Cloudyew - stock.adobe.comA cold water pre-flow occurs when cold water flows initially when the hot water tap is opened. This is often mistaken for a billing error, but it is a technical occurrence. The actual costs are usually very low and are fairly distributed through the legally regulated billing system.
When only cold water comes out of the tap
In most homes, cold water flows first when the hot water tap is turned on. This is due to the so-called cold water pre-flow – an unavoidable phenomenon caused by the length of the pipes and their insulation. Even modern hot water meters register usage regardless of the water temperature. Therefore, it can happen that cold water is recorded as hot water.
How does a cold water supply line form – and what are its effects?
Technical causes:
- On upper floors the pipe runs are longer, which increases the cold water supply.
- After longer periods of inactivity, for example in the morning, the water in the pipes cools down.
- Even well-insulated pipes lose heat over time.
Difference between systems:
risers (common in smaller buildings): Water cools down in the pipe, cold water supply time is longer.
Circulation lines (in larger buildings): hot water is available more quickly, but the energy required to heat it is significantly higher.
Billing systems:
- Hot water is usually charged proportionally according to consumption and living space.
- A distribution key of 50% consumption and 50% basic costs is common.
- This largely compensates for individual differences in the cold water supply within a system.
What are the actual costs due to the cold water supply?
One example shows that the actual costs are often overestimated:
- Daily cold water supply: 10 liter
- Annual consumption: approx. 3,6 m³
- Energy required for heatingApproximately 40 liters of heating oil or 40 m³ of natural gas
- Cost (at €0,60/liter)approximately 24 euros annually
- Consumption share (with a 50% distribution key): approximately 12 euros per year, or about 1 euro per month
Conclusion: Even with a generous estimate, the costs are very low.
Is the hot water bill unfair due to the cold water supply?
No. While the hot water meter does register the first few liters of cooled water, this water was originally heated and part of the central hot water system. The distribution losses are therefore part of the overall system and are fairly distributed among all users via the basic charges.
Is a hot water meter with temperature measurement worthwhile?
Hot water meters with temperature measurement not only record the flow rate but also the temperature. Theoretically, this allows them to distinguish between hot water and a cold water supply. In practice, however, such meters are rarely used. The main reason is their significantly higher purchase price compared to standard meters.
A cost-benefit analysis quickly reveals that the savings from more accurate consumption monitoring are often disproportionate to the cost. In many cases, the investment costs outweigh the benefits, as typically only around 10 to 20 percent of consumption costs would be redistributed – not actually saved.
Furthermore, the line losses remain technically unchanged. The distribution is merely altered, but the problem itself is not resolved. Therefore, the use of temperature-measuring meters is considered economically unviable by most landlords and property owners.
Conclusion: Is the cold water supply a problem?
No. The cold water supply is a technical necessity and is largely offset by the legally regulated billing system. This does not result in any significant financial burden for individuals.
What should landlords and tenants be aware of?
Suggestion:
- Regularly check the efficiency of the hot water distribution.
- If necessary, optimize the insulation of the pipes.
- Inform tenants transparently about the technical function of the cold water supply and the billing method.
Author
Quelle: Handbuch der Wärmekostenabrechnung | Frank Peters Minol Messtechnik W. Lehmann GmbH & Co. KG, 2019
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