I would like to turn off my AC when the Patio door and or windows are left open for 3 min. I am currently using Homeseer along with the Gemini Napco Plugin. I can detect the opening and closing of the door and window zones and then create an event to trigger an X10 command after 3 minutes to a Power Flash (which is an x10 relay). My main question is how to properly wire the Power Flash Relay to my Thermostat wires at the HVAC system so that I can make or break a connection that controls the AC withouth damaging my HVAC System.

Most A/C units have a pair of 24 V AC wires running from the furnace terminal block to the compressor relay coil. Usually the terminals are labeled Y and B or C, using yellow wire from the thermostat and blue from the transformer. You could tie in a NCC relay anywhere to break the circuit on command. If you tie it into the red wire between the furnace and thermostat, it would also shut off the furnace when the door is left open.

Thanks for the response. Using which method will provide constant power to the thermostat so that by using this relay to power off the furnace it will not cause power to be lost at the thermostat? Also, I have noticed that when the thermostat reaches the desired temperature the fan continues to operate for a short period and then shuts off. By placing a relay to control the HVAC will the blower operate the same way?

Usually there is a yellow wire running from the Y terminal at the thermostat to the Y terminal at the furnace. Break that connection, it is the same as turning the A/C off at the thermostat. The fan controls will still function as needed.

Thanks again. I will give it a try tonight

I ended up breaking the red wire connection. The system works fantastic, If the zones are open for more than 3 minutes it kicks off the A/C and the fan will continue to run for approx 60 sec like it normally should. If the zones are closed for 3 minutes the system kicks back on. This turned out to be a much cheaper solution then buying an X10 Thermostat when I was only looking for this particular function.

Thanks again!