I have a conundrum that has been quite vexing, and any advice would be appreciated. For a number of months, our indoor fan blower has been coming on at seemingly-random times, blowing room temperature air through the vents in our house. It usually comes on 2-3 times per day, for anywhere from 3-10 minutes each time, and the times at which it comes on can vary considerably (though late morning and mid-evening seem to be more common). When it does come on, it is usually several hours after the heater has last been on (and the heater is currently operating as it should when heat is called for). Turning the thermostat switch to “Off” does not shut this random fan blower action off, and there are in fact times when the fan comes on when the switch is already set to “Off”. Now, here is where it gets interesting. We have a York furnace in the attic (which came with our new Lennar home in 2006), and, the first time an HVAC specialist came to investigate this problem, he suggested that the circuit board was faulty (it was showing some error codes which indicated a needed replacement). So, in December 2012, he replaced the circuit board. The problem continued, however. So, last week, he came out again and found an error code indicating that a flame was being sensed - but no gas. Theorizing that the gas valve was responsible (and there was evidence of water having gotten into both it and the circuit board at some point in the past due to insufficient drainage), he replaced the gas valve. Both repairs may have been necessary, but the two combined ran us almost $800 - and neither solved the problem of the fan’s erratic operation. The air filter has been replaced multiple times through this process, and I have also replaced the batteries in the thermostat (though not the thermostat itself). I did notice when I was replacing the batteries a few weeks ago that the fan came on while I had the batteries removed. So, my question is, what could be responsible for this? Could the thermostat itself be at fault? It is a programmable model, but the HVAC specialist checked and cleared all programs. If the thermostat is not the likely culprit, what could be? And how serious is this problem? My wife has asked me if this is really the sort of thing that I should worry about at all, given that we have already replaced the circuit board and gas valve. Thanks!