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Central A/C problem-won't shut off
Old 07-14-2010, 02:12 PM   #1
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Central A/C problem-won't shut off

Our A/C is on and running and when it gets to it's desired temp the outside unit is not turning off. Even when I move the setting on the thermostat from COOL to OFF the outside unit continues to run.

Last night we left the A/C on (82 and humid outside) and in the morning the temp outside had dropped and the outside unit had some frost on the pipes and was running even though the temp inside was below the desired setting.

We shut off the outside unit at the circuit breaker and waited until all the frost had melted. Later I turned on the A/C again (it's high 80's and humid outside now) and the outside unit runs and runs even though it's getting really chilled in the house.

Just looking for suggestions as to what to look for. It's a digital thermostat and the A/C system is about 15 years old. I figure I'll need a pro, I'd just like to know what to expect.
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Old 07-14-2010, 02:48 PM   #2
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Not an HVAC guy here and the body of knowledge that I do not know is infinite, so anybody feel free to correct me. I'll take a shot. I am assuming that the blower (inside) is cycling on and off normally.

If I understand correctly the thermostat controls relays that turns the blower (inside) and the condenser (outside) on and off as required. So your symptoms point to the relay in the condenser or the thermostat, likely the relay in the condenser unit (outside).

Just guessing, you need a service call and relay replacement. Where I am that would be between $100 and $200 total. It is peak season for AC work, so it may take a while to get an appointment.

Edit to add: If the inside blower is *not* cycling on and off I would look at the thermostat first. A service call and thermostat from an AC place, again $100 - $200 installed, but changing a thermostat is an easy DIY job. Haven't shopped thermostats in 18 years but you can probably get a good one for $50 or so.
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Old 07-14-2010, 03:02 PM   #3
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I don't know anything about A/Cs except when they're not working right, I'm hot.

Maybe this will help....

Outside air conditioner unit keeps running even when not blowing inside? - Yahoo! Answers
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Old 07-14-2010, 03:11 PM   #4
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I don't know anything about A/Cs except when they're not working right, I'm hot.

Maybe this will help....

Outside air conditioner unit keeps running even when not blowing inside? - Yahoo! Answers
bbbamI, ya beat me to it! - here is what I came up with. (Great minds must think alike...)

Home ac outside unit will not turn on and off. It just keep running? - Yahoo! Answers
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Old 07-14-2010, 03:16 PM   #5
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Yes, that's what I'm finding when I read online, probably the contactor.

I called for an appointment and it will be sometime tomorrow. The scheduling person directed me to their online coupons. We haven't had the system serviced for a few years so it looks like a good time to get the whole thing checked out.
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Old 07-14-2010, 03:32 PM   #6
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Some thermostats use regular sized (AA) batteries. Could it be as simple as that? My wild guess is maybe the batteries are drained so the thermostat isn't functioning on/off properly. Remember, it's only a guess.

The bright side I suppose, in the middle of summer, better not turning off than not turnng on.
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Old 07-14-2010, 03:48 PM   #7
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I'd guess the contactor (relay) as well. If you know where it is, these sometimes respond to a good tap. If you are comfortable with electricity you can change it out yourself.

When mine was giving me problems, I opened it up and found it to be full of ants. For some reason ants are attracted to the electromagnet inside.
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Old 07-14-2010, 03:53 PM   #8
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I did heating & A/C work in the early '70s. I'd bet on the contactor, a 220 volt relay operated by the thermostat. Depending on your area $100-$200 to get it done. A guess is the part will be ~$50-80.

It's not a difficult DIY job. Use a meter to make sure the power is off (You'd turn the breaker off but check anyway. Don't ask how I know that.) and check for continuity through the contactor when it's turned off and the wires are disconnected so you're not measuring resistance through the compressor motor. To get the right contactor replacement you'll need the make, model, and serial # of the condensing unit.
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Old 07-14-2010, 03:55 PM   #9
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I'd guess the contactor (relay) as well.
Very well might be the culprit. Had mine replaced last year for $225 including the service call.
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Old 07-14-2010, 04:51 PM   #10
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Originally Posted by easysurfer View Post
Some thermostats use regular sized (AA) batteries. Could it be as simple as that? My wild guess is maybe the batteries are drained so the thermostat isn't functioning on/off properly. Remember, it's only a guess.

The bright side I suppose, in the middle of summer, better not turning off than not turnng on.
Usually if that battery is dead nothing works... at least that happens on mine..
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Old 07-15-2010, 09:43 AM   #11
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The technician just left. Yes, it was the contactor. He replaced that and we also had him do regular A/C maintenance since it had been many years. Everything checked out just fine and the charge includes a furnace service call in the fall.

We looked at doing this ourselves and decided this is something for the pros. DH has some electrical knowledge but nothing having to do with A/C systems. We didn't want to get stuck without A/C if it wasn't just the contactor or he didn't get it right the first time. Many other kinds of projects have started out with one trip to Home Depot and turned into multiple trips with assorted parts of varying sizes, taking more than one day. It's supposed to get in the 90's today and it's already humid. We're glad to have this done and know that our 17 year old A/C system on the original furnace is good to go for a few more years.

A new heating and A/C system is on our short list of things to take care of in the near future. This year we want to finish replacement windows. We've done 4 out of 14 and there is a nice tax credit this year. Next is the roof, then heating and cooling.
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