Last night, although it was set to AC the gas heat came on.
Today I replaced the thermostat. Same problem. If I turn on AC the gas fires up. If I turn on the heat nothing happens. Right now I am thinking that the thermostat was fine, and that there is something wrong somewhere with the air handler/furnace part of the system. Has anyone encountered this before? Any advice?
If it’s an electronic device then that is your problem. You’re talking about a computer that activates a switch. If it was a short in the wiring then the furnace would come on independent of the thermostat.
sounds like a prank. somebody switched the wires behind the thermostat, and when you replaced it you just put the wires onto the new thermostat right where the old ones were.
could be totally wrong, just my wild guess.
Definitely not a prank. I went to bed at 1:00 am. Everything is working fine. My wife wakes me up at 5:00 am. It is like one hundred degrees and the furnace is fired.
Here is where we are now. After two of us (one an engineer) spent about 2 hours trying to figure things out we decided it was the circuit board on the air handler/furnace. Now the question is, should we attempt to replace it ourselves. It is a carrier two zone system. The control board has already been replaced once and they left the box near the furnace. The part that needs to be replaced again is Fixed speed Furnace Control Board Part No. 325878-751. It has around 15 different wires that are plugged into the periphery, but it seems that I can buy the exact replacement on Amazon for $105. If I take some pictures before I start I think that it should be fairly straightforward. My guess is having Blue Dot HVAC come will set me out 500 or 600 dollars. Do I risk it? Am I courting disaster by trying to do it myself?
besides photos place a tape label on each wire with the terminal it goes to.
if you trust the vendor then go for it. if you’ve done replacement work on electronics or computer then you should be good.
in your other thread, before you started this thread, i was going to (but didn’t) suggest that after you disconnected the thermostat that you turn the furnace power back on in order to see if the problem might be there.
My job is to diagnose random problems on complicated industrial equipment. That’s different from hvac I realize, but the thought process is the same. So here’s my 2 cents.
Was the part that was replaced before done so because you were experiencing the same problem that you have now?
If your thermostat is calling for a/c based on the ambient temperature, and it fires the furnace, that seems like it’s sending the a/c signal to the wrong piece of equipment. Unless of course I misunderstood your OP - is the control circuit board in question responsible for both the heat and the a/c?
If you answered yes to both, then I think your diagnostic problems are done. The board in that case would be faulty.
As far as replacing it yourself, as long as the part you buy from amazon is the same manufacture, and the wire colors/locations are the same, it’s just a matter of patience. Do it one wire at a time, or label them one wire at a time. And don’t assume it’s not live just because you turned it off…ask me about capacitors. They bite too.
The last time the circuit board was replaced it was because the fan would not come on. Different problem.
And yes, the circuit board does control both the AC and the heat. In essence we have an air conditioner unit outside. Inside we have the furnace which is in the same piece of equipment that has the blower fan. The circuit board I speak of is what calls for the AC unit to turn on.
I did not check to see if the outside AC unit kicks on despite the fact that the furnace is turning on.
During my diagnostic travails I discovered that I cannot get the heat to turn on by turning the thermostat in the room to heat. If I do that, and set the desired temperature appropriately nothing happens. I do not know the significance of that fact.
Complicating things is the fact that this is a dual zoned unit with two thermostats. Either thermostat set to AC turns on the heat. Either thermostat set to heat does nothing.
Checking to see if both a/c and heat come on at the same time would be good information to have. You wanna rule out the possibility that there’s anything wrong with the a/c unit. That would lend more weight to the idea of a shorted connection, where though is anybody’s guess…could be in a physical wire between the thermostat and the unit or…inside the furnace or…inside the a/c unit…or any number of places depending on the way your system is wired. You ask for heat and get nothing, but you ask for a/c and you get heat. That sounds like a classic “oops the wrong wires are touching” scenario. And it’s possible that the problem is contained within the circuit board, but a visual inspection should reveal whether or not it’s melted together anywhere.
Unfortunately, unless you get lucky and figure this one out on your own, I really think you’re gonna need a technician to come out and diagnose it. There’s just too many variables, too many possible failure points. 
Just for completeness sake I will report that I gave up on trying to fix it myself. The technician diagnosed the problem as a faulty control board, not the main one but the one that controls dual zone functionality. He had to order it so it is not fixed as yet. He said he had never heard of this exact problem before.