Knowledge base
How to select the capacity of a buffer tank for a heating system?
An undersized buffer means energy losses and failures. Learn the methodology for selecting buffer tank capacity and avoid the most common mistakes.
Why capacity selection matters
This is how much energy a correctly sized buffer tank saves compared to a system without a buffer.
The estimated proportion of heating systems with a buffer tank of insufficient capacity, which reduces their efficiency.
The operating temperature range of the buffer tank that ensures the highest efficiency when working with a heat pump.
Steel buffer tanks, with proper maintenance, last for more than two decades.
The role of the buffer tank in a heating system
A buffer tank (accumulator tank) serves as a thermal energy store between the heat source and the distribution system. Its purpose is to compensate for the difference between the output of the boiler or heat pump and the building's instantaneous heat demand.
Without a buffer, the heat source must respond to every change in demand — leading to frequent cycling (switching on and off). In the case of solid-fuel boilers, cycling reduces combustion efficiency and increases emissions. Heat pumps lose performance with frequent compressor starts.
A buffer tank also enables the integration of multiple heat sources — such as a heat pump, solar collectors, and a peak-load boiler — into one cohesive system. The buffer then acts as the central hydraulic node.
In heat pump installations, a buffer is often required by the manufacturer as a condition for maintaining the warranty. The absence of a buffer can lead to compressor overheating and premature wear of the unit.
Capacity selection methodology
The basic formula for selecting buffer tank capacity is based on the output of the heat source: V = P × t / (c × ΔT), where V is the capacity in litres, P is the output of the source in kW, t is the minimum operating time in seconds, c is the specific heat capacity of water (4.186 kJ/kg·K), and ΔT is the difference between the flow and return temperatures.
In practice, simplified conversion factors are used. For solid-fuel boilers, 20–50 litres per 1 kW of output is assumed. For heat pumps — 15–25 litres per 1 kW. For gas boilers a buffer is optional but recommended at 10–15 l/kW when integrating with renewable energy sources.
Example: a heat pump with an output of 12 kW requires a buffer with a capacity of 180–300 litres (12 kW × 15–25 l/kW). A pellet boiler with an output of 25 kW requires a buffer with a capacity of 500–1,250 litres (25 kW × 20–50 l/kW).
When selecting the buffer, you should also take into account the water volume of the installation itself (radiators and pipework). An installation with a low water volume (e.g. underfloor heating) requires a larger buffer than one with conventional radiators.
The most common mistakes in buffer tank selection
Undersized capacity is the most frequent mistake. Installers select a buffer that is just about adequate based on minimum catalogue values, without accounting for the specific characteristics of the building. The result: the heat source continues to cycle, and energy savings are minimal.
Overlooking thermal stratification — a buffer tank operates efficiently only when a temperature layering is maintained (hot water at the top, cool water at the bottom). An insufficient tank height or incorrectly positioned connections disrupt stratification and can reduce usable capacity by as much as 30%.
Mismatch with the heat source's operating mode — a solid-fuel boiler with manual loading requires a significantly larger buffer (40–50 l/kW) than an automatic pellet boiler (20–30 l/kW), because it must store the heat from an entire fuel charge.
Failure to account for future expansion — if the addition of solar collectors or a heat pump is planned, the buffer capacity should take this into account at the design stage. Replacing the tank after a few years is an unnecessary expense.
The impact of a buffer on energy efficiency
A correctly sized buffer tank extends the operating cycles of the heat source, which directly translates into higher efficiency. A solid-fuel boiler operating in long cycles achieves an efficiency of 85–92%, whereas with frequent cycling this drops to 60–70%.
In installations with a heat pump, a buffer allows heat production to be shifted to off-peak tariff hours (e.g. the night-rate G12 tariff). The stored energy is then released into the installation during the day, without the need for the compressor to operate during peak hours.
A buffer also enables the efficient use of surplus heat from solar collectors. Without a buffer tank, collectors must be shut down once the set temperature is reached, wasting available solar energy.
In homes with underfloor heating, a buffer tank stabilises the supply temperature, preventing fluctuations and improving thermal comfort. Underfloor heating responds to temperature changes with a delay of several hours, which is why a stable supply is essential.
Installation aspects and material selection
The location of the buffer tank affects heat losses and installation costs. The optimum location is a boiler room or utility room close to the heat source, which minimises the length of connecting pipework and distribution losses.
Buffer tanks with a capacity of more than 500 litres require the load-bearing capacity of the floor to be taken into account (500 l of water equals 500 kg, plus the weight of the tank itself). In many cases it is necessary to place the tank on a foundation in the basement or at ground level.
The tank material should be selected to match the parameters of the installation. Carbon steel with an anti-corrosion coating is the standard choice for heating installations in closed systems. Stainless steel is recommended for open systems and domestic hot water (DHW) installations where the water is regularly replaced.
Pressure tanks (up to 6 bar) require UDT approval. During the design phase, it is worth planning the appropriate number and placement of nozzles upfront — for connecting the heat source, the heating circuit, temperature sensors, and the safety valve.
Related products from our offer
Pressure tanks for hot water
Buffer and accumulator tanks for heating and DHW installations, with capacities from 200 to 10,000 litres, with thermal insulation.
View details →Pressure tanks for liquids and gases
Pressure tanks with UDT approval for use in closed heating circuits, with a working pressure of up to 6 bar.
View details →Stainless and acid-resistant tanks
Stainless steel buffer tanks for DHW installations and open systems, providing the highest resistance to corrosion.
View details →Frequently asked questions about buffer tanks
Looking for a custom-sized buffer tank?
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