What is the difference between a volumiser, accumulator and buffer?

What is the difference between a volumiserm accumulator and buffer

Navigating the world of home heating systems can be challenging, especially when it comes to understanding specific components like accumulators, volumiser and buffers. Let’s demystify these terms, breaking down their functions, differences, and how they integrate into heating system.


As the name suggests, its main purpose is to accumulate energy. In this case, the energy is heat energy from a log wood, wood chip, or pellet boiler. For clean and efficient operation, combustion must proceed in a controlled and steady manner. A correctly sized accumulator ensures that the boiler maintains a high temperature, around 80°C, without needing to choke the fire. This prevents smoke and tar, which reduce the efficiency and lifespan of the boiler. The accumulated heat is then used on demand from the accumulator. In very cold weather, the property may use this energy in a matter of hours; most of the winter, it’s over the course of 24 hours; and in the summer, due to the accumulator’s high insulation, the heat will be available for heating domestic water for several days. Accumulator designs typically include stratification columns and snorkels for log boilers, with options for additional stratification plates, solar coils, and electric heating elements. Sizes range from 1,500 to 10,000 liters, and multiple accumulators can be connected in parallel.


As its name suggests, the vessel is used as a buffer of energy between the heat source and the heating system. Because automatic biomass boilers respond more slowly to heating demand than fossil fuel boilers, a buffer is used between the boiler and the heating system. When there is a demand, the heat drawn from the buffer signals the biomass boiler to ignite and produce heat. The buffer also ensures that when the automatic biomass boiler starts, it runs for an optimum length of time, irrespective of the amount of heat used by the building. The boiler then stops when the buffer is fully charged, waiting for the heating system to use the energy produced, and then repeats the cycle. This process, regulated by sensing the water temperature and aided by the buffer’s high insulation, can prevent the need to fire again for over a day during periods of light load. The buffer significantly impacts boiler efficiency, reducing wear and tear, emissions, and fuel consumption. Buffer sizing is based on the biomass boiler’s output but may be oversized for large district heating systems. Buffers are typically much smaller than accumulators used with log boilers and feature similar construction with stratification columns and snorkels. Buffers also come with options for additional stratification plates, domestic hot water coils, solar coils, and electric heating elements. Sizes range from 1,500 to 10,000 liters, and multiple buffers can be connected in parallel.

The term “buffer/accumulator” is often poorly explained and confusing. These devices are best described as small buffers performing many functions, including some buffering and accumulation. Their ability to do both is limited due to their generally small size, typically 200 to 500 liters. They are used for small heating demands and to produce domestic hot water via an internal coil or external heat exchanger. They also integrate multiple primary sources such as wood stoves, fossil fuel boilers, heat pumps, solar thermal, and electric heating. Though smaller, they share similar construction features with buffers and accumulators, including options for domestic hot water coils, solar coils, and electric heating elements.


Volumisers simply increase the volume of water in a system. In the case of a heat pump (HP), if the total water volume could starve the HP of the correct flow through the DX condenser/heat exchanger, a volumiser is inserted to correct this potential shortfall. The issue with HPs is that the flow temperatures are usually lower than those in fossil fuel or other fuels like biomass, making HP sizing critical to the actual calculated heating demand of the property. There’s much discussion on LinkedIn about removing unnecessary flow restrictions when applying HPs, with CoP being a crucial detail, but that’s a topic for another time.

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