I have a propane furnace and central air conditioning. I have a programmable thermostat now but I bought a newer one that had more programming features. I have a 4 wire system with a common (blue) wire that has never been used. When I hooked up the new thermostat, the instructions told me that I had to hook the common wire up to it along with all of the other wires.
The thermostat worked fine with one exception, the heat works fine but when I turn it on cool, it runs for about 3 minutes, then the display on the thermostat goes blank (it's a touch screen) and then the furnace kicks on. Unfortunately, the installation guide that came with the thermostat had some troubleshooting guidelines, but everything I tried hasn't helped. I checked the voltage to the thermostat and it's fine.
I put the old stat back on and it's works fine. Any help would be greatly appriciated. Thanks.