A new hot water cylinder is installed for 15 to 20 years or longer. If you choose it by gut feeling, you notice the consequences every day: if it is too small, there is not enough hot water for everyone in the morning; if it is too large, an unnecessary amount of water cools down and hygiene suffers. When choosing a hot water cylinder, the volume in litres is therefore only one factor. The heat generator, the heat exchanger surface, the insulation and the requirements for legionella protection matter just as much.
This guide explains the main cylinder types, from the classic domestic hot water cylinder through buffer and hygiene cylinders to the heat pump cylinder. You will learn how to size the cylinder sensibly, why heat pumps need a large coil surface, what the energy label tells you and which temperatures the technical rules require. A table with manufacturer data shows how much the cylinder types differ at the same volume.
Which types of cylinder are there?
The domestic hot water cylinder is the classic. It holds the water that is later drawn at the shower and washbasin. An internal heat exchanger, usually a coil, transfers the heat from the boiler or heat pump to the drinking water. Steel cylinders are enamelled on the inside and need a protective anode; stainless steel cylinders do without enamel.
The buffer tank, on the other hand, contains heating water rather than drinking water. It stores heat for the heating system, hydraulically decouples the heat generator and heating circuits and ensures that a boiler or heat pump achieves longer running times. Buffer tanks are typical for wood and pellet boilers, for solar thermal systems with heating support and for many heat pump systems.
A combination or hygiene cylinder combines both tasks. The vessel is filled with heating water, and the drinking water flows through a corrugated stainless steel pipe inside and is heated on the instantaneous principle. As a result, hardly any drinking water is stored in the cylinder, which makes hygiene easier. An alternative is a buffer tank with an external fresh water station that heats the drinking water on demand via a plate heat exchanger.
The heat pump cylinder is a domestic hot water cylinder with a particularly large heat exchanger. It is designed for the low flow temperatures of a heat pump and is explained in more detail in the following section. Cylinders with two heat exchangers, known as bivalent cylinders, combine two heat sources, for example a solar system at the bottom and the boiler or heat pump at the top.
Sizing the cylinder correctly
The right size depends on the household's hot water demand, the simultaneity of draw-offs and the output of the heat generator. A family in which three people shower within one hour in the morning needs more storage than a couple with spread-out draw-off times. A large bathtub significantly increases peak demand. At the same time, the higher the output of the heat generator, the faster the water is reheated and the smaller the cylinder can be.
For exact sizing, planners use recognised methods, such as DIN EN 12831-3, which covers the heat load and demand determination of domestic hot water systems. For your own preliminary planning, your previous consumption and the following factors are helpful:
| Factor | Effect on cylinder size |
|---|---|
| Number of people | More people mean higher daily demand and more simultaneous draw-offs |
| Bathtub, rain shower | High peak demand in a short time, rather a larger cylinder |
| Output of the heat generator | High reheating output allows a smaller cylinder |
| Solar thermal | Additional volume stores solar heat for cloudy days |
| Heat pump with low cylinder temperature | Less usable heat per litre, choose a slightly larger volume |
| Rarely used storage capacity | Longer residence time of the water, so do not oversize |
A cylinder that is much too large is not a safety margin but a disadvantage: it causes higher standby losses, and the water stands for longer. If you are unsure, have a specialist company calculate the size based on your system.
Heat exchanger: surface area to match the heat generator
The surface area of the heat exchanger determines how quickly heat passes from the heating water into the drinking water. A gas or oil boiler supplies high flow temperatures, so a comparatively small coil is sufficient. A heat pump works most efficiently with low flow temperatures. To load the cylinder quickly nonetheless and without a high-pressure fault, it needs a much larger transfer surface. The manufacturer's data from the TWL range for a volume of 300 litres each show how big the difference is:
| Cylinder type (300 l) | Heat exchanger according to manufacturer | Intended for |
|---|---|---|
| Domestic hot water cylinder S | 1.3 m² | Gas, oil or pellet boilers |
| Solar cylinder SO | top 1.1 m², bottom 1.3 m² | Solar system at the bottom, back-up heating at the top |
| Heat pump cylinder SWP | 3.4 m² | Heat pump |
| Heat pump cylinder SWP2 | top 2.9 m², bottom 1.3 m² | Heat pump plus solar system or second heat generator |
The manufacturer expressly points out that the domestic hot water cylinder S is not suitable for heat pumps. When switching to a heat pump, an existing boiler cylinder should therefore always be checked and usually replaced by a heat pump cylinder. Ultimately, the heat pump manufacturer's specification for the minimum surface per kilowatt of heating output is decisive. You will find further advice on combining heat pumps with existing buildings in the guide Heat Pumps in Older Buildings.
