The unit is installed and it works.
I put in the CAI unit plus an 8 relay board (which is posted in another thread).
At present, the relay interupts the "hot" wire to the thermostat which prevents the thermostat from turning on anything (heat/cool/fan). The thermostat still works as it derives its operating power from batteries. He can control it from a web browser or from Casa Remote on his iPhone. Casa Remote so far hasn't impressed me but it does turn the relay on/off.
We could wire it to interupt the thermostat output to fan/heat/cool for individual control or we could remove the thermostat all together and write plc programs turning the cai into a thermostat. But I don't really care to learn plc language at this particular time. If anyone can direct me towards a source of training that can get me up and running in less than an hour, I would love to hear about it.
Total cost, $40 for CAi, $25 for relay, $5 for temp sensors (not really necessary). Casa Remote was $25 but isn't necessary either. He already had a can to mount it in, we scavanged control wires from an old computer and a power supply from his box of extras.
Next step is to wire it to his DSC alarm so he can monitor the armed/disarmed status, as well as arm/disarm the system. We also plan on hooking it up to his magnetic door lock and at least one light. I haven't figured out how to get it to run x10 stuff but supposedly it will do that too.
For now, it is an absolute success at "what to use to control one thing via ip" for about $70.
Thanks for the cai suggestion.