Hot water circulation
Knowledge

Hot water circulation

Ensuring hygiene safety and comfort in hot drinking water in an energy efficient way!

Energy efficiency and safety for hot water circulation systems

A circulation system is always required when pipework contents exceed 3 litres in at least one flow path of a hot water system. The circulation volume flow must be able to transport the quantity of heat that is “lost” through the surface of the pipework. At the same time, the temperature must not fall below 55°C in any part of the hot water system (PWH and PWH-C). In systems with more than one circulation circuit, this requires hydraulic balancing. Static or automatic circulation balancing valves can be used for this purpose.

Why is it necessary to maintain the temperature above 55°C in the hot water?

The temperature of the circulating hot water must be kept at ≥ 55°C throughout. The water content of a hot water installation, which cannot be held at this temperature must be minimised in order to prevent the growth of Legionella.

Growth rate of Legionella pneumophila
Hot water circulation: Graphic showing Legionella growth

Source: M. Exner, Hygiene in Drinking Water Installations – Experience in Germany, expert discussion on Legionella, UBA/BfR [Federal Environment Agency/Federal Institute of Risk Assessment on 20.10.2009, Berlin

Choosing the right valves and systems for hot water circulation
Hot water pipework with circulation in rising and distribution pipes with MULTI-THERM
Hot water circulation: Hot water pipework with circulation in rising and distribution pipes
Hot water pipework with circulation up to every tapping point with ETA-THERM / MULTI-FIX-PLUS
Hot water circulation: Hot water pipework with circulation up to every tapping point
Hot water pipework with circulation up to every tapping point with flow splitter / MULTI-THERM
Hot water circulation: Hot water pipework with circulation up to every tapping point with flow splitter

Circulation balancing products

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Explore KHS Flow Splitters

Thermal separation

In systems with circulation to the tapping point, special attention must be paid to thermal separation in bathrooms as unwanted heat transfer from the hot water to the mixing tap and the connected cold water may occur.

Optimisation potential of thermal separation in bathrooms

As well as the insulation that is in place, the pipework routing has a decisive influence on the effective heat loads and therefore also on the maintenance of drinking water quality of the cold drinking water (PWC). Using simple optimisation possibilities, which can already be taken into consideration during the design stage, it is possible to significantly reduce the heat transfer from hot drinking water (PWH) to cold drinking water (PWC). The following pre-wall illustrations show typical sources of error and associated possibilities for improvement and optimised pipework routing with thermally separated valve connection.

Hot water circulation: Tall pre-wall with typical sources of error for increased thermal loads

Typical sources of error in a tall pre-wall

  1. The shaft is not thermally separated from the bathroom.
  2. Always lay cold drinking water (PWC) below the heat-carrying media.
  3. Separate the cold drinking water (PWC) pipework from the heat-carrying media.
  4. The pipework for the heat-carrying media should be laid through the upper region of the pre-wall and the cold drinking water in the lower region of the pre-wall. (Use thermal stratification, forming a cool zone in the lower region of the pre-wall.)
  5. The cold drinking water (PWC) ring installation should be connected to the fixtures from below using looped through drop ear elbows (reducing cold water non-branch pipes).
  6. The temperature transfers at the mixing tap, which is connected using double drop ear elbows, have a permanent negative effect from hot drinking water via the valve to the cold drinking water. (Temperature considerably > 25°C in the PWC drop ear elbow)

Hot water circulation: Tall pre-wall with vertical connection

Optimised installation in a tall pre-wall

  1. The heat-carrying shaft and the cold-carrying shaft are thermally separated from the bathroom.
  2. The pipework for the heat-carrying media was laid through the central and upper region of the tall pre-wall (forming a warm zone in the upper region of the pre-wall).
  3. The cold drinking water (PWC) pipework was laid in the lower region within the bathroom (forming a cool zone in the lower region of the pre-wall).
  4. The PWC ring installation was connected to the fixtures from below using looped through drop ear elbows and thermal separator.
  5. By using the thermal separator, no temperature transfers can occur from the hot drinking water to the cold drinking water via the mixing tap. (Temperature < 25°C in the PWC drop ear elbow)
  6. The PWC was connected to the washbasin from above and with a short branch pipe. A thermosiphon forms in this type of installation.

Preventing heat transfers at mixing taps

Heat transfers at the tapping fittings can be significantly reduced by thermal separation. The difference between connecting with a standard double drop ear elbow and a thermal separation component can be seen clearly in a thermographic image.

Hot water circulation: Double drop ear elbow versus thermal separation

Left: Mixing tap connected via thermal separator (temperature < 25° C).
Right: Mixing valve connected via double wall discs. (temperature > 60° C)

Thermal separation products

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