Understanding hot water cylinders

For reliable heat and efficiency

As a central component of the heating system, the buffer cylinder supplies continuous heat and domestic hot water for the bathroom and kitchen. Read on to learn more about the function of a cylinder and which types are available.

Sectional view of house with storage

What is a hot water cylinder anyway?

A hot water tank is a device that stores drinking water and/or heating water. A heating system with a hot water tank temporally separates the production and consumption of heat by having the tank store it for later. Depending on the type of device, this increases the efficiency of your heating and/or also ensures that there is always enough hot water for the shower and bath.

The savings potential of hot water storage tanks is often underestimated: a modern storage tank can save an average household several hundred euros per year, while also avoiding CO2 emissions.

01. Types of hot water cylinders

It is worth noting that the term DHW cylinder is sometimes used as a general term for a hot water cylinder, even if certain types are only relevant for storing heating water.

Below, we explain which cylinders are suited for the various applications.
  • DHW cylinder

    In many cases, it is sufficient for the cylinder to only store drinking water to ensure that hot water is available at any time for showering and bathing.

    The simplest DHW cylinder contains a heat exchanger that is charged via the heat generator (such as a gas or oil boiler). Depending on the hot water demand, a small wall-mounted cylinder with a capacity of less than 100 litres may suffice. If large amounts of hot water are needed, a larger floor-standing cylinder is used.


    To prevent Legionella bacteria, a cylinder used for drinking water should regularly be heated to 60°C. If a solar system is also used to produce hot water, a second heat exchanger is added for the solar system.


    Such a solar cylinder can hold between 200 and 400 litres, meaning it is always a floor-standing unit. A good solar cylinder should also always have a good stratified design. More detailed information on stratification and stratification cylinders can be found further on.

  • Heating water cylinder / buffer cylinder

    A modern heating system is generally equipped with a buffer cylinder. These are special cylinders for heating water.


    The buffer cylinder forms the interface between the various heat generators, such as a solar thermal system, heat pump or gas boiler, which supply warm heating water to the cylinder for later use. For this reason, they do not need a heat exchanger.


    The inner workings of buffer cylinders for solar systems serving as central heating backup are more complicated because these also need the solar heat exchanger.


    A buffer cylinder offers you maximum flexibility. It stores heating water but can also provide hot drinking water. This is done very efficiently and hygienically with a fresh water station which warms up the drinking water as it flows by, just like an instantaneous water heater.


  • Combi cylinder / stratification cylinder

    A combi cylinder combines the storage of drinking water and heating water. While the heating system could use two separate cylinders, this would unnecessarily increase the space required as well as the heat losses.


    There are two different types of combi cylinders.

    1. With the first kind, sometimes called a hygiene cylinder, there is a tank for drinking water underneath enclosed within the surrounding heating water tank.
    2. The second kind is a buffer cylinder or stratification cylinder with an integrated fresh water station. Heating water on demand gives Legionella bacteria no opportunity to take hold.


    Why is this so important?

    • If you have a solar heating system, it is important that the hot water for showering and bathing is only present in the upper section. In the middle and lower section, it is always cold enough in a good stratification cylinder for the heating system to make use of environmentally friendly solar energy as well. If the cylinder were fully heated all the way through, there would no longer be any place for the solar heat to go when the sun is shining.

    • In addition, a modern gas or oil condensing boiler needs a low return temperature to achieve a high level of efficiency. This means that the water temperature in the lower section of a hot water cylinder used with oil or gas heating must not be too hot.

    • A heat pump loses efficiency with every extra degree it heats the water. It is therefore ideal (not only for the hot water cylinder with heat pump) to have a temperature of 40 to 45°C in the middle section of the cylinder. This is entirely sufficient for underfloor heating. Hot water at 50 to 55°C for showering and bathing is then only present at the top of the cylinder.

02. The functions of a hot water cylinder

Unlike electricity, heat is very simple to store, which also makes storing it inexpensive. A cylinder is particularly important for a solar thermal system since the sun does not always shine. The cylinder ensures that you can make use of the energy from the sun even at night and on cloudy days.

