Cutting up-front investment and running costs with water-saving fittings – and for LEED, BREEAM and DGNB certification

DWD climate data analysis as theoretical basis

The topic for the bachelor’s thesis, for submission to the South Westphalia University of Applied Sciences, was provided by Arens: Namely: ‘Drinking water installations: impact of reduced drinking water consumption in the context of tighter drinking water hygiene regulations’. In the theoretical part of the thesis, Güsgen focused on analysing climate data from the German Meteorological Office (DWD).

Increase in hot days and greater demand for water

Among other things, climate data from the DWD show a significant uptick of +2.2 K in the linear trend for temperatures in the period 1881 to 2022 (fig. 1). In contrast, however, the trend for rainfall has yet to follow a negative trajectory: on the basis of the respective annual mean of precipitation anomalies in the period 1881 to 2022, rainfall in summer has declined in Germany in recent years, but this has been balanced out by a significant increase in winter (fig. 2 and 3).

While this does seem reassuring, the bachelor’s thesis goes on to note that current climate model calculations show precipitation decreasing in summer by as much as 40 percent, even as rainfall increases in autumn and winter. Overall, climate data authors Krahe and Nilson (2020) assume that precipitation will increase by some 2.5 percent, while available volumes of water will decrease by around 10 percent, since rates of evaporation from land and surface waters will increase due to higher temperatures. In this context, one should also remember that demand for water will also rise significantly in the agricultural sector.

LEED, BREEAM and DGNB certification – caution is advised for existing properties

While the three leading systems for ecological building certification – LEED, BREEAM and DGNB – do require lower levels of water consumption, both in new and existing properties, they do not consider any issues relating to maintaining water quality. Overall, water consumption savings make up between 2.3 and 10 percent of the total score, depending on the certification system. These are not standalone measures, however, as Arens cautions: “What should certainly be avoided is reducing the litre-per-minute flow at tapping points so that it actually drops below the calculated flow that was used to determine the dimensions of the piping system. If water consumption for an existing building is simply reduced by 50 percent, for example, this has consequences for drinking water quality. For an existing drinking water installation, this lack of adequate usage of the system would cause the water to stagnate in the piping. This could result in an increase in material shedding into the water, plus the excessive spread of bacteria.”

Arens also questions the wisdom of looking at flows in isolation: “We really need to consider whether a wash basin tap with a flow rated at 1.32 l/min (as with LEED) actually does save more water than a tap rated at 3 l/min – because people always need a certain amount of water to properly rinse soap off their hands, for example. In this example, handwashing with a tap rated at 1.32 l/min simply takes longer: no water savings are achieved at all, but the user does suffer in terms of convenience.” Accordingly, the savings required by such certification may not ultimately be satisfactory and may actually work to endanger drinking water hygiene in existing properties.

Planning needs to account for reduced calculated flows (DIN 1988-300)

Achieving significant potential savings in the context of new building projects requires taking into account reduced calculated flows from tapping points. These lead to ‘leaner’ and therefore more cost-effective drinking water installations. Table 2 from DIN 1988-300 defines three groups of minimum flow pressures and calculated flows. Of these, only the tabulated values are typically included in the planning software used for drinking water installations. In a footnote to table 2, however, a section marked ‘Important information’ does state that the calculated flows may in fact be higher or lower than the values in table 2, and that planners must use the real-world calculated flows and not simply copy the values stored in the software:

Practical example: recalculation for a residential building with six apartments

Güsgen’s bachelor’s thesis includes an impressive practical example that illustrates the corresponding calculations and potential savings. Based on the calculated flows for water-saving draw-off fittings and WCs available on the market (table 1) plus the values taken from table 2, DIN 1988-300, he completed a comparative calculation for a residential building with six apartments (figure 4).*

A detailed comparison between the two largest dimensions reveals the savings on initial investment that can be achieved for a larger-scale building. In this six-unit residential building, the dimension DN 25 becomes superfluous with lower calculated flows – and even the DN 20 dimension can be reduced in terms of length. In the case of these two dimensions, savings amount to no less than 78 percent: EUR 1,596 before and EUR 358 after. This is because the large dimensions for pipes, connectors, insulation and pipe clamps account for a disproportionate share of costs in large buildings. In summary, one can conclude that, despite the unfavourable general conditions as stated above, the initial investment for this building can still be reduced by 17 percent or around EUR 642. This comparison is based on gross prices for copper piping, press fittings, insulation and pipe clamps.

Further significant savings in running costs

In terms of day-to-day costs, additional savings can be achieved. First of all, reduced water consumption has a direct effect on annual utility charges for water and waste water. Energy costs are also reduced – since a smaller volume of drinking water needs to be heated and circulated at a specific temperature. These energy costs were not determined specifically as part of the bachelor’s thesis. “But they can be calculated indirectly from the reduced volume of water in the system,” Arens explains. “Accordingly, costs for this building would be cut by around 40 percent, calculated solely on the basis of metres of copper piping needed. This roughly 40 percent reduction in water volume also saves on the quantity of water needed for simulating specified normal operation with stagnation flushes during public holidays, for example.” To avoid having to conduct these flushes manually, a water management system like SWS from SCHELL can be used to perform these flushes automatically, which saves both time and water.

Planning with reduced calculated flows cuts initial investment and running costs

All three leading systems for ecological building certification – LEED, BREEAM and DGNB – include the topic of drinking water as a separate category in the context of ‘saving water’. None of these three systems takes the topic of drinking water hygiene into account, however. Arens: “This is the reason why we need to look at the bigger picture here.” In existing properties, for example, water withdrawals cannot simply be reduced below the calculated flows from DIN 1988-300 without compromising drinking water hygiene. This would result in drinking water simply stagnating in the installation – with potentially serious consequences for user health. As result, water withdrawals in existing properties can only be reduced to a point by water-saving facilities (see also here).