Heating water buffer cylinders – use heat with a time delay

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Viessmann heating water buffer cylinder with freshwater module

In principle, buffer cylinders store thermal energy that is not currently required so that it can be consumed later according to demand. Heating water buffer cylinders are used primarily where heat is not always consistently needed. For example, the peak times for heating and domestic hot water in an average household are in the morning and evening. However, a solar thermal system for central heating backup and DHW heating produces the highest yields in the middle of the day. A gas or solid fuel boiler, on the other hand, would be constantly switched on and off. A buffer cylinder reduces this cycling and uneven heat generation. It provides heat when there is a demand and ensures that the heat generator constantly operates at the optimal output stage.

Viessmann offers a wide range of buffer cylinders for domestic hot water and heating. These are equipped differently depending on their core tasks and to suit individual needs. The product range includes solutions for residential buildings as well as for commercial and local authority applications.  

The design enables optimal use of thermal energy

A special design is needed to enable temporary storage of thermal energy. A heating water buffer cylinder is basically a cylinder made of steel. It is equipped with hydraulic connection options. These vary in number depending on the model and application area. In addition, Viessmann heating water buffer cylinders impress with their all-round thermal insulation made from polyester fleece. This helps to minimise heat loss. It also renders additional insulation for the buffer cylinder unnecessary.

Matching to individual needs

Depending on the model and the circumstances, Viessmann buffer cylinders also have a connection for an immersion heater, an internal indirect coil for connecting solar collectors or an additional stratification system. In addition, a freshwater module can be supplied as an accessory, or a corrugated DHW pipe can be integrated if the buffer cylinder is also intended to supply domestic hot water.

Model house with heating system and heating water buffer cylinder in the basement.

How does a buffer cylinder work?

A buffer cylinder is always positioned between the heat generator and the heat consumers. A boiler heats the heating water, which flows through the buffer cylinder. The flow from the heat generator is mixed with the cold return from the consumers (radiators) in the buffer cylinder. This raises the temperature of the water so that it can be released as the flow to the radiators when required. The cold return from the heat consumers then flows back into the heating water buffer cylinder. If the cylinder also provides heat for domestic hot water, consumers include not just radiators but also draw-off points such as taps, showers, baths and dishwashers.

In relation to the volume, the density of the water is dependent on the temperature. Consequently, hot water is always found at the top of the heating water buffer cylinder. From there, it flows to the radiators. The cold return water from the radiators is at the bottom of the tank and flows from there into the boiler, where it can be reheated. The boiler, buffer cylinder and radiators therefore form a closed system.

Buffer cylinder for additional DHW heating

If the buffer cylinder also heats the domestic hot water, there is the option of supplementing the cylinder with the Vitotrans 353 freshwater module. This works according to the continuous flow principle. This means that the domestic hot water flows through the module, past the hot heating water that is in the buffer cylinder, and is heated up indirectly in the process. In this way, hygienic domestic hot water is heated according to the individual needs of the respective household. There is no longer any need to store domestic hot water, which makes for a very space saving solution.

The Vitocell 100-E range includes buffer cylinders with capacities ranging from 46 to 2000 litres. The Vitocell 140-E model can also be used together with the freshwater module. In addition, it features an internal indirect coil, which can be used for purposes such as connecting solar collectors. As such, this Viessmann buffer cylinder is also suitable for dual mode heating systems – i.e. systems which integrate two heat generators.  

Viessmann heating water buffer cylinders

Buffer cylinder with additional stratification system

As mentioned above, the density of water changes according to the temperature. Hot water is lighter in relation to its volume and therefore rises upwards in a vertical vessel. In a buffer cylinder, this effect generally leads to mixing. If a cylinder, such as the Vitocell 160-E, is equipped with a stratification system, the cold return water remains at the bottom, while the hot flow water can always be taken from the top. This has the great benefit that the cylinder does not need to be fully heated in order for draw-off points, such as showers or baths, to consume hot water.

In addition to the Vitocell 160-E with freshwater module, a supplementary stratification system is also available for the multi mode Vitocell 360-M heating water buffer cylinder.

Multi mode heating water buffer cylinders from Viessmann

Multi mode buffer cylinders not only combine heating water and domestic hot water. They can also be heated by multiple heat generators.

If, for example, a heat pump supplies too little heat to meet current demand, an additional heat generator is used. This is also similar to connecting the buffer cylinder to a solar thermal system. The Vitocell 340-M is a compact and space saving solution – a buffer cylinder with DHW heating via a flexible and stress-free corrugated pipe made of stainless steel. This pipe is integrated into the cylinder in a spiral formation, with fresh domestic hot water flowing through it. This water is heated up by the hot heating water. A separate cylinder for domestic hot water is therefore not necessary, since the water is heated as it is needed. This fact, together with the use of stainless steel as a material, ensures hygienic DHW heating with no risk of legionella formation. An integral internal indirect coil made of steel also enables the connection of solar collectors.

Ensure smooth operation

The operation of the heating water buffer cylinder is also monitored as part of heating system maintenance. This helps to ensure smooth running. During the visual inspection, the heating expert checks the buffer cylinder, pumps and valves for damage, corrosion, dirt and leaks. This is primarily a precautionary measure. The focus here is on the interfaces between components, to ensure that they are optimally coordinated with one another. In principle, however, the Viessmann buffer cylinder requires very little maintenance.

Air vent valves facilitate maintenance

The heat generator, buffer cylinder and heat consumers form a closed system. Nevertheless, there is always a risk of air penetrating the system. The Vitocell product range from Viessmann features air vent valves to allow the air to be released. With these, your Viessmann partner can easily vent the buffer cylinder.

Quality of the heating water ensures low expenditure

The buffer cylinder contains low-oxygen heating water, which means that additional rust protection is not usually necessary. If the water is of a certain quality and meets the specifications (according to VDI guideline 2035 – Requirements for heating water quality), a buffer cylinder does not need to be descaled. The situation is different with DHW cylinders. Here, tap water flows over the hot heat exchangers. Limescale may precipitate as a result, depending on the hardness of the water. Your Viessmann partner will also remove this.