What is a buffer Vessel

What is a buffer vessel?

As with any good renewable energy heating system, correct sizing of the primary heating components is a must. The buffer vessel is in essence a large insulated water tank that looks (but does not act) like an immersion heater tank you will find in the airing cupboard. This tank will vary in size dependent upon the requirements of the system and is fairly expensive because of the components inside and how it operates. This tank is in essence a thermal store; it is a battery or capacitor for your biomass system, allowing a burst of heat when your boiler is warming up and its function is to balance the system.Inside a buffer Vessle - biomass boiler

Even though some biomass boiler manufacturers state that one may not be required by their specific unit, this is not to say it won’t assist the overall solution to your heating requirements and increase the system’s efficiency in the long run.


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How does a buffer vessel work?

The buffer vessel acts as an energy store that releases its stored hot water into the heating system whilst the boiler gets up to temperature. The various states in which the system will operate are detailed below:

Buffer-Vessel-diagram no load storing energy

The biomass boiler will fire and get up to temperature, when it does the termostat will start the first pump (A) and begin filling the buffer vessel if there is no heating requirement. The thermostat controlling pump (A) prevents the water from circulating through the the system for 2 main reasons:

  1. To prevent condensation forming on the boiler
  2. Prevents the dilution of existing heat in the thermal store.

Buffer-Vessel-diagram load supplied from boiler

When heating is demanded and the main thermostat and timer kick in whilst the biomass boiler is on and operating, the thermal store will be, in escence, bypassed.

Buffer-Vessel-diagram load supplied from boiler discharges




If the main thermostat and timer come on and the boiler is not either in operation or up to temperature, then the thermal store discharges into the heating system to provide warmth whilst the boiler fires up.

Buffer vessel or thermal store?

The function of the buffer vessel tank can be used either as a simple ‘Buffer Vessel’ or as a ‘Thermal Store‘. The thermal store method generally requires a larger sized unit along with a variable speed pump and other temperature monitoring equipment to function correctly, however this method offers increased system efficiencies and a more ‘refined’ overall solution. Speak to your installer about what would be best suited to your installation.

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