Daox,
Just to prime the thought-pump...
...have you Googled: app note op amp differential
...such as here:
app note op amp differential - Google Search
...or Googled: differential controller circuit
...such as here:
differential controller circuit - Google Search
Also, as far as the ethernet monitoring part of your project, Ward Cunningham, the guy who invented Wiki (the structure upon which Wikipedia is built) has offered to help me get the Homemade Heat Pump Manifesto project on line, with monitoring data publicly available in real time. Obviously, I will be sharing these development details with you as they come available to me.
Best Regards,
-AC_Hacker