Insulation, energy label and standby losses
A hot water cylinder constantly loses heat to the room in which it is installed, even when no water is drawn. These standby losses depend on the insulation thickness, insulation material, cylinder size and temperature. Under EU Regulation 812/2013, cylinders up to 500 litres carry an energy label whose efficiency class is derived from the standing loss in watts. So do not compare only the class but also the loss value stated in the data sheet.
On large cylinders the insulation is often removable so that the vessel fits through narrow doors. According to the manufacturer, the TWL solar cylinder from 800 litres has 80 millimetres of Neopor and 20 millimetres of polyester fibre fleece, the smaller sizes 50 millimetres. Also pay attention to insulated connections: uninsulated pipe sockets and a missing thermosiphon loop allow heat to escape through natural circulation. A cylinder in an unheated cellar loses more heat than one in a heated plant room.
Legionella and temperature: what the rules require
DVGW worksheet W 551 describes the technical measures against legionella in drinking water installations. It distinguishes between small and large systems and specifies minimum temperatures at the water heater outlet and in the circulation for them; a value of 60 °C at the cylinder outlet is frequently cited. The current edition of the worksheet and the specifications of the design engineer are decisive. The German Drinking Water Ordinance (section 31) links the obligation to test for legionella, among other things, to storage water heaters with a content of more than 400 litres or to more than 3 litres of content in a pipe between the water heater and the draw-off point, with the circulation pipe not counted. Status: October 2026.
For the choice of cylinder this means: the cylinder and the heat generator must reliably reach the required temperature. With heat pumps this is often done by regular heating via the compressor or an immersion heater, as described in the guide Legionella Function. Hygiene cylinders and fresh water stations store little drinking water and are therefore often used in systems with low temperatures. Regardless of the cylinder type: water must not stand for long, and the cylinder should match actual consumption.
Corrosion protection, installation and connection
Enamelled steel cylinders are protected on the inside by the enamel layer. As enamel is never completely free of pores, a sacrificial magnesium anode or an impressed current anode additionally protects the defects against corrosion. A magnesium anode is consumed and must be checked according to the manufacturer's instructions. An impressed current anode made of titanium works with protective current via a control unit, for example the Westech anode for cylinders up to 1,000 litres, and is not consumed.
When installing, allow enough space for connections, maintenance and later anode replacement. Check the tilt dimension, weight and door widths, because a large cylinder is already considerably heavy when empty; the exact values are given in the manufacturer's data sheet. Every closed domestic hot water cylinder needs a safety group with a safety valve, often supplemented by an expansion vessel for drinking water. Installation, connection and commissioning are the job of a specialist company. On request, our installation partners take care of installation, commissioning and maintenance at the customer's premises.
Frequently asked questions
Can I keep using my old cylinder with a new heat pump?
Only if the heat exchanger surface is sufficient. Many boiler cylinders have a coil that is too small for the low flow temperatures. Have the surface compared with the heat pump manufacturer's specifications.
What is the difference between a buffer tank and a hot water cylinder?
The buffer tank contains heating water and stores heat for the heating system. The hot water cylinder contains the water that is drawn at the taps.
When is a cylinder with two heat exchangers worthwhile?
When two heat sources are to be integrated, for example a solar system in the lower part and a boiler or heat pump in the upper part. This also prepares the system for retrofitting solar later.
How often does the protective anode need to be checked?
The intervals are set by the cylinder manufacturer. With magnesium anodes, regular checks during maintenance are common; impressed current anodes show their status on the control unit.
Conclusion
Choosing the right hot water cylinder means matching the cylinder type, size and heat exchanger to the heat generator and actual demand. Heat pumps need a much larger coil surface than boilers, and solar systems benefit from a second heat exchanger. Good insulation reduces standby losses, and the temperature requirements of DVGW worksheet W 551 should be planned from the outset. Oversizing brings no advantage, only higher losses and longer residence times. You will find cylinders from TWL and Heizkraft as well as suitable accessories in our Hot Water and Buffer Tanks category.
