March 9, 201114 yr Actually, I don't see why not. I used to have an RCS X10 thermostat, but I replaced it with an ecobee just prior to upgrading to Insteon. The ISY can send and receive X10 messages, and the RCS thermostats just uses one X10 House Code with each of the Unit Codes (1-16) activating two functions (one for an ON and one for an OFF) for a total of 32 discrete commands. You would just have to issue the appropriate X10 Ons and Offs to control the thermostat.
March 9, 201114 yr While there isn't any technical reason why the ISY can't send commands to control an X10 thermostat, the real question in my opinion is whether or not it is able to receive temperature and other status events and store this data as it would an INSTEON thermostat. Without knowing the thermostat's status, its utility is somewhat limited. The answer to that question appears to be no. Provided the thermostat supports it, you could still set up programs to send X10 commands to affect its programming. You just won't know whether those commands took. The ISY can receive X10 events, so I suppose that you could utilize programs to set variables based on the messages it receives. Admittedly, we're getting a bit kludgey here, but it could be useful for HVAC management. Bottom line: If you want full thermostat support in the ISY, you'll need an INSTEON thermostat.
