My heating ac is a little strange in that the tranformer for the ac is outside in the compressor unit. Ever since I switched to a programmable thermostat I have had problem from time to time. There are 5 wires in two cables. Black which is RH and white which is W in one and , red which is RC, yellow which is Y and green which I think is G. I installed a new Hunter 5-1-1 and it worked fine for 2 months. I replaced it under and it worked fine for 2 years. When it did not work it was in controlling the ac. Recently it started acting up working most of the time but every once in a while it wouldn't turn the furnace on usually about 3 pm to 9 pm. For a few days if I reset the thermostat it seemed to work, then this method stopped working. I replaced it with a Honeywell 5-1-1. That work fine for two day now it stopped working. I confirmed that B&W turned the furnace on disconnected G and now at least the furnace started for right now. However fan in the on position does not work. Do you have any idea if G could be causing the problem? Could the furnace be overheating, I can't see that being the problem it's in the attic and it's 16 degrees outside. Even disconnecting the green failed but holding W&B together works. I don't know anything, but could it be too much or too little power from the transformer that is either burning out or not providing enough power to the thermostat?

Usually there is a 24 volt AC transformer in the furnace with the secondary winding connected to a red wire running to the thermostat and a blue wire, common, to the gas valve, A/C relay, and fan relay. From the thermostat there will be white wire to the gas valve, yellow to the A/C, and green to the fan. The thermostat is wired to switch the power from the red to the white, yellow, and green as needed with the blue completing the circuit. Most thermostats and furnaces have the contacts labeled R, B or C, W, Y, and G for the corresponding wire colors. It may be wired to have the A/C control wires return to the furnace and its controls and then a second wire goes to the A/C unit. Internal wiring may replace the green wire if the thermostat does not give you the option of fan only or continuous fan. Digital or programmable thermostats may need the blue wire connected to them.

From the above, you can see why the fan on doesn't work with G disconnected, and holding W and R together causes the heat to come on. Does either type of thermostat have a B or C terminal? What do the black and red connect to at the other end, do they both go out to transformer and connect to the same terminal? With 3 thermostats having gone bad, maybe it is time to look at the transformer. They are fairly cheap, $10-20. If it is an intermittent problem, even testing it for 24 volts might not find the problem. If you do test it, try to test it at the thermostat, red to white. Look at the rating , and get one at least as big, or bigger.

Ill try the transformer this weekend. Luckily this is the second floor furnace. It does seem to be deteriorating, even holding W to RC doesn't always work. It seems that the funace turns the fan on a lower speed when the heat exchanger come up to temp.

My heating ac is a little strange in that the tranformer for the ac is outside in the compressor unit. Ever since I switched to a programable thermostat i have had problem from time to time. There are 5 wires in two cables. Black which is RH and white which is W in one and , red which is RC, yellow which is Y and green which I think is G. I installed a new Hunter 5-1-1 and it worked fine for 2 months. I replaced it under and it worked fine for 2 years. When it did not work it was in controlling the ac. Recently it started acting up working most of the time but every once in a while it wouldn't turn the furnace on usually about 3 pm to 9 pm. for a few days if I reset the thermostat it seemed to work, then this method stopped working. I replaced it with a Honeywell 5-1-1. That work fine for two day now it stopped working. I confirmed that B&W turned the furnace on disconnected G and now at least the furnace started for right now. However fan in the on position does not work. Do you have any idea if G could be causing the problem?? could the furnace be overheating, I can't see that being the problem it's in the attic and it's 16 degrees outside. Even disconnecting the the green failed but holding W&B together works. I don't know anything, but could it be too much or too little power from the transformer that is either burning out or not providing enough power to the thermostat??
If u give me a day or two I can give u a good answer

Readjust your roll out switch next to your flame baffels