Buffer storage tank for heat pumps: Yes or no?

You are thinking about replacing your old heating system with a heat pump. Maybe you have already read up on the first technical details, and now one term keeps coming up again and again: buffer storage tank. What is it actually supposed to do, and do you need one at all?

A buffer storage tank is basically nothing more than an intermediate storage unit for heat. It absorbs surplus heating energy when it is not needed directly in the house and releases it again later. That sounds sensible at first, but it also has disadvantages.

Because: Every storage tank loses heat. And the larger its volume, the higher the heat losses. That is why it is not automatically the better solution, but rather a question of technology and planning.

In this article, you will learn when a buffer storage tank with a heat pump is absolutely necessary, what types there are, how large the heating water storage tank should be, and in which cases you can do without it. We also show you where typical mistakes happen and what must be considered when it comes to hydraulic integration.

Does a buffer storage tank make sense with heat pumps?

Whether a buffer storage tank makes sense depends above all on the type of heat pump and on how the heating system for heat transfer in the house is set up. In the past, almost exclusively so-called on/off heat pumps were installed.

These devices only knew two states: full output or none at all. As soon as they were running, they delivered 100% of their heating output, even if the building only needed a fraction of it at that moment. The result: The heating system constantly switched on and off. And that is exactly what is poison for the technology.

A heat buffer was therefore absolutely necessary with on/off heat pumps. It could temporarily store surplus heating energy and ensured that the heat pump cycled less often (switched on and off). This extended its service life and made operation more efficient.

Today, things are different. Modern heat pumps usually work with inverter technology. They continuously adapt their output to the actual heat demand. As a result, the system runs for longer at a time, but with reduced output. In many cases, this makes large buffer storage tanks unnecessary.

Nevertheless, a heating water storage tank can also make sense or even be required with inverter heat pumps. For example, if an additional second heat generator is used, such as a gas boiler, a solar thermal system, or a wood stove.

In such multivalent, so-called hybrid systems, the storage tank takes on an important role: It collects heat from several sources and makes it available when needed.

Another reason can be a special heat pump tariff. Some electricity suppliers switch off the power for the heating system at certain times, as they can earn money with interruptible loads. During this time, the buffer storage tank takes over the supply of the house. Without it, heating would not be possible during this phase.

There is another reason for a buffer storage tank with air-to-water heat pumps. At low outdoor temperatures, they have to be defrosted from time to time. For defrosting, heat is used that has previously been stored in a buffer storage tank.

But there are also counterexamples: With monovalent ground-source or groundwater heat pumps in combination with a well-balanced underfloor heating system, a buffer storage tank may not be necessary at all. Here, the buffer capacity of the underfloor heating is sufficient to bring the heat directly and evenly into the house without detours via the heating water storage tank.

6 clear advantages of a buffer storage tank with a heat pump

A buffer storage tank is no longer a must with an inverter heat pump. But in many cases it offers tangible advantages, especially when the conditions are right.

Here we show you six points that speak in favor of using one:

1. Less cycling – longer service life: Heat pumps react sensitively to frequent switching on and off. In unfavorably coordinated inverter systems, a heat buffer can extend the runtime of the heating system. This reduces wear on the compressor and increases service life. This is also one reason why on/off devices used to always be installed with a buffer storage tank.
2. Stable heating energy despite temperature fluctuations: On cold days or when outdoor temperatures fluctuate strongly, the storage tank buffers short-term output peaks. This keeps the heating system running more calmly and evenly, even when heat demand in the house fluctuates.
3. Access to favorable heat pump tariffs: Many grid operators offer discounted electricity tariffs. In return, however, they require permission to switch off the system temporarily: up to three times a day, for two hours each time. A sufficiently large heating water storage tank bridges these blocking periods without the house becoming noticeably cold.
4. Ideal for hybrid heating systems: As soon as additional heat generators come into play, such as a gas boiler, a wood stove with heat exchanger, or a solar system, the buffer storage tank becomes the central interface. It absorbs the temperature from different sources and then releases it again as needed.
5. Reliable defrosting for air-to-water heat pumps: When the outdoor temperature drops, the evaporator on the outdoor air heat exchanger freezes over. The heating system then needs energy to defrost itself. This heat then comes from the buffer storage tank.
6. Individual room control possible with underfloor heating: Normally, a heat pump runs at a constant flow temperature. But if you want to control several rooms individually, this can lead to problems. The buffer storage tank acts as a buffer zone between heat generation and heat distribution and gives you more flexibility without causing the system to fall out of sync.

What are the disadvantages of buffer storage tanks?

A buffer storage tank offers many advantages, but not for free. In some cases, the disadvantages outweigh them, especially if it is oversized or incorrectly integrated. Here are the most important points you should know:

• Heat losses are unavoidable: Every heating water storage tank loses heat, for example through the insulation and through the type of hydraulic integration. Especially with so-called parallel buffers, flow and return water mix. The result: The flow temperature must be raised additionally in order to provide enough heat at the radiator in the end.

• Space requirements can become a problem: A large heat buffer needs space. Not only for the tank itself, but also for the pipes and everything around it. In a single-family house, the space required by a heat pump with buffer storage tank is often a limiting factor. Especially when there is already little technical space available.

• The hydraulics become more complex: With a storage tank, the system becomes more elaborate. Additional pumps, more sensors, nested control, all of this increases susceptibility to faults and makes commissioning and troubleshooting more difficult if something breaks.

• Parallel buffers reduce efficiency: Many specialist companies rely on the parallel buffer because it can be integrated into almost all existing systems without problems. But it causes additional losses because the heat does not go directly from the heat generator to the heating system.

