The hot water demand accounts for around 10% of the energy consumption in existing buildings. Half of this is due to losses from heating pipes, distribution and storage (see relevant section).The consumption of hot water adds up over the year - after all, it is used all year round. If it is possible to save fossil-based gas early on, on a continual basis, then the natural gas storage facilities will be charged even more in autumn, ready for the winter. For this reason, it would be worthwhile to think about measures for saving hot tap water:
Water flow rate in a shower can be measured easily by anyone: you only need a bucket with filling level markings (many buckets have these) and a watch showing seconds (any mobile phone with a “Timer” app). Fill the water into the bucket using the shower head and time this for 30 seconds or one minute. More than 12 l/min is too much, but even with 8 l/min it would be worthwhile to use a water-saving shower head. By the way, you can still enjoy showering with a water-saving shower head: engineers have had some good ideas for their design4)
Legionella bacteria are found in potable water and can lead to illness (legionnaire's disease) if inhaled with the atomized water spray, particularly in persons with pre-existing conditions. These bacteria were discovered after the “Bellevue-Stratford-Hotel-event” in Philadelphia in 1976, there a spectacular outbreak among U.S. military veterans of the American Legion occurred. These bacteria proliferate particularly well at temperatures between 25 and 50 °C, exactly those hot water temperatures which are perceived as comfortable. However, at temperatures around 60°C these die off quickly; for this reason, supported by regulations relating to Legionella, in Germany it became common practice to keep all system components of a hot water system permanently above 60°C. On the other hand, such high temperatures naturally also result in correspondingly high permanent heat losses, especially with circulation pipes that operate 24 hours a day. A rough estimate shows that in Germany, circulation pipes account for around a half of the energy consumption for hot water (see also our article on “Thermal insulation of heat-carrying pipes”).
However, the risk is lower in small systems, such as those in most single-family homes, especially if there is regular use of hot water and if the water does not stand in the pipes for many days. For this reason, there are no binding regulations relating to minimum temperatures for these small systems. But: no-one can guarantee that transmission will not occur if temperatures drop below the 60°C threshold. Every user, including those in their own single-family house must therefore decide for themselves whether they consider the usually only substantially lower temperatures to be acceptable, e.g. in a hot water heat pump system.
Now the latest filter technology can be used for protection against legionella exposure: using microfilters with opening cross-sections considerably less than 0.2 µm, it is possible to eliminate these microbes which are larger than 1 µm. These are available as so-called “terminal filters” i.e. filters directly in front of the shower head. The filtration effect has been confirmed by independent laboratory measurements.
We tested one such shower head with regard to practical application:
Compared to frequently found shower heads with 12 l/min, water consumption can be reduced by around one half due to the lower flow rate5). The energy saving for usable heat (that which is made available by the shower head) is thus also halved. However, because a quite large share of the energy demand for hot water consists of circulation and storage losses, the total energy consumption is reduced “only” in the range of 25 to 30 %. In single family houses a very high level of safety can be assumed due to the terminal filter6): under these circumstances, the risk is reduced even further also with storage and circulation temperatures of 45° provided by heat pumps7).
The terminal filter inside the shower head is simply much more effective for preventing legionella exposure than permanent thermal disinfection. In this way the circulation losses can also be reduced by around 50 % compared to standard operation.
Water-saving shower heads are available on the market at the same prices as conventional ones. With the lack of information for most products, the only trouble is finding suitable shower heads which make a comfortable showering experience possible also with lower flow rates.
Extra filter costs of around 100 €/a may well be incurred for a terminal legionella filter. However, the saved energy costs alone are about 125 € each year, and in addition approximately the same costs are saved for potable water.
What matters much more is the fact that such a solution allows operation of the hot water system with a commercially available hot water heat pump as well. According to our assessment, it would be reasonable to aim for a rapid switch to shower heads of the quality described here; this would reduce the number of illnesses in this connection as well as the energy consumption and thus also CO2 emissions. At the same time, the switch to heat pumps (largely renewable energy based systems) for domestic hot water will also be enabled in this way in the medium term.
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