The idea was to start heating/cooling before arriving home via a browser on the android talking to a server running on an arduino with an ethernet shield
The Arduino control machine is not by the conditioners, it's in the 'electricity box' BUT, i tried and found that when the IR remote is operated by the PIRs wiring in the room, it's in the line of sight (although the incident angle is not optimal) and able to control the conditioner with some reliability. This wiring arrives to said 'electricity box'. The wiring is made of 4 telephone twisted pairs and i guess i could use one to modulate (a) powerful IR diode(s) placed by the PIR, the modulation would indeed be under the Arduino control and could send ANY command. I am happy to have posted the question here and thank you all, ronsimpson, Misterbenn, and KeepItSimpleStupid for having helped me to envisage this solution. If the reliability can not reach a 99% level with this incident angle, i'll have to use an RF bridge stuck somewhere near the IR receiver, this would imply more maintenance to change the batteries