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Old 08-13-2019, 06:59 PM   #1
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AC thermostat battery


My central AC thermostat display went off. Not showing the temperature or settings.


But the switches are working, I can turn the system to OFF or COOL or HEAT, and the fan to AUTO or OFF, and they do respond.


I took the front panel off and noticed the batteries were inserted backwards, so I put them back on correctly and the display came back on.


Which leads to my question. Isn't the battery inside the thermostat just used to preserve the custom settings in the event the AC power goes out? Or is it used in some way to power the display?
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Old 08-13-2019, 07:40 PM   #2
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Re: AC thermostat battery


Some stats can be powered directly by the system when there's a common wire and the batteries are just for backup, others are 100% battery powered.

if you have a common wire, labelled C, the connection on either end may be bad. There may have been enough to power the electronics but the display.

some old stats are power stealing with battery backup, they trickle current through the heating or cooling control circuit (wire y or w), using it as a common. they can wreck havoc with newer units that have sensitive electronics.
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I am not in the business of any trade or topic I give advise on. I have previous diy experience plus good knowledge of theory. Attempt repairs at your own risk. Never jump out safeties - especially pressure switches - on a furnace for testing with fuel supply on; use a meter. Do not troubleshoot with live line voltage present unless there's no alternative.
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Old 08-13-2019, 08:16 PM   #3
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Re: AC thermostat battery


*but not the display.
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I am not in the business of any trade or topic I give advise on. I have previous diy experience plus good knowledge of theory. Attempt repairs at your own risk. Never jump out safeties - especially pressure switches - on a furnace for testing with fuel supply on; use a meter. Do not troubleshoot with live line voltage present unless there's no alternative.
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Old 08-14-2019, 11:18 AM   #4
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Re: AC thermostat battery


If they have been always backwards, then your thermostat was likely using a common wire. Since it's off, we should assume that your AC is no longer working, unless you confirm otherwise. (maybe a family member thought they were helping by replacing the batteries and put them in backwards.)

Cheers!
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Old Yesterday, 06:35 PM   #5
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Re: AC thermostat battery


I think someone changed the batteries, but in the process of changing them, inserted the new batteries backwards.


I just took off the front panel to examine the wiring. Here is what I have.





It's an 18-5 behind the wall, but only four conductors are connected.


Green conductor goes to G
Yellow conductor goes to Y
White conductor goes to W
Red conductor goes to R where R and RC are jumpered


There is a blue connector tucked back inside the wall that I teased back out.








Should I connect the blue conductor to B?
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Old Yesterday, 06:41 PM   #6
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Re: AC thermostat battery


there's no option to use a common on that stat, don't connect it to anything.

B and O are for heatpumps.

impossible for it to have worked with batteries backwards unless it's a power stealing stat - which i doubt it is. maybe someone messed with it recently?

Heat cool shouldn't have worked, fan switches on the lower end ones are strictly mechanical, joining R and g internally when set to on.
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