When requesting a quote for the installation of a heat pump system, we may well come across solutions that appear to contradict one another. One contractor plans to install a buffer tank alongside the heat pump, whilst another maintains that a modern inverter-controlled unit does not require one at all. Furthermore, there isn’t necessarily agreement on whether, if a buffer tank is required, it should be 50, 100 or even 200 litres in capacity.
There is a simple reason for this: With a heat pump, a buffer tank is not always necessary, and there is no single buffer tank size that applies to all heat pump systems.
Whether a buffer tank is required for a heat pump depends, amongst other things, on This is determined by the manufacturer’s specifications for the selected heat pump, its minimum capacity and required flow rate, the water volume of the heating system, the type of heat emitters, zone control and the hydraulic design of the entire system. The selection of the buffer tank is therefore part of the design of the heat pump system.
What is a buffer tank, and what does it do in a heat pump system?
The buffer tank Essentially, it is a water-filled, thermally insulated tank that forms part of a heat pump heating system – or, if designed appropriately, a cooling system. It can be used to increase the volume of water circulating in the system, but depending on the design it can also perform hydraulic tasks.
This is important because, for the heat pump to operate, the system may require a sufficient quantity of water that can be continuously circulated, as well as an appropriate flow rate. A properly designed buffer tank can help to ensure that these conditions are maintained even when, for example, several heating circuits are shut off due to room-by-room control.
It is important, however, that The buffer tank is not the same as the domestic hot water tank (DHW tank). The buffer tank contains the heating/cooling system’s working fluid, whilst the DHW tank is responsible for supplying hot water to taps, showers and other water supply points.
Buffer tank vs. DHW tank
| Buffer tank | HMV tank |
|---|---|
| It stores the fluid used in the heating/cooling system | Stores hot water for domestic use |
| It supports the operation of the mechanical system | It provides a hot water supply |
| It can increase the system’s effective water volume | It is sized to meet the household’s hot water requirements |
| In certain configurations, it can also perform hydraulic isolation | That’s not his job |
Why is a buffer tank installed alongside the heat pump?
In a heat pump system, there may be several different reasons for using a buffer tank.
Ensuring an adequate system service capacity
The manufacturer of the heat pump may specify the minimum volume of water that must be available for the unit to operate. As there is already water in the heating system, the question is therefore whether the volume of water guaranteed to be available in the system meets the value required by the unit in all relevant operating conditions.
There may be significant differences between different heat pumps in this respect. Manufacturers’ documentation should therefore always take precedence over general sizing guidelines found on the internet.
Reducing the frequency of switching the power on and off
The second major problem is short-cycle operation, known in technical terms as ‘short cycling’. Let us assume that the heat pump is capable of delivering at least 3 kW of heat output at any given moment, whilst the building requires only 1 kW of heat output. The excess energy must be transferred somewhere. If there is little water in the system, its temperature rises rapidly, the heat pump reaches the cut-off condition and switches off. It may start up again shortly afterwards.
State-of-the-art inverter heat pumps are capable of reducing their output within a certain range, but there is a lower limit to this. Particularly during transitional periods, or where there are many small heating zones, it may happen that the current heat demand falls below the heat pump’s minimum output.
However, this does not mean that a buffer tank can be used to correct any incorrectly sized heat pump. A buffer tank can, at best, provide only a partial technical solution to the problem of a significantly oversized unit.
Support for defrosting in air-to-water heat pumps
In cold, damp weather, condensation and then ice may form on the outdoor heat exchanger of an air-to-water heat pump. The unit must defrost this from time to time, a process which requires energy that is drawn from the system. An adequate supply of water may therefore also be important for the defrosting process.
Hydraulic isolation
The buffer tank may also have another function, distinct from simply increasing the water volume: if connected correctly, it can hydraulically separate the heat pump’s primary circuit from the heat exchanger’s secondary circuit. This may be necessary in more complex systems: for example, where there are several independent heating circuits, multiple circulation pumps, or heat exchangers requiring different flow rates.
