I'd like to hear your thoughts on a Thermostat kit design

The power stealing best I could figure was using the furnace relays and common as a power source.

I saw a "device" that is powered by proximity of the mains wire (msp430 + glass lcd), you put it on the wall and it works, no batteries only a torus like thing inside... it was some demo I saw ~1y ago but no schematic / explanation ... I hoped there's something like that in general use .. (as that what "power stealing" triggered in my mind) ...

anyhow, I do not believe such way of powering the device would be suitable for temp sensor