  1. To achieve this, the heating system needs little more than a well-insulated tank and some water.
  2. If plenty of heat is available, the water in the cylinder is warmed up. With a solar thermal system, this is particularly the case on sunny days, for example.
  3. By the way: in summer, the solar system can easily heat the buffer cylinder up to 90°C. This allows all other heating systems to take a break for days or weeks at a time.
  4. With a solar thermal system, the heat carrier medium that has been warmed by the sun flows through a heat exchanger in the solar cylinder, transferring the heat energy to the water in the cylinder. At night and on cloudy days, the heated water is then available for supplying your house with heat or hot water.
Hot water supply with a stratified tank

03. Benefits at a glance 

A cylinder isn’t only useful with solar thermal heating. Hot water cylinders offer many advantages in combination with a heat pump, gas heating system or oil heating system as well:

Save costs

Large savings potential 

The savings potential of hot water cylinders is frequently underestimated. With a modern cylinder, an average household can cut several hundred euros from their budget per year while also lowering CO2 emissions.
Hot water tank - Bathtub

Sufficient domestic hot water

Heat generators are generally designed for space heating purposes. This avoids inefficient overdimensioning, which would result in unnecessarily high costs. However, this also means that the heating output is insufficient for supplying a large quantity of hot drinking water within a short time.

This requires an especially powerful instantaneous water heater or a sufficiently large cylinder with a capacity in the hundreds of litres. In other words, hot water cylinders can meet the need for large quantities of hot drinking water in the bath and kitchen.
Indoor climate - family at home on the couch

Less wear

Storing domestic hot water or heating water reduces how often the burner in a gas or oil boiler has to start up. This means less wear and a longer lifespan for the heat generator.

04. Hot water cylinders in new and existing buildings

  • Hot water cylinders in new buildings

    In new buildings, there are a great many options when it comes to hot water cylinders. In this case, you can plan space for an especially large unit with several thousand litres of capacity. This would allow solar heating to provide over three fourths of the heat needed for heating and domestic hot water, for example. 


    On the other hand, a heat pump paired with underfloor heating that covers a large area already has plenty of storage capacity itself, meaning that a cylinder is only needed for drinking water.

  • Hot water cylinders and modernisation projects

    Space requirements for a cylinder play a larger role when modernising a heating system. However, it is often possible to find a corner in the utility room for a slim stratification or combi cylinder. Modern units are dimensioned to allow installers to carry them down narrow basement stairs.

    In other words, even when modernising, you won’t have to do without efficient heating with a modern cylinder for domestic hot water and/or heating water.

05. Heating with a hot water cylinder pays off

The costs of a hot water cylinder depend heavily on the type of cylinder. A simple buffer cylinder without heat exchanger costs less than a solar stratification cylinder with a fresh water station.


In all cases, however, the efficiency gains with a good stratification cylinder will bring energy savings. And together with a state subsidy, the investment will pay off quickly.

Frequently asked questions about hot water cylinders

  • The terms domestic hot water (DHW) and drinking water are generally used synonymously. This is water that comes from the tap or the shower head. It is hygienic, pure water that you can use without concern for brushing your teeth, cooking and drinking.

    The word service water is used in various ways. Some people differentiate between hygienic domestic hot water / drinking water versus service water (such as for flushing the toilet). Most homes simply use regular drinking water for flushing, which is why service water is also sometimes used synonymously with domestic hot water and drinking water.

    The difference between DHW, drinking water and service water on one hand and heating water on the other is quite clear, however. Heating water is found in the heating pipes. It is specially prepared for heating and is not suitable to be ingested or for washing. For the same reason, the heating system must never be filled with normal drinking water.

  • The size of a hot water cylinder depends heavily on the heating system. For drinking water cylinders, you can expect at least 40 litres per person. If a solar system is connected, it will need at least twice as much per person. Heating buffer cylinders or combi cylinders start at a capacity of 500 litres and can even be as large as 1,000 to 2,000 litres with a solar system.

  • Heating cylinders contain oxygen-poor heating water so that they won’t rust. They can last for 40 years. In order to ensure a long lifespan of 20 years or more for drinking water cylinders, it is important to have the cylinder regularly maintained by an expert. They must be delimed, if necessary, and the corrosion protection (magnesium anode) may need to be replaced.

  • If an existing buffer cylinder still has unused connections, it is possible to add a fresh water station for producing domestic hot water. If that is not an option, a new combi cylinder is a good choice – either in the form of a buffer cylinder with good stratification plus a fresh water station or in the form of a hygiene cylinder.


Our recommendation - Stratification cylinder BSP

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