On 9/13 I replaced an old Rite-Temp thermostat with a Honeywell RTH8580WF thermostat (just came out a few weeks ago). We have a 2.5 ton single stage AC (electric) and radiator heat with a boiler/water pump (natural gas), both older systems that have been working without fail for the entire time we have been in the house, but we wanted a more accurate and advanced thermostat.
The AC seemed to be controlled perfectly by the new thermostat until yesterday, when I realized the blower was running continuously even when it should have shut off (we always keep our fan on Auto).
The compressor is acting appropriate, only kicking in when the temperature/settings dictate, but even when I have the Fan set to 'Auto' and actually turn OFF the system from the thermostat the blower keeps running. I turned off the AC blower at the breakers and that shut it off, I then turned on the breaker and the blower came right back on.
So I turned it off again at the breaker, popped the thermostat off the wall (sticks onto a wall-mounted plate which connects to the wires) and rechecked the green wire (going to G), took it out and screwed it back in to it's spot. I turned the blower power back on at the breaker and voila! The blower did not turn on.
I then went to the thermostat again (still set to system off with fan auto), and put the fan to on. The fan powered on immediately, but when I switched the fan back to auto, the fan stayed on, even with the system set to off. I tried turning off the power at the breaker again, then turning it back on and the blower stayed off again (this time I didn't mess with the thermostat wires).
I flipped the breaker power again, unscrewed and reseated the G wire again, and the blower stayed off with power restored. To test it again I tried lowering the set temp so the AC would try to kick on the whole system. It kicked on the blower/compressor the way it should, but when I dialed the temp back up, the compressor turned off but the blower again stayed on.
I also tried removing the thermostat from the wall plate and disconnecting the G wire, but the blower still runs continuously.
I am 99.99% sure that things worked fine the first 1-2 days and that this started a couple days AFTER we installed it. I don't know if that means if the thermostat somehow got fried or if installing the thermostat fried something else. I am having them send me a replacement thermostat, but is that likely to be the problem?