Furnace air luke warm, are these burner cycle timings normal?


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Old 01-04-14, 02:18 PM
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Furnace air luke warm, are these burner cycle timings normal?

Hi there,

I have quick question for a HVAC pro. While the furnace is running, I've noticed the heated air (at the register) never gets all that warm. I went down to the furnace to see how long the burners were staying lit and came up with the following timings:

Burner On - 2 min 52 sec
Burner Off- 1 min 47 sec
Burner On - 2 min 51 sec
Burner Off - 1 min 45 sec
Burner On - 2 min 53 sec

Before timing this, I set the thermostat very high, to 78 degrees. The outside temp is just 25 degrees and the house is a bit drafty.

The filter was changed today.

All registers are wide open.

I am using a Nest thermostat.

The main blower remains on at all times. The smaller fan by the burners also stays on except for a short moment while the burners are off, right before the burner element heats up again, it will reset.

Do these timings sound normal to you or would it be worth exploring further?

Thanks!
Jon
 
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Last edited by hawkinsjon; 01-04-14 at 02:49 PM.
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Old 01-04-14, 04:26 PM
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Since you are using a Nest & I hear all kinds of horror stories about them, I suggest disconnecting the stat & simply jumpering the red & white wires. If the same pattern continues it's not the thermostat.

Most furnaces have a small window in the blower compartment door thru which you can see a (usually red) light. If it is blinking, count the blinks between pauses then open the blower compartment door & on the inside of the door you should find a key to the blink code. Please let us know what the key says about your blink code.
 
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Old 01-05-14, 12:04 PM
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Thanks for the quick reply. I checked the two sight glasses at the main blower and saw no red lights flashing.

Do these timings sound a little too quick to you or does that seem like it could be normal?
 
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Old 01-05-14, 01:53 PM
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If these run times were during one call for heat & the thermostat was not satisfied between runs, something is certainly amiss. During a given call for heat, regardless of how long that call is, the burners should not cycle off.
 
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Old 01-07-14, 06:08 PM
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Thanks Grady, that's good to know. I thought it seemed a little weird. Kind of annoying to have our house take so long to heat up, makes sense now after watching the burners.

Any idea towards next steps, testing or what the cause could be? I'll try running a jumper from the red to white wire tonight and see if the burners have the same behavior.

Much appreciated,
Jon
 
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Old 01-07-14, 09:40 PM
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Update: I jumpered the red and white thermostat wires, went downstairs and watched the furnace behave in the same manner. So its definetly not the nest.
 
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Old 01-08-14, 07:36 PM
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My guess is the furnace is "cycling on limit" which means it is heating to the point where the high limit switch is shutting down the burners.
 
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Old 01-10-14, 10:22 AM
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Is that normal operation or maybe the switch calibration is off thinking its hotter than it is and killing the burners?
 
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Old 01-10-14, 06:46 PM
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It is certainly not normal operation. If it is cycling on limit, it should give you a code when the burners go off. If it is or isn't throwing a code the sensor calibration could be off but not likely. The only way to tell is to first find out which sensor is tripping then measure the temperature very close to the sensor & compare the sensor value to the actual temperature.
 
 

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