Same as subject line! Something inbetween the little heater that goes under the thermostat (doesn't help with AC) and the very expensive models that are available from other manufacturers.
Ultra
You can use the X10 heater for controlling AC. It just works the opposite of the heat set back. You program your set back for the daytime hours instead of nightime and set the HVAC thermostat to your desired setback temperature. The X10 TH2807 heater operating during the day, warms the thermostat, causing the home to be cooler that the set temperature. Then at night the heater is turned off and the house then goes to the thermostat set point (the "setback" temp).
Another cheap (but more difficult) way of getting X10 HVAC setback is to use two thermostats; one for "normal" temp and one for "setback" temp. They need to be located side by side. If your original thermostat wires are not secured inside the walls, you can use your existing thermostat wire to "pull" two new wires for the two thermostats. Then you can use X10 Appliance Modules driving 4PDT relays to control "Normal Temp", "Set back Temp", System ON/OFF, and Fan ON/OFF.
The May 1999 issue of
Popular Home Automation magazine has the DIY plans, parts source, 1999 costs, etc. for the two thermostat X10 setback project. It was written by yours truly, so if you want to try it, PM me with your email address and I can email the article or my original draft...which ever I can find.