Thermostat has no power

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Old 10-16-18, 08:02 PM
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Thermostat has no power

Went upstairs tonight and noticed that the air conditioning wasn't running. I have a smart thermostat and the display was dark as if there was no power. Went to the breakerbox and the furnace breaker wasn't tripped. I power cycled the breaker and when I went back upstairs the thermostat was powered up and was going to start cooling.

I won't get a chance to take a deeper dive until the weekend but not sure what would cause that to happen; i.e. no power to thermostat? Issue with the controller board? How would I test if that is a likely candidate? What else should I look for?
 
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Old 10-16-18, 08:19 PM
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I would check for a possible condensate issue especially if the air handler is in a place where it can cause water damage if it leaked. Many times a float switch is installed that shuts the system or parts of the system down to prevent flooding.
 
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Old 10-16-18, 08:28 PM
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Will check. Over the past several months this, the blower motor has started a slight hum indicating an issue with the bearings and I have considered replacing it but I never put much emphasis on this since it was more of an annoyance.
Could the motor load get high and trip out the furnace controller. Right now the ac is working fine and all I did was power cycle the furnace
 
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Old 10-16-18, 09:26 PM
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Could the motor load get high and trip out the furnace controller.
Very doubtful. Could be a relay going bad on the control board for the blower.
You'd need a voltmeter to check for 24vAC at the R and C terminals to see if it's a low voltage issue first. If that is ok then you'd need to check for 120vAC to the blower motor. It would most likely be the N (neutral) and Cool (H) terminals.
 
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Old 10-17-18, 04:45 AM
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Thanks.
Of course now that I powered cycled the furnace, everything is working normal. Just don't know for how long. Thinking back over the last several months, this is the second time that this has happened. No power to the thermostat, turning breaker off and on, suddenly everything works again.
I am guessing that since the thermostat having no power is the key symptom, I am guessing that the issue is at the converting from 120V to 24V?
 
 

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