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.