%
• Less energy required for hot water heating systems
• More user-friendly, more hygienic

A simple test helps to identify potential risks: Don’t use the fitting for at least four hours. Then draw off three litres of cold water and measure the temperature in 250 ml.

• Above 25 °C? → Pipe routing is unacceptable.
• Below 25 °C? → A green light for hygiene.

In low-use areas – typically found in nursing homes, hospitals or hotels – relying on users for the water exchange is often inadequate for maintaining the water quality. Instead, automated water management systems and electronic fittings can handle these activities to provide targeted and fully documented flushes. This not only prevents water stagnating but also avoids wasting precious resources.

With forward-looking planning, professional installation and targeted management, drinking water installations can stay hygienic, efficient and economical into the long term. Separate shafts, streamlined T-piece installations and intelligent fittings form the basis for a sustainable, hygienically safe and resource-friendly drinking water system that complies in full with modern standards for drinking water hygiene.