Heater flame going on and off

[pedirob]

So my furnace is acting up:
1. It starts normally.
2. After a few seconds it ignites and you hear the roar of burning propane.
3. After a variable interval of 2-30 seconds, flame shuts off. This interval seems to get shorter the more it repeats.
4. Goes back to #2 and repeats cycle. Fan stays on throughout. No error code displayed, because the heater doesn't shut off.

The troubleshooting guide on this forum was very helpful, and suggests this is most likely "cycling the limit switch" resulting from overheating in the heat exchanger causing the high limit switch to engage.

There are no visible obstructions, but I can't see very much. Air is coming out of the vents. How hard is it to access the combustion chamber and heat exchanger?

Is there a way to test the high limit switch? (I'm handy with a voltmeter)

Another curious observation - a few tiny pebbles of crumbly material coming out of the exhaust port. I'm wondering if those are the incinerated remains of a mud dauber nest?

Appreciate any help!
Rob

 

[Dahillbilly]

check the sail switch, if it isn't closing the contact system will shut down as you described. Good luck

 

[pedirob]

check the sail switch, if it isn't closing the contact system will shut down as you described. Good luck

Already checked. It does this even when I bypass the sail switch. Besides, it wouldn't ignite if the sail switch were open.

 

[jerryjay11]

If you place voltmeter leads on both terminals of the switch when turning on the burner (burner powered) then you shouldn't read any voltage. If the burner is running and the switch is closed, you should not read voltage. If you read voltage, then the switch is open. Just hold them there until the burner shuts down to determine if the switch is opening on high heat. Could be a restricted heat exchanger or maybe the burner assembly needs cleaning, particularly the pilot orifice.

 

[pedirob]

If you place voltmeter leads on both terminals of the switch when turning on the burner (burner powered) then you shouldn't read any voltage. If the burner is running and the switch is closed, you should not read voltage. If you read voltage, then the switch is open. Just hold them there until the burner shuts down to determine if the switch is opening on high heat. Could be a restricted heat exchanger or maybe the burner assembly needs cleaning, particularly the pilot orifice.

Which switch are you referring to?

 

[Dahillbilly]

the high limit switch is normally closed(NC), voltage will pass through it when working correctly; it will open when it reaches the rating of switch & no voltage passes through then. Can be tested using OHM meter, if closed will read some value--- if open no value detected. When testing for voltage make sure tester is set for DC volts; with one lead on a good ground & system turned on, use other probe & check each switch both terminals, should have voltage present until excessive heat causes switch to change states(goes from NC to open). Hope this helps

 

[BigFry]

My personal experience with a failed high limit switch was that the furnace would run about 2-3 minutes, not seconds, but would also go into "lockout" mode and have to be power cycled to run again. My experience with the flame running for about 10 seconds, and then restarting was a failed control board. If you suspect the limit switch, they are not expensive and easy to replace.

 

[jerryjay11]

If you place voltmeter leads on both terminals of the switch when turning on the burner (burner powered) then you shouldn't read any voltage. If the burner is running and the switch is closed, you should not read voltage. If you read voltage, then the switch is open. Just hold them there until the burner shuts down to determine if the switch is opening on high heat. Could be a restricted heat exchanger or maybe the burner assembly needs cleaning, particularly the pilot orifice.

pedirob​

Since you suspect the limit switch... But this works on any switch as electricity follows the least resistance. If any switch is closed (path of least resistance) the voltage will go straight through the switch to the load it is powering (assuming it is good sw). If the switch is open, then the path of least resistance is through voltmeter.

An ohmmeter with power on can damage meter. And it you take time to turn off system, pull wire connector off switch terminal to avoid any interference from circuit the limit switch may have cooled and reset. This is why I would use a voltmeter in the manner I described.