• Costs rise – not only during installation: A heat buffer costs money not only to purchase, but also to operate. Investment costs are higher, as are maintenance and control effort. In addition, there are ongoing and higher heat losses.

How to find the right size for your buffer storage tank

A heating water storage tank should always be as small as possible and only as large as necessary. Because every liter of water stores heat, but also brings losses with it. The right size depends on the output of your heat pump and the type of heating system.

The rule of thumb is: 15 to 30 liters per kilowatt of heating output.

The exact value depends on the intended use:

• approx. 15 l/kW is often sufficient if you only want to reduce cycling of the heating system.
• approx. 30 l/kW is needed if additional blocking periods in the electricity tariff are to be bridged or several heat generators are integrated.

With a sole heat pump system (monovalent) with low output, a small storage tank of 100 liters is often sufficient, especially if the system is not oversized but rather slightly undersized and there are no other special requirements.

In hybrid systems, the need increases significantly. Here, the buffer does not only serve to prevent cycling, but also as an energy store for additional sources such as solar thermal energy or wood heating. It also becomes important when using favorable heat pump tariffs if power shutdowns have to be factored in.

For orientation, here are a few example sizes according to heating output for monovalent systems:

Series or parallel buffer? The better choice for your house

Not every buffer storage tank works the same way. What matters is how it is hydraulically integrated. Two variants have become established: series buffer and parallel buffer.

Both fulfill the same task: They store heat, but they do it in different ways.

With the parallel buffer, the heating circuit and heating system run side by side via the storage tank. The heat pump charges the buffer, while the heating system simultaneously draws heat from it.

That sounds simple, but it has a catch: In the buffer, the warm flow water mixes with the cooler return water. As a result, less temperature reaches the radiator. To compensate for this, the heating system has to generate a higher flow temperature and loses efficiency in the process.

Nevertheless, the parallel buffer is today the most common solution in existing buildings. The reason: It allows clean separation of heat generation and heat distribution, even when the existing pipe network or heating surfaces are not optimally designed. For many specialist companies, the parallel buffer is the safest way to guarantee a stable system.

The series buffer works differently. Here, the heating water from the heat pump first flows through the heating surfaces and only then through the buffer. There is no mixing, and the flow temperature remains constant.

However, the series buffer requires that the hydraulics in the house are well balanced. It is especially suitable for new builds or renovated systems in which only one monovalent heating system is responsible for heat supply.

If your house already has an evolved heating structure, the parallel buffer is usually the more pragmatic choice, even if it works somewhat less efficiently.

5 typical mistakes and how to avoid them

A buffer storage tank can increase the efficiency of your heating system – or do exactly the opposite. What matters is how it is planned and integrated.

Here are five mistakes you should avoid:

1. The storage tank is too large or too small: A heating water storage tank that is too large loses unnecessary heat. One that is too small cannot fulfill its task. Use the heating output of your heat pump as a guide and calculate with 15 liters per kilowatt. More only makes sense if blocking periods are to be bridged or additional heat generators are used.
2. The hydraulics are incorrectly designed: The heat buffer must be correctly integrated, otherwise unnecessary losses may occur. Especially with parallel buffers, incorrect placement of the connections quickly leads to mixed temperatures. The result: The heating system has to run hotter than necessary.
3. Integration into the SmartGrid is missing: Many electricity suppliers offer favorable heat pump tariffs with blocking periods. Without a suitable storage tank, however, this advantage cannot be used. Anyone who overlooks this during planning gives away long-term savings potential.
4. The control is not optimized: After installation, the work is not done. A poorly adjusted system rarely runs efficiently. Have the specialist company show you how to adjust the most important parameters or arrange regular remote maintenance directly.
5. Night setback continues to be used: What made sense with gas heating interferes with a heat pump system. Night setback leads to strong load peaks in the morning, exactly what you want to avoid. Anyone heating with a heat pump should simply leave it out.

Bonus tip: Combine buffer and storage smartly

Many systems use two storage tanks at once: a buffer storage tank for heating and a domestic hot water tank for hot water. It is important that both fit together and match the output of the heat pump.

If domestic hot water preparation runs via the heating heat pump, the buffer storage tank should not be too small. Because during domestic hot water charging, it is not available to the heating circuit. Depending on the system, this can lead to comfort losses. That is why many rely on a combination of buffer storage tank and separate hot water tank.

An alternative is the hot water heat pump without storage tank, meaning a compact hot water heat pump. It works independently of the heating system and can also be combined with existing buffer storage tanks. This brings advantages especially in summer, because the large heating heat pump can then simply pause. This also reduces noise emissions at the outdoor unit of the air-to-water heat pump.

If you also use a PV system, you can use the buffer storage tank to convert surplus electricity into heat. This is technically simple and improves self-consumption, provided that the heating water storage tank is designed for it and the control system cooperates.

Heat pump buffer storage tank – Yes, but well planned

In many cases, the buffer storage tank makes the heat pump more efficient, longer-lasting, and more flexible. What matters is whether your system benefits from it and how you integrate it into the system.

Too large, incorrectly integrated, or unnecessarily complex, and the buffer quickly becomes an energy guzzler. Correctly sized and well planned, on the other hand, it can reduce cycling, lower electricity costs, and distribute heat better.

Important: There is no off-the-shelf solution. Every house, every system needs an individual assessment. That is exactly what our free heat pump check helps you with. With a few details, you get an initial assessment of whether your house is even suitable for a heat pump and what you should pay attention to.