Intermittent heat cutting off and power loss at thermostat


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Old 01-27-22, 04:49 PM
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Intermittent heat cutting off and power loss at thermostat

Hello,

I have a 9 year old Bryant gas furnace model 986ta42060v17-a. It had professional inspection last year.

2 story house, with separate unit for each floor. This unit is on the first floor, the furnace is in the crawl space, and has fan-assisted exhaust.

Intermittent trouble with heat cutting off and thermostat goes blank - the blower still runs until thermostat comes back on, which is usually within a minute. A few times it would not come back on, while I was at work of course, and I had my wife turn off the breaker for a minute and everything came back on.

There is a c wire and the thermostat does not use batteries.

I reseated all connections on the board, checked and tightened thermostat wires at both ends.



Since the problem is intermittent and I wanted to rule out the thermostat I ran wires off the R & C terminals at the thermostat, put the faceplate back on and connected a meter so I could watch the voltage and the thermostat display at the same time.

The blower door switch is taped closed and the blower door is in place.

When the system is sitting idle, or just the blower is on, there are 27 volts at the thermostat. When the heat is on, 25 volts. When the display goes blank, volts drop to 0.

I then remove the blower door to check the status code light and it is solid; no codes.

Because itís intermittent and short duration, or cuts off when Iím not home, Iím not able to test anything at the furnace during the power cut.



This leads me to a power supply to the thermostat issue, but Iím not sure where to go first.

Iíd rather not replace parts without a test, but donít see any other option.

What part should be swapped out first?



Thanks for your help.




 
  #2  
Old 01-28-22, 10:23 AM
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Welcome to the forums.

Intermittent trouble with heat cutting off and thermostat goes blank - the blower still runs until thermostat comes back on
That is telling you that the furnace is overheating. One of the high limit switches is opening and resetting. That board may only allow that to happen several times before going into lockout.

You need to locate what is reducing the air flow.
Check/replace filter(s), check for too many closed registers and blocked returns.
You may need to clean the blower wheel.
 
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Old 01-29-22, 06:51 AM
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Hi Pete,
Thanks for the quick reply.
There is one filter, and it is changed regularly, last time was 2 weeks ago.
I verified that all the vents are open, and I looked in the blower this morning ind it isn’t dirty. Air flow through the vents has been normal.
This morning the board finally gave me a code, 1 short 3 long. Looked it up and it’s a limit switch situation.
Good call!
Since air flow is good does that point to the limit switch?
Thanks,
Jim
 
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Old 01-29-22, 08:16 AM
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It's possibly a defective limit switch but poor air flow is more common.

Usually to confirm that we'll drill a small hole in the return duct and another in the supply plenum where it joins the ductwork. We'll measure the temperatures. The difference is called heat rise. Maximum heat rise is posted on the ID tag inside on the side wall near the burner.
 
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Old 01-29-22, 02:53 PM
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Pete,
I drilled and checked temps. Incoming is 66 deg (we keep the house cool), after the burners is 115 for a rise of 49 degrees.
This is within the ranges on the label. (Low 30-60 deg High 40-70).
 
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Old 01-29-22, 05:37 PM
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Based on that I'd replace the limit switch.
 
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Old 02-02-22, 05:15 PM
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Thanks, Pete.
The part was supposed to be here today, but no idea now.
I’ll post results when it gets here.
 
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Old 02-12-22, 05:32 AM
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New limit switch installed this morning and furnace is now running perfectly.
Thanks for the assist, Pete!
 
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Old 02-12-22, 11:38 AM
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Glad to hear you got it fixed. I had a similar problem a couple of months ago, and it also turned out to be a defective high limit switch. Everything's been fine since I replaced it.
 
 

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