this project is in development, what I've got so far is most of the web interface ( http://heizung.servehttp.com/
is the direct link to the Arduino with Ethernet shield and SD card, when it's not running you can take a look at the interface on http://goo.gl/3rxFK
). Notice it's in german language, if requested it's not a problem to translate it to english.
The heating to be controlled is not fully installed yet. There will be a heating circuit fed with hot water from a tank by a mixer. This mixer is driven by a motor to be controlled via relays. To this end, a sensor reads the temperature on the heating circuit, positioned directly behind the mixer. This temperature is compared to a target value (dependent on the outside temperature). The relays then move the motor either to mix more or less hot water to the heating circuit.
Temperature sensors are Dallas 16B20, I don't know yet how many I might need, but since they are connected serially to the One Wire Bus, I can add as many as I like.
Total costs: Arduino Uno 30€, Ethershield 30€, relays (4 on 1 board) 10€, each 16B20 3€.
The heating circuit is circulated by an intelligent pump (I don't interfere there with my control).
Simplified, that's it. If anyone wants to look at the source: Just ask, I'll post it.
Thanks for any feedback.