The HIU (Heat Interface Unit) belongs to E.On Heat. This unit is responsible for hot water and heating in the flat. If you have issues with any of these, you need to call E.On Heat.
Contact details on their support page:
Useful info
The normal pressure in HIU is between 1 and 2 bars.
All valves should be in open state.
The Unit is locked by key and only an E.On Engineer should open it.
