I bolted some 10A 5V relays onto the GPIO pins via a 3V3-5V converter board to switch the heating circuits on and off.
There are some standard 1-wire temp sensors - docs are easy to find online. The 1-wire protocol allows for really long cabling and doesn't need external power, so it's ideal for this kind of project
There's some incredibly simple Python code controlling the relays and reading the sensors, and GNUPLOT runs off a cron job to plot the temperatures to image files that get served in a web page via lighttpd.
Because I don't have a static IP at home the Pi pings my Linode server with its IP address every minute, and the Pi server web pages are wrapped in an iframe that pulls them off the current home IP.
The whole thing is a total hack, but it works well enough. Bottom line is I can see the indoor/pipe/tank/outdoor temps from anywhere, and the heating turns on if it thinks the pipes are going to freeze, and turns off if the water in the tank and/or room temps are warm enough. I can also turn the heating on and off remotely.
It wasn't quite trivial to do, but it wasn't a hair-tearing challenge either. So far as I can tell, it was easier than it would have been with alternative boards.
Basically anyone who knows beginner-level Linux server set-up and some very basic electronics should be able to build something like this around the Pi. It's perfect for this kind of embedded micro-server/controller project.
Are you controlling the heating in each room separately?
From your description saying "relay", I'm presuming this is electric heating?
Do you find that this keeps all the rooms around the right temperature? The usual problem with a normal heating system is that you have parts of the house that regularly are hotter or colder than the rest, so I was wondering if you had managed to solve this.
I'm looking to control radiator valves electronically, which of course relies on someone actually making electronically controlled radiator valves that don't cost a stupid amount of money.
Not a heating project but maybe this will inspire you. My RPi monitors of temperature and humidity in- and outdoor of my apartment: http://pi.tafkas.net/temperature
