Dual mode DHW cylinders for comfort & security of supply

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

Dual mode DHW cylinders and how they work

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

The solar DHW cylinder

How does a dual mode DHW cylinder work when connected to a solar thermal system? The solar medium 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 is higher than in the DHW cylinder. The process medium flows through the lower indirect coil and heats the water in the cylinder. The solar medium, now cold, flows back to the collectors, where it can be reheated.

Indirectly heated by the solar medium, the hot water then rises in the cylinder. From there, the corresponding amount of DHW can be extracted.

Use of the upper indirect coil: additional heating

The upper indirect coil comes into play when there is no solar yield over a longer period of time – i.e. when the DHW cylinder cannot be heated via the solar thermal system. The second indirect coil, which is supplied with heat via the central heat generator such as an oil or gas boiler, switches in to ensure an appropriate security of supply. The hot heating water flows from the boiler into the indirect coil. This then transfers the thermal energy to the dual mode DHW cylinder.

The hot water can now be conveniently drawn from above via pipes and transported to the draw-off points such as radiators or taps. As soon as solar energy can be harnessed again in the course of the day, the DHW cylinder with solar connection is heated from bottom to top.

Combi cylinder and buffer cylinder for solar operation

To ensure DHW convenience and security of supply, combi and buffer cylinders can also be supplied via a solar thermal system. The Viessmann dual mode heating water buffer cylinder combines these two heat sources and uses them for heating. In this case, any energy from the heat generators that may at times be excess to need is temporarily stored and held ready for use later on.

The combi cylinder is a buffer cylinder with integrated DHW heating. A dual mode buffer cylinder, on the other hand, is equipped with a freshwater module mounted on the outside. For example, the Vitocell 100-E cylinder for the storage of heating water can be extended by adding the Vitotrans 353 freshwater module. This module is used for hygienic DHW heating and operates according to the instantaneous water heater principle. A combination of this kind is particularly space saving. In addition, the freshwater module enables a high draw-off rate. Due to the instantaneous water heater principle, no storage of DHW is required.

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