It is important to note, however, that the concepts of a buffer tank and a hydraulic isolator are not the same. Whether a tank also provides hydraulic isolation depends on its design and connection.
Do you need a buffer tank for underfloor heating?
Underfloor heating is particularly well suited to heat pumps, as it operates with a large heat-emitting surface area and a low flow water temperature. Furthermore, due to its extensive pipe network, the underfloor heating system itself can contain a significant volume of water, whilst the building structure acts as a large thermal mass.
That is why In a properly designed underfloor heating system, a separate buffer tank may not be necessary simply to increase the water volume. However, the issue becomes more complicated if each room is fitted with its own thermostat and the thermostats use motorised valve actuators to shut off the underfloor heating circuits. In such cases, even if the theoretical water volume of the entire system is large, the amount of water and the flow rate actually available to the heat pump may be significantly reduced.
Daikin’s installation documentation specifically draws attention to the fact that, where remote-controlled valves regulate the individual heating/cooling circuits, the minimum required water volume must still be guaranteed even when the valves are closed.
If you’d like to know more about why these two technologies work particularly well together, you can read about it in the Underfloor heating with a heat pump vs radiator heating: design and benefits We cover this in detail in our article entitled ….
The situation may be different with a radiator or fan coil unit
In radiator-based systems, and particularly in fan-coil systems, the available water volume may be smaller than in an extensive underfloor heating system. For this reason, the use of a buffer tank may be considered more frequently.
Heat exchangers and the need for a buffer tank
| Heat sink | Typical system property | From Puffer’s point of view |
|---|---|---|
| Underfloor heating | Large surface area, considerable water displacement and inertia | This can often be achieved without the need for a separate buffer |
| Radiator | Water volume that depends heavily on the design | A case-by-case assessment is required |
| Fan coil | Typically, a smaller water volume and rapid regulation | There may be a need for additional water volume more frequently |
However, this is merely a general trend. It is no substitute for the design of a specific system.
Heat pump buffer tank size: what capacity tank is required?
If, for example, we choose an 8 kW heat pump, what capacity buffer tank do we need? It is not possible to give a technically sound answer based on kW.
This is clearly illustrated by a few real-life examples. The Daikin Altherma documentation specifies a minimum water volume of 20 or 80 litres, depending on the design of the system in question, whilst other Daikin models may have different values. In other words, it is not even possible to derive a universal „X litres/kW” rule from the manufacturers’ specifications.
What, then, determines the size of the heat pump buffer tank?
Among other things, the designer must be familiar with:
- the heat pump’s power range and minimum deliverable power
- the minimum system service volume specified by the manufacturer
- the minimum flow rate specified by the manufacturer
- the volume of water in the pipework and heat exchangers
- the recommended minimum operating time for the heat pump
- the permissible change in water temperature
- the number and regulation of zones
- heating and, where applicable, cooling operations
- the complete hydraulic design of the system
That is why Nor is it possible to determine the size of the buffer based on the floor area of the building. Of two detached houses, each measuring 120 m² and with similar heating requirements, one may have a single large underfloor heating system, whilst the other may have several separately controlled radiator or fan-coil circuits. From the heat pump’s perspective, the two hydraulic systems behave quite differently.
How can the required buffer volume be calculated?
To understand the physics behind the sizing, it is worth starting with water’s heat storage capacity: Q = m × c × ΔT
where Q is the stored thermal energy, m is the mass of the water, c is the specific heat of the water, and ΔT is the change in the temperature of the water.
One litre of water weighs approximately one kilogram, and the specific heat capacity of water is approximately 4.19 kJ/(kg·K). It follows that 100 litres of water can absorb approximately 0.58 kWh of thermal energy when its temperature rises by 5 °C.
Let’s look at a simplified example.
Let us assume that, under the given conditions, an inverter heat pump is capable of regulating its output down to a minimum of 3 kW. Meanwhile, the active heating zone draws 1 kW of heat. The system must therefore temporarily absorb a „surplus” of 2 kW.
