Problem with my AC


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Old 08-09-17, 10:08 AM
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Problem with my AC

About two weeks ago, something happened and the fan would not shut off when the AC was no longer running.

The first thing I tried was turning the power off at the breaker to try to reset it. This worked in the short term. The fan would shut off when the AC turned off. But after a day or two, the fan would run constantly again.

I did this a couple times, then the thermostat said it wasn't getting power. I have two units, one upstair and one downstairs, both running off Nest thermostats. I thought it might be an issue with the thermostat, so I swapped them out. Neither thermostat was getting power at the upstairs unit, so I don't think it's a defective thermostat.

At this point, I'm stumped. I'm not sure why I'm not getting power to the unit upstairs. I've checked the breaker and reset it and it looks fine. The only thing I can think is there's a short somewhere, but wouldn't that trip the breaker?
 
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Old 08-09-17, 10:33 AM
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When you swapped the thermostats did you swap the bases as well? Nest uses electronics in the base that can go bad.
 
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Old 08-09-17, 11:28 AM
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I didn't, but I moved the power from the RC to the RH connector in case the connection was bad. The display registered where the wires were connected. I can swap the bases tonight if that doesn't eliminate that possibility.
 
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Old 08-09-17, 11:57 AM
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You need to get your meter out and check for control voltage.
Is the nest using a common wire?
 
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Old 08-09-17, 01:17 PM
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Off the top of my head, I remember a red power wire, green fan wire, yellow cooling wire and black heating wire. I don't remember a common wire. I can check the voltage when I get home.
 
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Old 08-09-17, 01:33 PM
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The Nest isn't a very good thermostat. They have been known to cause all sorts of bizarre problems, especially when they are used in a power stealing setup.
 
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Old 08-09-17, 03:21 PM
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If I swap the bases tonight and the problem persists, while the other unit continues to work, will that eliminate the thermostat as a culprit?
 
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Old 08-09-17, 03:52 PM
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I doubt it's the base. If I had to guess because I didn't have a meter I'd say it's a problem with an overflow float switch. I'd check the overflow pan on the upstairs air handler. Look for the overflow switch. It may be stuck tripped.
 
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Old 08-09-17, 04:49 PM
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I'll check that when I get home.

I also forgot to mention that I discovered yesterday that rain water was getting in. I live in the desert, so we don't get rain often. But we have been getting quite a bit over the past couple weeks, so that might have something to do with it.

I got on the roof and sealed the gap where it was getting in.
 
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Old 08-09-17, 05:57 PM
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I don't see an overflow float switch. Where would it be located in this?



 
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Old 08-09-17, 09:29 PM
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Yes... it's built into the condensate pump.
That's what those low voltage wires are that are connected there.
 
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Old 08-09-17, 10:31 PM
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I think you either have wire related problem (may be wet) or control board problem. Remove the thermostat, jump (connect) Red wire to Green wire and see if that starts the fan, then disconnect R from G, and see if that stops the fan. If the fan won't stop, you may have a control board problem. Also check transformer to see if it generates 24V (you need a meter for this) for the thermostat.
 
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Old 08-09-17, 11:07 PM
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Don't forget.... this is a furnace and when many furnaces lose their 24vac the fan automatically comes on.
 
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Old 08-11-17, 03:14 PM
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I checked the control board and found a blown fuse. Replaced the fuse and the board glowed orange where you see the black marks. So I guess it needs a new control board.

 
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Old 08-11-17, 03:22 PM
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I would agree that the board is no longer usable now.
 
 

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