What matters during the initial adjustment of a fresh water station
Reading time: 7 Min
Fresh water stations have been an integral part of hygienic and efficient domestic hot water preparation in many European countries for years. In a fresh water station, the energy from an external heat source is transferred to the water pipe via a heat exchanger using the continuous-flow principle. The primary and secondary sides remain hydraulically separated from each other.
This method is used equally for centralised and decentralised fresh water stations, which are increasingly being used in heat interface units in multi-storey buildings.
Since the desired heat is thus to be provided in real time, accurate to the degree, the components of a fresh water station must be carefully balanced with each other before they are ready for the market. In addition to thorough component selection and intelligent design of the compact unit, series of measurements and tests are carried out on a test stand for this purpose – the so-called adjustment of the fresh water system.
Since the controller receives the measured values from the necessary sensors and at the same time controls the actuators such as the circulation pump and valves, it has the central task of coordinating all components in different situations in favour of a fast and stable extraction temperature.
The result of the adjustment should be a market-ready fresh water station that ensures a fast and stable tap temperature in a variety of installation situations (e.g. different heat sources and system structures) as well as tap profiles over the entire range of intended flow rates.
Setting up the fresh water test stand
To carry out the adjustment, a test stand with the following minimum equipment is required:
- a heat source, e.g. a buffer tank with sufficient and stable temperature
- devices for simulating the desired range of tapping volumes
- the test specimen, e.g. the prototype of the fresh water station
Some manufacturers attach importance to additional measuring instruments and recording of the measured values. According to our experience, these are not absolutely necessary, since a modern controller now knows and provides all the necessary information needed for optimal control (see info box).
Infobox: The evolution of fresh water sensor technology
Fresh water technology became popular in the mid-2000s. In the early days, fresh water stations were controlled only by simple temperature controls with paddle switches. However, due to the insufficient control results, this technology was quickly developed further. The difference between current and desired hot water temperature is still the reference variable today, but it is extended by additional measurements. For example:
A practical procedure for initial adjustment
Before starting the adjustment, it is advisable to make an unbiased assessment of the arrangement of the components in the station. An unfavourable placement of sensors in the station or the inappropriate selection of sensor types can lead to a variety of misbehaviour. Even experienced designers and established manufacturers are not immune to this, as fresh water stations are complex. In my 20 years of practical experience with fresh water adjustments, many weak points have been discovered at an early stage.
Likewise, one experiences time and again how small errors in the test set-up lead to incorrect measurement results and conclusions. It is therefore advisable to thoroughly check not only the station itself, but also the test stand before starting the adjustment. An outside view from a third party can be very helpful here.
As soon as the test series begins, the first goal should be to determine the performance limits of the system: how does it behave with small and large tapping volumes, as well as large and small temperature differences between system flow and hot water setpoint. Here, the pump or the control valve and the heat exchanger are of essential importance.
Unfortunately, a glance at the data sheets is not enough. A PWM pump, for example, which according to the data sheet runs from 13%, may only produce a reliable volume flow in that particular station from 17%. This can be the case, for example, if gravity brakes or check valves lead to pressure losses.
On the basis of these key points, the most important control parameters can already be preset good enough to maintain an approximately optimal speed or valve position already at the start of tapping or load changes. Experience has shown that going into the fine-tuning tests with carefully determined presettings saves many hours or even man-days of time – and usually produces significantly better results!
In the last step, the PID values can be fine-tuned on the test stand in order to regulate the hot water temperature with the utmost precision. As a good sense of judgement is built up over the years as to which factor is the right adjusting screw in each situation and in which magnitude, extensive previous experience also brings a clear time and result advantage here.
Troubleshooting is natural part of the adjustment process
Since fresh water stations are complex hydraulic systems, deviations from the desired behaviour and problems during adjustment are the rule rather than the exception – which is precisely why the test series are carried out.
Unfortunately, not all potential weak points are obvious. A laboratory test cannot possibly simulate all conceivable tapping and installation situations, and yet it should avoid blind spots in the test series as much as possible, which would otherwise lead to customer complaints later.
An important and by no means easy task in the adjustment process is therefore to identify problems in the first place. This requires a good hand in the selection of test scenarios and the utmost attention to indications of where one needs to dig deeper.
Once a potential problem has been identified, the right questions need to be asked at the right moment in order to track down the actual cause. Sometimes a mix of luck and gut feeling makes the difference between a breakthrough and long headaches. A deep understanding of the interrelationships of the system is essential here, as cause and effect are often linked in several ways.
Often, supposed problems also turn out to be errors in the measurement setup. In addition to system knowledge, it is also advantageous to have already seen a certain range of error patterns and causes. While even after 20 years of experience in setting up fresh water stations one occasionally experiences something new, many causes of problems resemble each other.
Once the cause has been identified, a range of solution options must be generated. This often involves the above-mentioned fine-tuning of control parameters such as the PID values.
Infobox: PID Control
Nowadays, a PI or PID controller is often used for the control algorithm. The proportional component P ensures the correct reaction speed by amplifying the deviation between the setpoint and actual values. The integral component I can compensate for a control deviation and the optional differential component D enables fast readjustment in the event of disturbances. Through the interaction of the P, I and D values, the control behaviour can be fine-tuned experimentally. [i]
Software can also compensate for, circumvent or mitigate minor weaknesses in hydraulics and hardware. If, for example, small tapping volumes cannot be realised because the control range of the pump does not reach down far enough, this can be solved by special operating modes or by using additional system components. An unfavourable placement of a temperature sensor can, if necessary, be intelligently reinterpreted by adapted algorithms, etc.
Whether a hydraulic problem is better solved by construction design or by software must be decided on a case-by-case basis. However, in order to be able to include software-based solutions in the selection, close cooperation with the controller manufacturer is recommended in any case, up to carrying out the initial adjustment together.
A look into the future of fresh water control
Since the advent of centralised and decentralised fresh water stations, fresh water controllers have undergone very dynamic development and have now reached a very good state of the art. Nevertheless, further evolutionary stages are around the corner.
Self-learning systems correspond to the generally growing importance of artificial intelligence and also offer interesting potential for fresh water stations. At SOREL, we started researching neural networks with the Fraunhofer Institute back in 2016. However, in addition to the opportunities and the general hype, the use of self-learning systems also requires a careful approach and a rational weighing of the advantages and hurdles that need to be overcome.
The possibility of transmitting system data to a data server via the internet is another interesting perspective. The laboratory situation is supplemented by diverse and real data from the field. In addition, this data extends over the entire life cycle of a product, for example to reflect material wear or limescale deposits. The potential benefits for quality assurance, customer service and product development are manifold.