If we wish to ensure at least 10 minutes of continuous operation, this amounts to 2 kW × 10/60 hours = 0.333 kWh of thermal energy to be stored.
If we allow for a temperature change of 5 °C, then approximately: 0.333 ÷ (1.163 × 5 / 1000) ≈ 57 litres of effective water volume is required.
However, this still does not mean that you need to buy a 57-litre buffer tank. If, for example, there are already 35 litres of water guaranteed to be available in the pipes and the constantly open heat emitters under the given operating conditions, then the shortfall in effective volume is considerably smaller.
The aim of this example is not to help you calculate the size of the tank at home, but to demonstrate that buffer tank sizing is based on specific thermal and hydraulic principles, rather than simply a rule of thumb of one litre per kW.
Could the buffer tank be too big?
Yes. Just as with heat pumps, the saying „it’s better to have a larger buffer tank – that can’t do any harm” does not apply here either. A larger tank takes up more space, is more expensive, and loses heat over a larger surface area. Furthermore, An unnecessarily large volume of water may alter the system’s dynamics and response time.
With heat pumps, the aim is generally for the system to operate efficiently and reliably at the lowest possible required water temperature. The correct size is the one that fulfils the specific hydraulic and operational requirements.
The connection of the buffer tank also matters
The same tank may behave differently depending on how it is connected to the system.
One possibility is that the buffer is connected in series to the system, and its primary function is to increase the available water volume. In other designs, the heat pump and the heat dissipation system form a separate hydraulic circuit, whilst the buffer also provides a hydraulic isolation between the two.
If the only requirement is to increase the effective water volume, the design need not necessarily be the same as that required when the primary and secondary circuits also need to be hydraulically isolated.
Can a buffer tank reduce the efficiency of a heat pump?
A properly designed buffer tank may be a necessary or useful part of the system, but this does not mean that the buffer tank increases the COP or reduces the electricity bill.
A poorly designed system It may even have the opposite effect. The tank loses heat to its surroundings, and in the event of hydraulic separation, mixing may occur if the flow rates are inadequate. If, as a result, the heat pump has to produce a higher water temperature, this may reduce its efficiency.
In a heat pump system, the purpose of the buffer tank is to ensure favourable operating conditions for the heat pump whilst maintaining the lowest possible required water temperature.
7 questions you should ask your contractor about the buffer
If you’re comparing several heat pump quotes, it’s worth asking:
- Why is a buffer tank necessary – or why is it not necessary – in this system?
- What is the minimum water volume specified by the manufacturer of the selected heat pump?
- What is the guaranteed available water volume of the system without a buffer?
- What is the maximum capacity of the selected heat pump?
- What happens when the thermostats shut off some of the heating circuits?
- Does the buffer serve solely to increase volume, or does it also provide hydraulic isolation?
- On the basis of what calculations or manufacturer’s specifications was the size of the tank selected?
The most common mistakes when choosing a buffer tank
The most common misconceptions centre on two extreme claims. One is that a buffer tank is essential for every heat pump; the other is that a modern inverter heat pump never needs one. Neither is universally true.
It could also be a fault, if we select the buffer tank based solely on the heat pump’s rated power in kW, whilst disregarding the heat pump’s modulation, the system’s existing water volume and zone control.
This, too, could be problematic the „better safe than sorry” approach, as is the case when a tank of the correct size is fitted to the system but with incorrect hydraulic connections.
In every case, the most important starting point is the manufacturer’s documentation for the specific heat pump and the mechanical design of the entire system.
If you’re still in the process of choosing a heat pump, to compare the different technologies, the A comprehensive guide to heat pump types – air-to-water, air-to-air, geothermal, hybrid Our article entitled … provides further guidance.
To sum up: does a heat pump need a buffer tank?
Not all heat pumps require a buffer tank, and there is no single buffer tank size that applies to all systems.
First, it is necessary to determine the building’s heat demand, the heat pump and its operating range, the heat emitters, the system’s water volume, the flow rates and the control system. Only then can a decision be made as to whether a buffer tank is required, what function it should fulfil, and what the optimum tank size is for this purpose.