Quantity   Component name
1 × Raspberry Pi I think that no further introduction is need !
1 × Custom but simple electronic "brain" Specific modules that control 8 "muscles", less that 20$ each.
1 × Custom but simple electronic "muscles" Generic modules, less that 15$ each per radiator.
1 × 12V electric valve Two for each radiator. Prices depend on the source. I know a guy... :))
1 × The Internet ! (or a local network) What would we do without them ?!