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In principle, a dual-mode DHW cylinder has two available resources to meet the demand for heat or domestic hot water. This means there are two heat sources for hot water production. Dual-mode systems are also equipped with two indirect coils (heat exchangers). This allows for the easy integration of a solar thermal system to support DHW heating. The second resource, i.e., the second heat generator, is typically a gas or oil boiler. Of course, other heat generators can also be used, such as a solid fuel boiler or heat pump.

Vitodens 200-W with dual-mode DHW cylinder Vitocell 300-W

Dual-mode DHW cylinders and their operating principle

As mentioned, the dual-mode version of the DHW cylinder uses two indirect coils and is therefore ideal for integrating solar thermal technology into a heating system. The cylindrical body contains a lower and upper heat exchanger (indirect coil). Both are supplied with thermal energy independently of the heat generators.

Solar cylinder for hot water supply

How does a dual-mode DHW cylinder work when connected to a solar thermal system? Solar water, heated in the solar collectors, flows from the collectors into the lower heat exchanger—the lower indirect coil. This occurs as soon as the temperature in the collectors becomes higher than that in the DHW cylinder. The working fluid passes through the lower indirect heat exchanger and heats the water in the cylinder. The solar water, now cool, flows back into the collectors, where it can be reheated.

Hot water, indirectly heated by the sun, rises into the cylinder. From there, the corresponding amount of hot water can be produced.

Using the upper indirect heat exchanger: additional heating

The upper indirect heat exchanger comes into play when solar energy is absent for an extended period—that is, when the DHW cylinder cannot be heated by the solar thermal system. A second indirect heat exchanger, which receives heat from a central heat generator, such as an oil or gas boiler, is activated to ensure adequate supply reliability. Hot heating water flows from the boiler to the indirect heat exchanger. The thermal energy is then transferred to the dual-mode DHW cylinder.

Hot water can now be conveniently collected from above through pipes and delivered to draw-off points such as radiators or taps. As soon as solar energy is available again during the day, the solar-powered hot water cylinder is heated from the bottom up.

Combined solar tank and buffer tank

To ensure a convenient and reliable hot water supply, combination and buffer tanks can also be supplied via a solar heating system. The Viessmann dual-mode water heating buffer tank combines these two heat sources and uses them for heating. Energy from the heat generators, which may occasionally exceed demand, is temporarily stored and ready for future use.

A combination cylinder is a buffer cylinder with integrated DHW heating. A dual-mode buffer cylinder, on the other hand, is equipped with an externally mounted freshwater module. For example, the Vitocell 100-E heating water storage cylinder can be expanded by adding a Vitotrans 353 freshwater module. This module is used for hygienic DHW heating and operates on the principle of an instantaneous water heater. This combination is particularly space-saving. Furthermore, the freshwater module ensures high water withdrawal rates. Thanks to the instantaneous water heater principle, there is no need for DHW storage.

Viessmann also offers cylinders for hot water heating and DHW storage, equipped with two separate cylinder zones, such as the Vitocell 120-E.

Viessmann Vitocell 100-W-CVUD, 300 liters

Vitocell: Dual-mode DHW cylinders to suit every need

For residential, commercial, or municipal needs, Viessmann offers a suitable dual-mode hot water cylinder for any hot water requirement, with cylinder capacities ranging from 300 to 900 liters. Indirect heating coils extending to the bottom of the cylinder ensure heating of the entire volume of water. Thus, dual-mode solar-inlet hot water cylinders offer a high level of hot water convenience. This is facilitated by large indirect heating coils, which ensure uniform heating of the entire cylinder.

But when it comes to hot water supply, convenience isn’t the only deciding factor. Dual-mode hot water cylinders also boast excellent energy efficiency with minimal heat loss. Furthermore, they take up minimal floor space and, thanks to their modern design, can be installed in multi-purpose spaces.

Drinking water quality for the highest demands

Drinking water quality is also guaranteed. The Vitocell 100-W, Vitocell 100-B, and Vitocell 100-U/W models are equipped with Ceraprotect enameled steel and meet the requirements of standard EN 12897. They are among the best enameled DHW cylinders available. Viessmann also uses high-quality stainless steel for these dual-mode DHW cylinders (Vitocell 300-B). This is not only hygienic but also food-safe thanks to the high-quality surface.

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