Measuring hot water consumption with water meters is essential in modern buildings. Especially with complex pipe systems and circulation lines, the question arises as to how consumption can be measured accurately. While the use of water meters in circulation lines may seem sensible at first glance, it presents significant measurement problems and often leads to inaccurate billing. This article explains why water meters in circulation lines are unsuitable from a metrological perspective and what the consequences are.
What is the idea behind differential measurement?
In complex hot water systems with many branches, the total consumption of a single unit often cannot be measured with just one water meter. To avoid the cost of multiple individual meters, a workaround is sometimes used: Two water meters are installed in the hot water circulation line – one before and one after the unit. The difference between their readings is intended to reflect the actual consumption.
Why is this measurement method problematic?
Every water meter has inherent tolerances due to its construction. According to calibration law, hot water meters are allowed deviations of up to ± 3%, and in practical operation, even up to ± 6%. If two meters are used for differential measurement, these inaccuracies are compounded – even with low flow rates.
For example, a deviation of just 0,1% can already result in a billing error of over 16%. In the worst case, a negative consumption value will even be reported.
Overview: Why water meters in circulation lines are problematic
- The use of this technology contradicts metrological and legal requirements.
- Water meters have legally permissible measurement errors.
- The difference between two counters increases measurement errors instead of correcting them.
- High flow rates in the circulation system exacerbate the effects.
- This makes reliable billing of consumption impossible.
More about How water meters work
Are there technically reliable alternatives?
The only technically sound solution remains the targeted installation of individual hot water meters at each relevant draw-off point within the unit. Even though this option incurs higher installation costs, it ensures accurate consumption recording and legally compliant billing.
However, a difference-based solution using circulation lines is not only unsuitable from a measurement technology perspective, but also represents an economically useless investment.
Conclusion: There is no room for water meters in circulation lines.
Water meters are not designed for installation in circulation lines. Attempting to determine consumption via differential measurements leads to unacceptable discrepancies and makes a fair distribution of costs impossible. Anyone who values legally compliant and precise consumption recording must rely on accurate individual measurements – even if this means higher initial costs.
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