H20 Log boiler buffer

Used for storing or buffering the heated water from the boiler so it is available for either central heating and or domestic hot water.
 

A Buffer is used as a buffer of energy between the heat source and the heating system. It ensures an automatic boiler runs for an optimum length of time, rather than starting and stopping as demand is created. 

The purpose of an accumulator is to accumulate energy. In this case the energy is heat energy from a log boiler, as a log boiler needs to burn an entire combustion chamber of fuel just once a day, it needs store all the heat energy so it can be drawn on thoughout the day.