Buffer vessels are simply a duplex stainless steel tank that contains a volume of water, increasing the overall volume of the heating distribution system. This extra volume of water is designed to absorb any extra heat generated by the heat pump in low load conditions. If the building cannot absorb the extra heat generated the return temperature to the heat pump will increase and will potentially cause the heat pump to turn off. If the temperature drops in the water returning to the heating appliance, then it will turn itself back on. If this cycling occurs at a greater frequency than the minimum run time for the appliance, this is classed as short cycling. The aim of a buffer vessel is to remove the possibility of any short cycling of the heating appliance. These buffer vessels are designed to work at up to 3.5 bar pressure for primary water only. They should not be used for potable domestic hot water and are not approved for this purpose.