So, what does it look like when an A/C breaks? The graphs below tell the story.  I'm hoping I can use this data to write a script to detect this condition. I'll add more detail once I know what the actual breakage is.  Maybe it just iced up?  Those spikes are a little odd on the 1hr graph.

This is the AC trying to run, me giving up on it, and turning it to fan only.

Looks like it broke around midnight:

Now you can see below my vacation, returning, AC fires up as normal, and then clunk?

You can see the last working run, and then it starts to diverge

Here you can see vacation mode, and then the failure.

Why the Venstar T5900 is the answer to DIY HA thermostat needs

So, I recently picked up a Venstar T5900.  I've been looking for a good "high-end" thermostat for awhile now, that would work with DIY Home Automation.  I think the Venstar is about the only answer out there.

There is of course the 3M one from RadioThemostat, and yes, thats a pretty good choice.  It has an open API, and a few advanced features.  But it's not very pretty.  Old style LCD.  Meh.

Nest is cute, but they will probably never make an API.  Screw them.

The Venstar is color LCD, touch screen, tons of features, scheduling, has the web-connect interface stuff, easy to install, and best of all, a documented API.  They even did a pretty good job of documenting it.  Now admittedly, the API won't let you fiddle every single bit of the t-stat, but it's good enough for most HA uses.  It lets you fiddle the set-temps, and turn the unit on/off/auto/etc as you please.  It also lets you query the onboard sensors.

So what are the quirks?

1) The SSDP discovery routine is wonky.  It doesn't work to the spec.  Normally you send an SSDP query on the multicast, and get a direct response.  The venstar replies back to the multicast.  This can be worked around by just listening for the NOTIFY messages instead.

2) If you want to shut the unit OFF, you have to turn off the schedule first.  No big deal really.

3) If you send it too many queries too quickly it ignores them.  Just slow down your sends.

Which should you pick?

The T5800 is nice, but the T5900 is the one you should buy.  $10 more, and has a humidity sensor.  Even if you don't have a humidity system to control, for $10 you get one more sensor to read.  Yay data.  You definitely want the skyport wifi bit, without that, no API access for you.  Total cost shipped was about $225.  Took about 30 minutes to install, and 20 minutes to program locally.

The API is a simple JSOM interface for grabbing data, and a simple HTTP POST interface for changing settings.  No big deal to write code for at all. They even supply sample code in a few languages.