The use of the buffer tank recommended optional for the following reasons
failure to use any other source (eg an existing boiler)
the Hot Water e.g. solar energy,fireplace,biomass
Available ranges from 100 to 1000 lt
RG suitable for storage of cold water 7-12οC (for cooling applications)
PS suitable for storage of hot water 16-95οC (for heating applications)
PSR with coil for use of an additional source of heating.