Freshwater modules for hygienic DHW heating

The Vitotrans 353 freshwater module from Viessmann offers an alternative to the DHW cylinder. The freshwater module renders storage of domestic hot water unnecessary, as water is heated according to the instantaneous water heating principle. The heat comes from the heating water buffer cylinder, which can in turn be heated by different heating systems – solid fuel boilers, solar thermal systems, oil or gas boilers, heat pumps and more. The modules are prefabricated stations that are suitable for wall mounting as well as for installation directly on the cylinder itself.

Design of the freshwater modules

Unlike conventional DHW cylinders, the water is not used for storing thermal energy, but instead is only heated when required, by means of a powerful plate heat exchanger. This forms the core of the compact station. In addition, the following components are indispensable for the Vitotrans 353 freshwater module:

• Control unit
• Pipework with safety valve
• Flow sensor
• Shut-off valves
• Heating water side: high efficiency circulation pump

An optional return valve and DHW circulation set can also be added to the freshwater module. Another accessory is a heat-resistant valve for germ-free water sampling according to the Drinking Water Ordinance [Germany] (only for type PBMA/PBMA-S, PBLA/PBLA-S and PZMA/PZMA-S).

DHW heating by means of a plate heat exchanger

The mode of operation of the freshwater module is characterised by indirect heating of the domestic hot water according to the instantaneous water heating principle. How does the process work in detail?

If there is demand for domestic hot water at a tap somewhere in the building, fresh domestic hot water flows through the freshwater module, where it heats up as it passes the hot heating water. At the same time, the heating water from the buffer cylinder cools down. Both fluids flow in separate systems inside the heat exchanger. They never come into contact with one another and do not mix, as heating water does not meet the quality requirements for domestic hot water and has a much lower oxygen content. The process of heat transfer from one medium to the other occurs for as long as domestic hot water is needed and drawn from the taps.

Draw-off rate and other key data

There are five different types of Vitotrans 353 freshwater module, suitable either for wall mounting or for mounting directly onto the heating water buffer cylinder itself. A distinction can be made between the basic and the DHW circulation series. Depending on the type, the draw-off rate is between 25 and 68 litres per minute. This applies under the following conditions:

• Set domestic hot water temperature of 45 degrees Celsius
• Heating water flow temperature of 60 degrees Celsius
• Cold water inlet temperature of 10 degrees Celsius

The draw-off rate can also be increased by connecting up to four identical modules together in cascade formation. This can be done without the need for additional control. Consequently, the freshwater modules are also suitable for higher demand, for example in local authority applications.  

The maximum heating water temperature for all models is 95 degrees Celsius. This way, the freshwater module can be combined with different heating systems. The maximum DHW temperature, however, is 75 degrees Celsius. This is high enough to prevent germ formation. It makes this form of DHW heating particularly hygienic.

Consistent outlet temperatures ensured

Carefully matched components ensure a consistent outlet temperature at the tap with different draw-off rates. High efficiency circulation pumps of the latest generation are speed-controlled by an integral control unit so that the heating water flow rate is always at the optimum level for the current draw-off rate.

Integration in low temperature heating systems

Freshwater modules require only low flow temperatures to function optimally. This makes them ideal for integration in low temperature heating systems. These modules therefore round off the options for energy saving operation of heat pumps, gas or oil condensing boilers and systems with solar backup. In the case of solar backup systems, it is possible to increase the efficiency of the solar circuit, as the average temperature of this circuit can be lowered due to the cold return.