Using arduino to control honeywell thermostat changeover through ip

Well, I have an issue that I think an arduino can solve, but the problem is that I don't know how it would interface with Honeywell's password protected IP control site. I have a temperature sensor that I can use on the heat/cool pipe to sense the temperature and I want to have it control the changeover automatically. This is their site:

It has a javascript log in and then javascript controlled buttons once logged in. Is there any way that I can use the post command to log in and switch the changeover from heat to cool?

I don't have an account on that web site so I can't see what it does. Are you really saying that you need to go to an internet web site to control your heating and cooling systems? That seems bizarre.

That's correct! The thermostat uses Honeywell's proprietary redlink wireless protocol, so unfortunately, my only option is IP control.

Do you know how that web-based control mechanism works? I suspect that the web page includes a control which communicates directly with your Honeywell installed equipment via your LAN. If you can monitor that communication using Wireshark or similar and reverse-engineer the protocol, it may be possible to implement your own client. This seems like the sort of job that would be a lot easier to do on a PC, perhaps using sensor input collected from an Arduino. You might even find that the pages use a COM service with an API which you can reverse engineer by inspection of the HTML source, and access programmatically on the PC.

In any case, if it works as I suspect, then having your Arduino download pages from the internet site will not do you any good since you would need the page to be rendered and the controls on it to be loaded and executed in order to achieve anything, and obviously your Arduino isn't going to do any of that.