Collection of links:

• (Not openhr20) Sygonix via bluetooth: http://torsten-traenkner.de/wissen/smarthome/heizung.php
• 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
• PoE PSE: https://www.maximintegrated.com/en/products/power/power-switching/MAX5980A.html/tb_tab0

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.

Using open HR20:
http://www.mikrocontroller.net/articles/Heizungssteuerung_mit_Honeywell_HR20
https://www.smarthomatic.org/wiki/doku.php?id=hr25_thermostat
De facto standard firmware:
https://github.com/bruce33/openhr20
Latest source is now here
https://github.com/OpenHR20/OpenHR20
Antenna for 433/866MHz:
https://github.com/OpenHR20/OpenHR20/wiki
RF module:
HopeRF RFM12B 868Mhz Wireless Transceiver Module
Differences between RFM12 and RFM12B and functional description:
http://www2.htw-dresden.de/~wiki_sn/index.php5/RFM12
Connection between controller and RFM12B:
http://www2.htw-dresden.de/~wiki_sn/index.php5?title=Datei:Sn_master.png&page=1
Forum:
http://embdev.net/topic/118781?page=3
Bridging RS-232 to Wifi:
http://www.esp8266.com/viewtopic.php?p=19952
Bridging RS-232 to powerline:
http://thelastinstance.de/elek/project03/project03_1.phtml
http://www.amateurfunkbasteln.de/230vmod/230vmod_ger.html
http://www.mikrocontroller.net/topic/322092
Ebay: USR-TCP232-T-EN.pdf
openhr20 mit Ethernet-Controller ENC28J60 von Microchip:
http://www.mikrocontroller.net/topic/17603
Powerline adapter with firmware:
http://wiki.openwrt.org/toh/devolo/dlan-usb-extender
Programming Thermostat with AVRDRAGON in linux:
https://piontecsmumble.wordpress.com/2013/02/25/openhr-20-tutorial-part-0-introduction/
https://piontecsmumble.wordpress.com/2013/03/03/openhr-20-tutorial-part-1-mechanics/
https://piontecsmumble.wordpress.com/2013/03/06/openhr-20-tutorial-part-2-flashing-the-thermostat/
https://piontecsmumble.wordpress.com/2013/03/11/openhr-20-tutorial-part-3-soldering-the-rfm12b-radio/
https://piontecsmumble.wordpress.com/2013/03/11/openhr-20-tutorial-part-4-building-the-master-board-using-arduinoatmega328p/
https://piontecsmumble.wordpress.com/2013/03/20/openhr-20-tutorial-part-5-deploying-the-control-interface-on-openwrt/
POE Power over Ethernet:
https://store.arduino.cc/product/X000002
https://www.arduino.cc/en/uploads/Main/PoE-datasheet.pdf
=====
I've build a HM-11 Module into a "prototype" HR20-BTLE Version.
====
FYI,

I just got around to programming the rest of my thermostats, and had to 
do the following:

sudo avrdude -p m169 -c dragon_jtag -e -B 12 -U flash:w:hr20.hex
sudo avrdude -p m169 -c dragon_jtag -U hfuse:w:0x99:m
sudo avrdude -p m169 -c dragon_jtag -e -B 12 -U flash:w:hr20.hex -U 
eeprom:w:hr20.eep -U hfuse:w:0x91:m

If I didn't flash the hex first, I was not able to set the fuse.
====
Security of HR20:

The best description I know of is in repository, 
rfmsrc\doc\hr20-security.pdf
====
HR25:
https://sites.google.com/site/slangey/misc/honeywell-hr25
(Compile with make hr25. The resulting hex file will be hr25.hex)





http://www.mikrocontroller.net/articles/Sparmatic_Heizungsthermostate
http://www.mikrocontroller.net/articles/Heizungsthermostat
http://www2.htw-dresden.de/~wiki_sn/index.php5/HR20
http://www2.htw-dresden.de/~wiki_sn/index.php5/HR20/flashen
http://forum.fhem.de/index.php?topic=39897.0


https://www.domoticz.com/forum/viewtopic.php?f=12&t=9122

http://www.mikrocontroller.net/topic/237375

Android on virtualbox:
http://www.android-x86.org/releases/releasenote-5-1-rc1

Bluetooth Gatt devices:
(enable communication with Bluetooth Smart or Smart Ready devices)
http://www.instructables.com/id/How-to-Create-an-Arduino-Compatible-Bluetooth-40-M/
http://ladvien.github.io/robots/HM10/
http://developer.android.com/reference/android/bluetooth/BluetoothGatt.html
http://www.jnhuamao.cn/index_en.asp?ID=49
http://torsten-traenkner.de/wissen/smarthome/heizung.php   <--- mit Beispiel

Hersteller eurotronic:
http://www.eurotronic.org/produkte/comet-blue.html
App runterladen bei:
https://apps.evozi.com/apk-downloader/
com.eurotronic_technology_gmbh.europrog.app