Heat doesn't come on below freezing - using Nest thermostat

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Old 12-30-14, 10:50 AM
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Question Heat doesn't come on below freezing - using Nest thermostat

I'm still a homeowner (condo) newbie, so forgive me for my stupidity.:-)

I have a Bryant gas furnace (upflow), model #383KAV. I hooked up a Nest thermostat last year, and all seemed to be well until the outside temperature went below freezing. Once that occurred, the thermostat would call for heat (and light up orange), but the furnace wouldn't start. I would have to flip the circuit off and on; once I did, the furnace would operate normally.

I thought I had solved the issue this year, but once again, it has returned. We had our first sub-freezing night last night (about 28 degrees), and I awoke to a chilly interior. The Nest said it was heating, but no heat came out until I restarted the furnace by flipping the switch. I spoke with the Nest tech support on the phone, and he said that the problem is not with the thermostat but with something in the furnace.

I just find it hard to believe that it works fine until the temperature drops, but then again, I am no expert with this kind of stuff. Do any of you have suggestions as to what I can do? I can't imagine it is a major issue, but I still would like to solve it. Thanks in advance for your help!
 

Last edited by Matthewmarino77; 12-30-14 at 10:55 AM. Reason: Typo
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Old 12-30-14, 10:55 AM
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Check for flashing code on the control board.
 
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Old 12-30-14, 11:14 AM
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Thanks for the tip. I just checked the board and didn't see any flashing lights.
 
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Old 12-30-14, 12:55 PM
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Since it resets with a simple power cycling (it would likely also reset by cycling the thermostat from a heat call to satisfied) you have what is called a "soft" lockout. It is, most likely, a problem with the furnace and not the thermostat.
 
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Old 12-30-14, 03:44 PM
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I cleaned the flame sensor, so hopefully that will help the situation. If not, I'll look to get someone in here to check it out.
 
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Old 12-30-14, 06:08 PM
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Your furnace has a control board in it that has a diagnostic LED. The LED should always be blinking unless you have loss of 120v or 24v. The LED will not work with the blower door open. There should be a sight glass where you can see it without opening the blower door.

The diagnostic codes are printed on the backside of the blower door.
 
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Old 12-30-14, 07:52 PM
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I checked the LED, and it's solid red with no flashing of any kind. The panel says that a continuous light means it has 24V power.
 
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Old 12-31-14, 01:18 AM
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It was probably the dirty flame sensor.
If it locks out again check for a trouble code before resetting the furnace.
 
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Old 12-31-14, 06:31 AM
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Sure enough, it went into lockout mode this morning, though at least the heat lasted through the night. Code 14 sends you to 34, which says that the furnace resets in 3 hours and to check that the sensor is clean and also mentions other things to check: the gas valve being off; the green wire being connected to furnace sheet metal; rough ignition; and enough power (microamps) to the sensor.

I usually reset it on my own, but perhaps I will let it reset itself this time and monitor it from there. At some point, I suppose I can try cleaning the burners, too. Would that be an option? If so, how could I do it?
 
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Old 12-31-14, 09:20 AM
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If I were servicing that furnace the first thing I'd do is to check the flame sensor for proper operation. It requires a volt/ohmmeter with a microamps scale. The meter gets connected in series with the flame rod. The flame rod must be IN the flame. If the burners had an issue then possibly the flame is not reaching the rod every time.

To clean the burner properly it needs to be removed from the furnace.
 
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Old 12-31-14, 04:54 PM
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Check the wire from the flame rod to the circuit board. Because the current is so low these connections must be clean and tight. Also check the green grounding wire by removing it and cleaning the terminal and the sheet metal before reattaching.
 
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