Furnace cycling - not producing heat - no lockout

[danemayer]

Woke up this morning with the furnace fan running continuously but not pushing heat.

Went outside and found that the burner was igniting, pushing hot air out the exhaust vent. After maybe 10 seconds the burner would shut off. Fan continued to run. After another 10 seconds or so the burner would ignite again. Then after 10 seconds shut off again. It just kept doing this.

Normally, if the burner tries to ignite but doesn't sense flame, the furnace shuts off and goes into lockout after 3 tries, also shutting down the blower until the thermostat is reset. This covers a low propane flow situation. If voltage is low, or blower airflow is poor (or restricted) the sail switch won't close, and ignition will not occur. 3 tries and lockout. So the fact that it wasn't going into lockout argues against a number of possible failures.

With the burner igniting and staying on for about 10 seconds, I think the flame sensor was satisfied that ignition had occurred.

That leaves the high-limit switch as the likely culprit. If it opened shortly after the burner ignited, it would shut off gas flow, but if the flame sensor had already signaled successful ignition, it wouldn't put the furnace into lockout. Blocking vents can cause this, but we don't have any vents blocked and airflow is pretty good at all vents.

Of course, shortly after I began investigating, the furnace began to operate normally. I didn't do anything to fix the problem.

Another possibility is a malfunction on the control board. Since the control board was replaced about 3 weeks ago, the board is certainly a suspect.

It'd be nice if these failures would happen during the day when the sun's out and it's 30 (F) outside, rather than in the middle of the night when it's -20 (F) or so.

I have a spare high-limit switch and a spare control board on hand. Just have to see how it goes.

 

[kakampers]

You're right Dan...always happens when it's cold as "you know what"....my neighbor is having issues too....his will come on and run for about ten seconds, then shut off...any ideas??

 

[danemayer]

You're right Dan...always happens when it's cold as "you know what"....my neighbor is having issues too....his will come on and run for about ten seconds, then shut off...any ideas??

The blower should come on for about 20 seconds and then there should be an ignition attempt. Failure to ignite will result in lockout after 3 tries. If that's what's happening, first thing to check is for 12V DC to run the blower, operate the gas valve, and fire the igniter. Also have to check propane flow. Make sure there are no vent obstructions inside and outside.

Standing outside the furnace, you can easily hear if there's an ignition attempt. If there's no ignition attempt, it's probably the sail switch, loose wires, or the control board. Maybe the gas valve. If it attempts to ignite but shuts off almost immediately, it's probably the ignitor/flame sensor, or maybe the board. All these scenarios result in a lockout after 3 tries. Resetting the thermostat clears the lockout.

 

[Silverado23]

I've had a situation where when the temperature is low and the tank is low, A low line pressure situation where the furnace can fire up but then shuts down after a short time because the overall propane line pressure drops and the tank cannot provide enough pressure fast enough. If you have dual tanks, turn on both tanks and even with the low tank as the primary tank selected, the pressure will recover fast enough for the furnace to continue to run until empty.

 

[kakampers]

You're right Dan...always happens when it's cold as "you know what"....my neighbor is having issues too....his will come on and run for about ten seconds, then shut off...any ideas??

Thanks Dan! We'll check this out....we've got close to a foot of snow...no chance to get an RV tech out here for several days...

 

[kakampers]

Dan....our friend's did the same thing...came on and started working. These people are not RVer's and we tried to tell them they need a battery on the rig, but they don't get it...could no battery be causing this?

 

[danemayer]

Dan....our friend's did the same thing...came on and started working. These people are not RVer's and we tried to tell them they need a battery on the rig, but they don't get it...could no battery be causing this?

If the power converter is working correctly, it would supply about 13V DC to the fuse box and from there to the furnace. Battery shouldn't be necessary unless shore power went out or the converter wasn't working correctly.

 

[kakampers]

If the power converter is working correctly, it would supply about 13V DC to the fuse box and from there to the furnace. Battery shouldn't be necessary unless shore power went out or the converter wasn't working correctly.

Thanks! wasn't sure...thought it would work on converter...

 

[JohnDar]

It isn't just RV's that experience heating system failures at inconvenient times. Our S&B furnace blower motor gave up the ghost earlier this month on a Sunday night with temperatures heading for single digits. We did get a service tech out that evening to replace the motor, but it wasn't cheap. After some consultation, we're getting the entire 22 year old HVAC system replaced.

 

[rxbristol]

I've had a situation where when the temperature is low and the tank is low, A low line pressure situation where the furnace can fire up but then shuts down after a short time because the overall propane line pressure drops and the tank cannot provide enough pressure fast enough. If you have dual tanks, turn on both tanks and even with the low tank as the primary tank selected, the pressure will recover fast enough for the furnace to continue to run until empty.

I've seen this before...just like you said.

 

[dbbls59]

I wonder if silverado23 in onto something. If one tank was nearly empty and not providing enough gas to sustain the flame it would not continue to burn. The auto change over finally switched over to the full tank and the furnace started working as you were looking at it wondering what was wrong. Check as see if one tank is empty. Just a thought.

 

[kakampers]

I've had a situation where when the temperature is low and the tank is low, A low line pressure situation where the furnace can fire up but then shuts down after a short time because the overall propane line pressure drops and the tank cannot provide enough pressure fast enough. If you have dual tanks, turn on both tanks and even with the low tank as the primary tank selected, the pressure will recover fast enough for the furnace to continue to run until empty.

I would agree with this, except our friends, like us have a rented 150 gallon tank which we are running off...the dual tanks are not being used.

 

[Silverado23]

Thanks Dan! We'll check this out....we've got close to a foot of snow...no chance to get an RV tech out here for several days...

Do you have snow built up on the regulator or on any exposed propane lines?

 

[stratcat]

had similiar situation, determined that when the tanks are switched an air bubble in the line works its way to the furnace. the burner won't fire until the air is out of the line. i start the furnace numerous times to force the air out of the line. sometimes the problem won't sow up until 1-2 days after tank change over. just an annoyance of the system design and plumbing.