Anyone who is currently building a house or renovating an apartment will inevitably be faced with the question: instantaneous water heater or DHW cylinder? We show what makes the most sense when and where. We first look at the difference and then compare the advantages and disadvantages of both devices.

What is the difference?

In a instantaneous water heater, the water is heated by electricity or natural gas as it flows through the appliance. A hot water tank, on the other hand, is almost always connected to a heating system or a solar thermal system, which heats the water stored in the boiler.

The instantaneous water heater at a glance

Small but oho! The instantaneous water heater takes up little space due to its compact shape. Below you will find the advantages and disadvantages of the device.


The instantaneous water heater requires significantly less space than a hot water cylinder. This is because no water is stored in the instantaneous water heater. Rather, as the name suggests, it is heated as it passes through the appliance. Since no water tank is required to collect the water to be heated, the risk of germ formation is also extremely minimised.

In addition, a instantaneous water heater is considerably cheaper than a hot water tank and can be installed for just a few hundred euros. Installation is also much simpler. The non-existent water storage tank is also an advantage in terms of energy efficiency, as no energy can be lost if it is not used.

Flow-through heaters are usually installed close to the tap so that no pipe losses can occur. Furthermore, it does not require heating times like a boiler and supplies an unlimited amount of hot water. If several bathtub fillings are poured in one after the other, the capacity of a hot water tank is quickly exhausted.

Thanks to its compact design, the instantaneous water heater can also be easily integrated into any bathroom – of course, you can also adapt your shower cubicles flexibly to these conditions.


The supply of several taps is traditionally difficult for instantaneous water heaters. For example, cold surprises can occur during showers when someone takes hot water from another tap. In extreme cases, the water flow is so low that the instantaneous water heater switches off completely and only cold water comes out of the shower.

However, it is normal for only cold water to come out of the shower for a short time when it is first turned on, as it always takes some time for the appliance to start up. It can take some time to quickly remove warm water for a cat wash. The phenomenon that it takes some time for warm water to flow is also called cold water drain.

But not only temperature fluctuations belong to the disadvantages of the instantaneous water heater, also a high primary energy consumption counts to it. To heat water, the device converts electricity into heat. This means that more energy is lost than, for example, with gas heating, where a fossil fuel is converted directly into heat. Furthermore, a continuous-flow heater cannot be operated with oil, district heating or renewable energies.

The hot water cylinder at a glance

Although the hot water tank is more expensive to purchase, it can often save you money in the long term.


The biggest advantage of the DHW cylinder is that it can be connected to an existing heating system. This makes it much more environmentally friendly than the instantaneous water heater. This is referred to as an indirect DHW cylinder. This means that the heat from a heating system, such as a boiler, a heat pump or a solar thermal system, is transferred to the stored water. In order to avoid supply and radiation losses, you should place the hot water storage tank near your boiler.

Hot water tanks have a high water recall capacity at a constant temperature. For example, several people can shower with hot water at the same time. Provided you have heated enough water beforehand. In principle, however, it is possible to operate several taps without any problems.

The device is also suitable for storing water with a high water hardness, which is particularly the case with water containing lime. Furthermore, the use of renewable energy sources is possible with a hot water storage tank.


In general, the hot water storage tank impresses with its high energy efficiency, but energy can be lost if there are supply losses due to radiation. Adjust the appliance accordingly to your needs. This will save you money if you are on holiday, for example, and do not need the hot water tank. If you have switched off the storage tank for a longer period of time, you will have to expect a long heating time.

The DHW cylinder is much more bulky than the instantaneous water heater. When installing the appliance, you must therefore take the higher space requirement into account. You should also not underestimate the risk of germ formation. To prevent this, you should heat the water at least once a day above 60 degrees.

Although the hot water cylinder reliably supplies several taps with hot water at the same time, at some point even the largest tank will reach its limits. You should therefore consider exactly how many people need to be supplied in your household beforehand.

We hope that we were able to give you an overview of the most important advantages and disadvantages. The decision for or against one of the two systems should now be somewhat easier for you, so that you can soon relax with sufficient warm water in your own shower. For more water heater information and tips from Water Tech Advice see their tumblr updates today.

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