AC comes on when it isn't supposed to

We recently had a few 85 (and even 90) degree days here so I popped on the A/C. It seemed to be running fine and cooling the house as usual. Then a couple of days ago it got colder again, hovering in the mid 60s. At night, I was pretty cold. I got up and noticed the thermometer on the thermostat was about 10 degrees colder than the setting (not a digital) - but the A/C was running.
It's been steady coming on and off as if it was cycling like it would be on a hot day (let's say a day hot enough for it to come on every once in a while instead of run constantly like when it's 95 outside). But there's no real need for it to. So I bump up the thermostat to 85 or 90 or something to get it to cut off and it doesn't - it's still coming on.
I turned the switch on the Tstat to Off (yes, the Fan is set to Auto, not On) but it still runs! Yikes. I can turn it to heat and turn the heater to 50 degrees so it doesn't turn on and that seems to shut the thing down. - Whoops, wait - Dammit - it just came on again - it seems that the blower fan is coming on but the Heater's not on (it's still on 50).
Maybe the fan has just been running (cycling) but the A/C itself was only turning on when it tripped - and just that fan air was enough to cool the house down on those colder nights. But either way, if the fan is coming n - OK, it' just turned off again - automatically when the thermostat is in the Off position, something's obviously funky.
Should I flip the circuit breaker? Fortunately it's supposed to be 70s in the days, and not below 50 at night the next for as far as my 10 day forecast shows me.
Damn. It just came on again. The Tstat has been on Heat, Auto, and the lever all the way back to 50 degrees this whole time (and the Thermometer reads 71-72 in the house).
Could it be the thermostat? I've been pondering getting a digital anyway. Or is this something to get the HVAC guy in for? It really pisses me off that our heater had to have a service call the first year we owned the house and we've already had to buy a new AC 8 years in. Now here's another problem with an AC that's 1-2 years old. I'm sure I'll need shingles on the roof next.
Ok, it's off now and I flipped the switch to off. Let's see if it actually stays off.
Any help on this is much appreciated,
Thanks, Steve
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wrote:

That will certainly turn it off. Probably. :)

Yes, it could be, but before just replacing parts that aren't known to be bad, I'd test to see if it the thermostat. The simplest test would be to disconect the thermostat. While you are doing that, I'd look close to make sure that no two wires that go to the tstat are touching. The metal parts, and if insulated parts are touching, I'd look closely to see if maybe the insulation has a hole letting one wire touch the other. I'd look at the tstat end and the furnace end, if you can. If the right two wires are touching, the AC will run, as if the thermostat told it to.
Then note which color wires are connected to which screws on the tstat and then disconnect it. If the ac doesn't go on, (and you've set the breaker is back on)it might well be the tstat which is the problem. Worth buying a new one, but don't throw away your old one until the new one actually fixes the problem. Even then you can give it to a neighbor kid and let him take it apart. (I guess these days you should warn him about the mercury.)
If the new one doesn't fix the problem, you might be able to return it and reinstall the first one, or at the very least, the one you removed is probably good and you can save it as a spare, or give it to someone. I'm going to use my old thermostat on a temp-zone for my burglar alarm, so that if I go away in the winter, the alarm will notify the monitoring company if the house temp goes too low. I'm sure it has other uses too.

Not yet. See what results you get from above. (I mean from the text above, not from Heaven.)

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Steve Latham wrote:

If you have a heat pump, it may be warming you when it is cold and cooling when hot.
--
Joseph Meehan

Dia \'s Muire duit
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Do you have a HP? Gas or Electric heat? It could be a high temp limit in the heater section going/gone bad. They will force the indoor fan on to cool down the furnace.. you probably should replace the stat just because it can save you money in the long run... May solve your problem but I doubt it.

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wrote:

Do you mean the fan in the house, or do you mean the compressor and fan outsdie was also running. You say that that one night the house was colder than the setting, but you don't say iiuc that that continued to be true ever since then. I assumed that it was true, but maybe shouldn't have.

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