Collection of links:

• Prosa about programmable thermostats: http://www.oeko.de/oekodoc/1594/2012-450-de.pdf
• Controlling thermostats via Ethernet, powered via PoE: https://www.arduino.cc/en/uploads/Main/PoE-datasheet.pdf
• Network interface with microcontroller (From Elektor): http://www.rs-online.com/designspark/electronics/blog/networker---an-advanced-web-server-with-a-microcrontroller
• PoE Injector: http://www.elv.ch/Der-Ethernet-Injector-PoE-I-%E2%80%93-Power-over-Ethernet-Einspeisung/x.aspx/cid_726/detail_40470
• PoE DP: http://www.elv.ch/Power-over-Ethernet-Splitter-PoE-S/x.aspx/cid_726/detail_39662

Advantage: Power is fed via Ethernet cable. Disadvantage: Occupies Ethernet cable.

[HR20] <--- Ethernet cable ---> [Converter module]

Converter module supplies both power and serial. If two HR20s should be controlled, then the same Ethernet cable can be used. Four wires per module suffice (Gnd, +3.3v, Tx (TTL), Rx (TTL) ).
HR20a:
Power(Gnd) - Rx: T568A/B White/Blue (4) -  Blue (5) cable pair
Power(+3.3) - TX: T568B White/Orange (1) - Orange (2) cable pair

HR20b:
Power(Gnd) - Rx: T568A/B White/Brown (7) -  Brown (8) cable pair
Power(+3.3) - TX: T568B White/Green (3) - Green (6) cable pair

Advantage: Power is fed via PoE Module. Can be routed further if a PoE Router is being used. HR20 with PoE PD module integrated

HR20 without PoE PD module. Power for the HR20 and Ethernet/wifi to RS232 converter is fed via a separate power supply.

Each HR20 is per default connected to the master connecter. An additional slave output can be used to connect another slave.

Adapter cables convert RJ45 to DIP10. This requires a small module with multiple RJ45 connectors.