Notice this for the first time today. I haven't switched over from Heat mode to Cool mode yet because my house is perfectly fine with the windows open and the HVAC fan running. However, now when I try to run only the fan, it will turn the AC compressor on. When I go to the thermostat to see what it's doing, I can see the blue "cooling" background for a split second before it goes black again. My understanding was that it should not be running the AC unless it's in Cool or Heat-Cool mode. The only way I can prevent it from running the AC is if I turn it completely off, which is annoying in that I can't run the fan that way either. Whatever it's doing can't be right, in that I use a dust cover on the AC unit until I switch to cooling and it is NOT a good idea to run the AC with the cover on. Who knows what could have happened if I hadn't been home to catch it...
Is there any way to prevent the AC from coming on unless it's in Cool or Heat - Cool mode? And yes, it has been wired correctly for 5 years and has never done this before this spring.
