I will be able to give you exact details about the temperature sensors on sunday evening, right now I don't have the datasheet upon me. What I can tell you though is, that the sensors are pipe mounted rtd s and have a resistance of 2 kOhm at 25 degree celsius. In order to measure the temperature, the sensors are installed in line with a 2kOhm resistor between 0 and 5 V and the voltage drop over the sensor is measured. Not necessary but a nice feature: A circuit with operational amplifiers is then used to stretch the voltage output to the input range of the arduino, in order to exploit the full resolution of the analog inputs.
Why didn't the pt1000s work for you?
As of the rest of the pumps and valves and so on, they are all 20 years old, but I can give you the specification though on Sunday. The whole system was created by an enterprise formerly known as Sandler Solar and is still operational and working. At the time the system already shipped with a small lcd display and a nice scheme of a house with leds indicating which rooms were actively heating!
We are trying to create our own controller, because the enterprise/its successor is not selling our type of system anymore and after all those years it is probably a matter of time until something breaks that cant be easily fixed anymore.
In the first place, we want to be able to run the heating system headless with hardcoded setpoint temperatures. But in the end I want to have at least a web interface just like the one you are describing! It would also be interesting to maybe integrate the heating system into an open source home automation suite likehttp://fhem.de/fhem.html
As the project is kind of a hobby, it is progressing quite slowly, but steady. I have already written a significant amount of documentation and will try to upload it to the wiki on the github page when I got more time.
So let's keep in touch - and I wish you great success with your attempts to build your system!