I have come across exactly what you are looking for (not with arduino) in the smartbus home automation website. However good theyre automation systems are they do not sease to be overly complicated, expensive and tricy to use. The arduino is your best bet here.
I would suggest installing Tesmp and pressure-or barometric sensors to your HVAC sys's, Any other sensors and stuff that you will need in order to be able to obtain your data. You will need to be able to SEND data around so you should be interested into investing in some ethernet on bee modules.
One thing that could be important here in terms of engineering is to let the data be as simple as possible when being sent, meaning that if for example you want to obtain voltage data and plot that it might be a better idea to send the data RAW (i.e numerically) to an applicable spreadsheet and then just plot it from there using excel or anything else - using this as an idea. This will make the xbee or whatever other module you are planning to use to communicate with the board faster since the data it has to send will ont involve simple hex numerals
It might sound complicated but should be so much. all tou need is some imagination and good choice on what you will use....use lesser elements and modules - less stuff means not only less stuff to code but mostly less stuff that can go wrong...
Looking forward to hearing how this turns out. Please keep us posted on the subject.