Domestic hot water circulators have two key functions in drinking water units: first, they ensure convenient supply of domestic hot water without waiting time during extraction. And second, to avoid stagnation and as a result safeguard drinking water hygiene. The growth of knowledge on the scientific side regarding microbiology in drinking water is however increasingly changing concept and installation practice. Interactions between flow, extraction and temperatures in the drinking water system for instance make it clear that an exact alignment of all components is required for a hygienically safe domestic water installation – including circulators for domestic hot water. Since they can aid temperature maintenance in the system significantly.
Three headlines in just two weeks prove the necessity of the investigations into drinking water tanks in existing installations, required under the German Drinking Water Ordinance (TrinkwV): “Legionella in two Friedberg gymnasiums” (Bavaria, Augsburger Allgemeine). “Legionella alarm in Dormagen primary school showers” (North Rhine-Westphalia, WDR). “Shower ban due to bacteria in drinking water” (Brandenburg, Märkische Allgemeine). This list could be extended indefinitely. Any legionella discovery has serious consequences, even if fortunately, in very few cases people fall ill to dreaded pneumonia as a possible risk of infection. Since as soon as the technical action value of 100 colony-forming units is exceeded per 100 millilitres, the German Drinking Water Ordinance (TrinkwV) requires a cause analysis with subsequent and often cost-intensive cause elimination.
Lacking temperature maintenance due to stagnation
Lacking temperature maintenance in stagnating drinking water is the real reason for microbial contamination. Since most microbes, legionella included, multiply especially rapidly at water temperatures of between 25 °C and 45 °C. And especially when water is standing in pipes and bacteria thus have the time to help themselves to the nutrients available, for instance biofilm on the interior surfaces of the pipes. In the Friedberg gymnasiums legionella colonies are said to have formed in an extraction fitting since a dripping washbasin fitting was decommissioned and as a result was no longer rinsed out. In the other case, the cause appears to be an isolated and rarely used shower. Regular water exchange through the intended operation of a drinking water distribution and measures for maintaining the temperature of the drinking water under 25 °C as well as domestic hot water above 50 °C are also crucial.
In order to avoid dangerous legionella growth, domestic hot water circulations are the main focus. Since legionella access the human body as a rule when contaminated water is atomised and breathed in – like for instance when taking a shower. It often goes unnoticed however that domestic hot water circulation pumps can make a significant contribution to temperature maintenance.
Flow rate consistent with temperature level
In order not to provide growth-inducing temperatures for the legionella and in accordance with DVGW worksheet W 551 the water must leave the drinking water heater when it is > 60 °C . The return temperature at the inlet into the drinking water heater may be a maximum of 5 K lower. In order to ensure this temperature, spread, the correct setting of the circulator, among other things, plays an important role.
An example: The specific heat requirement of a hot water pipe which is fitted in unheated rooms, for instance in the basement, amounts to 11 watts per metre (in accordance with the DVGW worksheet W 553). When all pipes are correctly insulated in accordance with the German Energy Savings Ordinance (Energieeinsparverordnung (EnEV)) the estimated heat losses of the distribution pipes amount to 4 K. The specific volume flow to ensure the required temperature spread of 60 °C/55 °C consequently totals 2.3 litres per hour. Should the thermal insulation of the drinking water pipes be interpreted lower than the EnEV then the volume flow must be increased in order to compensate for the higher heat loss. On the contrary, if pipes are fitted in shafts then the specific heat requirement drops by 7 W per metre. The required volume flow for the temperature maintenance is as a result less and is estimated at about 1.5 litres per hour. These values  are to be taken into account for the calculation of total flow rates for pump design, but also for the adjustment of balancing valves (see table 1).
Temperature-dependent speed control
The DVGW worksheet W 551 underlines the direct connection between dependence of the temperature maintenance and volume flow in the domestic hot water circulation: “In hygienically faultless conditions circulation systems can be operated at lower temperatures to save energy for a maximum of 8 hours every 24 hours, for example due to deactivation of the circulator” (section 6.4). In relation to this specification the EnEV 2016 stipulation is also to be interpreted: “For installation in hot water systems circulators must be equipped with automatically operating devices for switching on and off” (section 14, para. 4).
Two “unknowns” in both of these specifications make it difficult to find the optimal point of intersection between hygiene and energy saving: First, “hygienically faultless conditions” in domestic hot water circulations cannot be clearly judged in the inventory. Second, it is not always clear to identify the right time to shut down the pump without loss of convenience. On the one hand, because user behaviour can change. And on the other, because in buildings like multi-family houses, hospitals, care homes and hotels there is virtually a constant demand for hot water.
Circulators with a built-in temperature sensor offer a practical solution. In the temperature-controlled mode, the speed is regulated depending on the water temperature so that the return temperature always remains above the previously set minimum temperature. The pump also maintains the set minimum flow, if the temperature control were to reduce the pump's flow below the set minimum. As a consequence, the pump adapts the pump output automatically, energy-efficiently and exactly without compromising the temperature maintenance.
Furthermore, some circulators with temperature-dependent speed control like for example the “Wilo-Stratos PICO-Z”, aid thermal disinfection in the drinking water system: If the pump detects via the built-in temperature sensor that a thermal disinfection has infiltrated, then it runs at maximum operational speed and provides fast distribution of hot water to the hydraulically most unfavourably located extraction point.
The pump logic of the “Wilo-Stratos PICO-Z” recognises a thermal disinfection first when the water temperature exceeds 70 °C or deviates from an average temperature. This is also how thermal disinfections can be carried out to fight legionella below 70 °C. This is particularly sensible when considering from a scald protection point of view.
Temperature maintenance, perfusion and finally the intended extraction of hot water are the decisive parameters in order to avoid an endangering multiplication of bacteria in drinking water. In circulating hot water lines the configuration of the pipes, the correct adjustment of the circulator and the provision of the optimal promotional volume are significant influencing factors. For energy-saving temperature maintenance in the system the temperature-dependent speed control of the pump is very effective – if the pump capacity matches the distribution network. The pump manufacturer Wilo for instance therefore has developed circulators with these functions in differing sizes for installation in multi-family houses, smaller commercial properties like hotels and care homes or widespread drinking water networks like in hospitals.
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