How DHW cylinders work

Topics at a glance

Direct and indirect heating
Mono and dual modes of operation
Vented and unvented systems

In simple terms, the function of a DHW cylinder is to temporarily store water which has been previously heated and only release it again when required. This ensures that water flowing out of the tap is available at the right temperature as quickly as possible. A DHW cylinder is usually thermally insulated, so that the heat is stored for a long time. Different methods are used for this purpose and sometimes combined. Rigid foam and fleece are often used for the casing. In addition, vacuum panels are attached to increase the insulation performance. These achieve the same effect as a Thermos flask, with which you are familiar. As a result, advanced DHW cylinders work with almost no heat loss.

The difference between direct and indirect heating

When it comes to DHW cylinders and their function, the distinction between direct and indirect heating must be taken into account. The application areas and the degree of insulation vary, depending on the type. The economic efficiency of these DHW cylinders is also linked to this.

Directly heated DHW cylinders and how they work

As the name suggests, the water in this type of cylinder is heated directly. Options include a flame or an electrically operated indirect coil. A directly heated DHW cylinder is often called a hot water storage heater. It is, so to speak, a water heater and hot water storage tank in one. These appliances usually have only minimal thermal insulation –– and in some cases none at all. This means they can be operated separately from the existing heating system. Directly heated DHW cylinders are mostly used for decentralised DHW heating. You can read more about the other features of this type of cylinder and when it is economical to use one on the following page: Electric hot water storage heaters.

Vitodens 200 wall mounted gas condensing boiler with Vitocell 300-W

Indirectly heated DHW cylinders and how they work

An indirectly heated DHW cylinder works quite differently from its direct counterpart. Instead of electricity or an open flame, the heating water from the heat generator flows through the integral indirect coil. The heating water releases its heat in the process, and the water temperature in the DHW cylinder rises steadily. Heating water flows through the indirect coil until the required temperature is reached. The DHW produced in this way is then drawn from the upper section of the DHW cylinder. Important: the heating water never mixes with the DHW. Both conventional boilers and heat pumps can be used as heat generators.

Mono and dual modes of operation in DHW cylinders

If the water in a DHW cylinder is heated by only one indirect coil and one heat generator, experts call this a mono mode DHW cylinder. However, since it is more economical and environmentally responsible to leave a large part of the DHW heating process to a solar thermal system, a dual mode DHW cylinder is the appliance of choice. There are two indirect coils inside a dual mode DHW cylinder. In this case, the indirect coil in the lower section heats the water with the energy from the solar thermal system. If the solar energy available is not sufficient to bring the water to the required temperature level, the second indirect coil is switched on. With the help of a heat generator, this heats until the required temperature is reached.

Subdivision into vented and unvented DHW cylinders

In addition to direct/indirect heating and mono/dual mode models, there is another distinguishing criterion: namely vented and unvented systems. The operating principle of the DHW cylinder itself – i.e. the process used to heat the water in this cylinder – also plays a role.

Experts distinguish between vented and unvented DHW cylinders. A vented DHW cylinder, such as an electric hot water storage heater, is independent of the water pipe pressure. When DHW is drawn off, the DHW valve opens the inlet for cold water. In the process, the cold water flows into the cylinder and displaces the lighter hot water, pushing it upwards to the draw-off point. Vented DHW systems are only intended for individual draw-off points.

With unvented DHW cylinders, the situation is different. These can supply several draw-off points at the same time. Inside this type of DHW cylinder, there is a constant water pipe pressure which pushes the hot water to the draw-off points. No special non-pressurised fittings are required for the operation of an unvented DHW cylinder. With vented DHW cylinders, on the other hand, the use of commercially available pressure fittings can lead to pressure misalignment and cause material damage. The choice of suitable fittings is therefore of great importance.